stj axis location rotational equilibrium theory foot function

Volume 91 • Number 9 • October 2001 465

The subtalar joint is a functional joint of the humanfoot that acts as a mechanical link between the footand the lower extremity. Transverse plane rotationsof the leg are converted into frontal plane rotations ofthe foot, and vice versa, by the oblique triplanar orien-tation of the subtalar joint axis. Two of the more im-portant functions of the subtalar joint are: 1) to allowthe foot to pronate and act as a mobile adapter whenbearing weight on irregular surfaces; and 2) to allowthe foot to supinate into a position of increased sagit-tal plane stability during the propulsive phase of gait.Other functions of the subtalar joint have been dis-cussed previously by other authors.1-4

The characteristics of the axis of rotation of thesubtalar joint have attracted the attention of re-searchers over the years. Manter,5 Hicks,6 Root et al,7

and Isman and Inman8 all suggested in their experi-mental studies that the subtalar joint axis is a solitaryaxis that passes through the talocalcaneal joint and is

directed obliquely from posterior-lateral-plantar toanterior-medial-dorsal. More current research by VanLangelaan,9 Benink,10 and Lundberg and Svensson11

investigated the axis of rotation of the subtalar jointusing more accurate methods of subtalar joint axisspatial location with roentgen stereophotogram-metry. By implanting metallic beads into both thetalus and the calcaneus and then taking standardizedx-rays of the feet in two cardinal planes, these re-searchers all demonstrated that the subtalar jointaxis is not a solitary axis but rather is better de-scribed as a number of discrete axes of rotation that,together, form a bundle of axes passing through thetalocalcaneal joint (Fig. 1).9-11

Therefore, the most accurate research to date hasdemonstrated that the subtalar joint axis no longercan be accurately described as a solitary axis of rota-tion but is best described as a multitude of axes ofrotation that all have different spatial locations. Inturn, each discrete spatial location of the subtalarjoint axis is dependent on the rotational position ofthe subtalar joint axis.9-11 As such, the results fromthese researchers suggest that the morphology of thearticulating surfaces of the talus and calcaneus,

Subtalar Joint Axis Location andRotational Equilibrium Theory of Foot Function

Kevin A. Kirby, DPM, MS*

A new theory of foot function based on the spatial location of the subta-lar joint axis in relation to the weightbearing structures of the plantar footis proposed. The theory relies on the concept of subtalar joint rotationalequilibrium to explain how externally generated forces, such as groundreaction force, and internally generated forces, such as ligamentous andtendon tensile forces and joint compression forces, affect the mechani-cal behavior of the foot and lower extremity. The biomechanical effect ofvariations among individuals in the spatial location of the subtalar jointaxis are explored, along with their clinical consequences, to offer an ad-ditional theory of foot function, which may improve on existing podiatric bio-mechanics theory. (J Am Podiatr Med Assoc 91(9): 465-487, 2001)

*Diplomate, American Board of Podiatric Orthopedicsand Primary Podiatric Medicine; Assistant Clinical Profes-sor, Department of Podiatric Biomechanics, California Col-lege of Podiatric Medicine, San Francisco, CA. Mailing

address: 2626 N St, Sacramento, CA 95816.

ORIGINAL ARTICLE

466 Journal of the American Podiatric Medical Association

along with their three-dimensional (3-D) relation-ships to each other at each point within the subtalarjoint range of motion, determines the spatial locationof the subtalar joint axis for that rotational positionof the subtalar joint. (In this paper, “spatial location”is defined as the 3-D location and the angular orienta-tion of the axis of rotation in relation to anotherpoint of reference. “Rotational position” is defined asa point within the joint’s range of motion, such as“maximally pronated,” “neutral,” or “3° from maximal-ly pronated,” in the specific case of the subtalar joint.)

It has been determined that closed kinetic chainsubtalar joint pronation causes plantarflexion and in-ternal rotation of the talus and closed kinetic chainsubtalar joint supination cause dorsiflexion and ex-ternal rotation of the talus in relation to the plantarcalcaneus and the ground.1, 3 Previous anatomical in-vestigations also have noted that the subtalar jointaxis consistently pierces the talus anteriorly at thesuperior aspect of the talar neck.2, 5, 6, 8, 12 Therefore,as the talar head and neck rotate and translate withinspace in relation to the plantar foot and ground dur-ing closed kinetic chain rotational motions of thesubtalar joint, so does the subtalar joint axis rotateand translate in relation to the plantar foot andground during these same motions (Fig. 2).13-16 Theimportant link between spatial location of the subta-lar joint axis and the rotational position of the subta-lar joint axis has been discussed in detail in the liter-ature by both the current author13-18 and Nester.19

In 1987, this author described a clinical method ofdetermining the representation of the subtalar jointaxis on the plantar foot. How alterations in subtalarjoint axis spatial location can affect the relativepronation and supination moment arms that groundreaction force has available on the plantar foot toproduce subtalar joint moments were described. Theconcepts of medial and lateral deviation of the subta-lar joint axis were introduced to explain the author’sclinical observations that variations in the spatial lo-cation of the subtalar joint axis affected the overallbiomechanics of the foot during weightbearing activ-ities (Fig. 3).20 The author also devised a method forquantifying the plantar position of the subtalar jointaxis, which was first described in a study by Ruby etal21 that measured the effect of certain anatomicalvariables on the peak knee-loading forces duringseated cycling (Fig. 4).

In 1989, the author discussed how pronation andsupination moments are produced across the subta-lar joint axis by the effects of ground reaction forceand muscular contractile force and also describedthe concept of rotational equilibrium across the sub-talar joint axis. A foot with a medially deviated subta-

Figure 1. Research using roentgen stereophotogram-metry 9-11 has demonstrated that the subtalar joint axisis not a solitary joint axis but a continuously movingaxis that is better represented as a bundle of axespassing through the talocalcaneal joint. Here are ex-amples of how the subtalar joint axis may have differ-ent spatial locations in relation to the talus andcalcaneus while in the A, subtalar joint pronated posi-tion (STJP); B, subtalar joint neutral position (STJN);and C, the subtalar joint supinated position (STJS).

A

B

C

STJN

STJS

STJP

STJN

STJS

STJP

STJN

STJS

STJP

STJ pronated

STJ neutral

STJ supinated


lar joint axis causes a net increase in subtalar jointpronation moment, which results in a foot that ispronated in standing and is likely to resist supinationmotion when supination moments act upon it. In ad-dition, a foot with a laterally deviated subtalar jointaxis causes a net increase in subtalar joint supinationmoment, which results in a foot that is neutral to su-pinated in standing and is likely to resist pronationmotion when pronation moments act upon it. Theconcept of rotational equilibrium across the subtalarjoint axis also was used to describe how the rotation-al position of the subtalar joint axis during relaxedbipedal stance is the direct result of the balancing ofthe pronation and supination moments acting acrossthe subtalar joint axis at that time. In addition, theproduction of interosseous compression forces inthe sinus tarsi, which are the result of excessive sub-talar joint pronation moments, was used to describethe biomechanics and orthotic treatment of sinustarsi syndrome.14

The purpose of this paper is to propose a theoreti-cal model of foot function that is based on the previ-ously described concepts of subtalar joint axis loca-tion and rotational equilibrium and that is compatiblewith the findings from current biomechanical re-

search and the author’s clinical observations.14, 20 Theconcepts of subtalar joint axis spatial location andsubtalar joint rotational equilibrium will be describedin detail and will be used to explain the response ofthe foot to the external effects of ground reactionforce and the internal effects of osseous compres-sion forces, ligamentous tensile forces, and musculartensile forces. This theoretical model also will beused to describe how alterations in the rotational po-sition of the subtalar joint and various structural de-formities of the rearfoot and forefoot affect the spa-tial location of the subtalar joint axis in relation tothe foot and therefore also affect the balance of sub-talar joint pronation and supination moments actingon the foot during weightbearing activities.

Normal Location of the Subtalar JointAxis and Its Biomechanical Effects

In his classic 1941 paper on the experimental deter-mination of the spatial location of the subtalar jointaxis in 16 cadaver feet, Manter5 demonstrated thatthe average subtalar joint axis in his samples was an-gulated 16° from the sagittal plane (the sagittal planebeing defined as a line from a bisection of the poste-

Figure 2. When a foot that functions normally is in relaxed bipedal stance, resting slightly pronated from neutralposition, the subtalar joint (STJ) axis passes through the posterior-lateral calcaneus posteriorly and above the firstintermetatarsal space anteriorly (center, B). As the subtalar joint undergoes pronation motion, the talus internallyrotates and medially translates in relation to the plantar foot, causing the subtalar joint axis to internally rotate andmedially translate (left, A). As the subtalar joint undergoes supination motion, the talus and subtalar joint axis ex-ternally rotate and laterally translate in relation to the plantar foot (right, C).

A B CSesamoids

Metatarsalheads

STJ axisSTJ axis

Talus

STJ pronated STJ slightly pronated from neutral STJ supinated


rior heel to the first intermetatarsal space) and 42°from the transverse plane (the transverse plane beingdefined as the plantar aspect of the foot). Manteralso found that the medially directed angulation ofthe subtalar joint axis ranged from 8° to 24° and thesuperiorly directed inclination angle of the subtalarjoint axis ranged from 29° to 47° in the 16 cadaver feet.

In addition to Manter, other authors have per-formed similar experimental studies that demonstratelarge variations among individuals in the angulationof the subtalar joint axis. Therefore, even though an“average” spatial location of the subtalar joint axiscan be quoted for the samples examined by each re-searcher, it remains clear in the review of these ex-perimental studies that each researcher found a largevariation among individuals in the spatial location ofthe subtalar joint axis in the samples studied.6-11

The author has used the palpation method of sub-talar joint axis location for the past 16 years on more

than 2,000 feet to determine the plantar representa-tion of the subtalar joint axis.20 In line with whatother researchers have noted,5-11 the author has alsofound that there is a large variation among individu-als in the angulation of the subtalar joint axis in rela-tion to the sagittal plane of the foot. In addition, alarge variation among individuals in the medialand/or lateral position of the subtalar joint axis in re-lation to the plantar foot also has been found consis-tently.13-17, 20 The author has discovered a large varia-tion among individuals in the representation of thesubtalar joint axis on the plantar foot that was ob-served in many feet examined by means of the palpa-tion method of subtalar joint axis location, and theauthor has hypothesized that this represents a largevariation among individuals in the spatial location ofthe subtalar joint axis.

To determine a normal location for the subtalarjoint axis, the author has relied on the basic hypothe-

Figure 3. Ground reaction force acting on the plantar calcaneus (GRFC) in a foot with a normally positioned subta-lar joint (STJ) axis will cause a subtalar joint supination moment, and ground reaction force acting on the lateralforefoot (GRFFF) will cause a subtalar joint pronation moment (center, B). In a foot with a medially deviated subta-lar joint axis, the moment arm that GRFC has available to cause a subtalar joint supination moment is decreasedand the moment arm that GRFFF has available to cause a subtalar joint pronation moment is increased, which pro-duces a net increase in subtalar joint pronation moment (left, A). In a foot with a laterally deviated subtalar jointaxis, the moment arm that GRFC has available to cause subtalar joint supination moment is increased and the mo-ment arm that GRFFF has available to cause subtalar joint pronation moment is decreased, which produces a netincrease in subtalar joint supination moment (right, C).

A B CGRFFF

STJ axis

STJ axis

GRFC

GRFFF

Medially deviated STJ axis Normally positioned STJ axis Laterally deviated STJ axis


sis that feet that function the most normally1, 3, 4 alsowould have a normal subtalar joint axis spatial loca-tion. When using the subtalar joint palpation method,the author has consistently found that the feet thatfunction the most normally during walking gait havea subtalar joint axis that passes through the posteri-or-lateral heel posteriorly and through the first inter-metatarsal space area of the plantar forefoot anteri-orly (Figs. 2 and 3).13-18, 20 It is this normal position ofthe subtalar joint axis in relation to the plantar foot,first noted by the author 16 years ago, that has

formed the biomechanical basis for the subtalar jointaxis location and rotational equilibrium theory offoot function.

Since the plantar aspect of the foot is the onlyweightbearing aspect of the human body duringwalking activities, the ground reaction force that re-sults from the mass and movements of the humanbody above the foot will have a very significant effecton the function of the foot and lower extremity. Atthe rearfoot, ground reaction force acts mostly at themedial calcaneal tubercle since the lateral calcanealtubercle is largely nonweightbearing, on averagebeing angulated 21° from the weightbearing surfaceof the foot.13 In addition, the plantar aspects of thefirst through fifth metatarsal heads, and to some ex-tent the base of the fifth metatarsal, are the primaryweightbearing structures in the forefoot.22

When the subtalar joint axis is in its normal posi-tion, ground reaction force acting on the medial cal-caneal tubercle causes a supination moment acrossthe subtalar joint axis since the medial calcaneal tu-bercle is medial to the subtalar joint axis. Ground re-action force acting on the second through fifth meta-tarsal heads and the fifth metatarsal base will cause asubtalar joint pronation moment since these plantarstructures are lateral to the subtalar joint axis (Fig. 3).The longer the distance between these plantar struc-tures and the subtalar joint axis, the greater the mo-ment arm will be to produce subtalar joint moments.Therefore, the longer the moment arm and the greaterthe magnitude of ground reaction force acting onthese plantar structures, the greater will be the mag-nitude of supination or pronation moment actingacross the subtalar joint axis.13-17, 20 (“Moment arm” isdefined as the perpendicular distance of the line ofaction of the force to the axis of rotation. “Moment”is the mathematical product of the magnitude of theforce and the length of the moment arm.23)

The rotational effects of the contractile forces ex-erted by the extrinsic and intrinsic muscles of thefoot also are affected by their position relative to theosseous structures of the foot. All extrinsic musclesthat insert or have osseous pulley systems medial tothe subtalar joint axis will exert a subtalar joint supi-nation moment with their contraction. All extrinsicmuscles that insert on the dorsal aspect of the footlateral to the subtalar joint axis or that have osseouspulley systems lateral to the subtalar joint axis willexert a subtalar joint pronation moment with theircontraction. Therefore, in a foot with a normal subta-lar joint axis location, the posterior tibial, gastrocne-mius, soleus, anterior tibial, flexor hallucis longus,and flexor digitorum longus all will have varyinglengths of supination moment arms across the subta-

Figure 4. Illustrated above is the method for quantify-ing the plantar representation of the subtalar joint(STJ) axis as described by the author.20 The longitu-dinal bisection of the foot is first determined by takingthe width (W) of the posterior heel and bisecting it(W/2). The line from the bisection of the posteriorheel to the center of the second metatarsal head de-termines the longitudinal bisection of the foot. Thefirst subtalar joint axis measurement parameter de-termines the angular relationship (θ) of the subtalarjoint axis to the longitudinal bisection of the foot (inthe example above, θ = 23°). The second measure-ment parameter of the subtalar joint axis uses thedistance from the posterior heel bisection to the sec-ond metatarsal head (b) and the distance from thepoint where the subtalar joint axis crosses the longi-tudinal bisection of the foot to the second metatarsalhead (a) to derive the ratio (a/b) indicating the rela-tive position where the subtalar joint axis crosses thelongitudinal bisection of the foot (in the exampleabove, a = 110 mm, b = 203 mm, and a/b = 0.542).

Center of second metatarsal head

STJ axis

W/2W


lar joint axis. In addition, the peroneus brevis, per-oneus tertius, and extensor digitorum longus willhave varying lengths of pronation moment armsacross the subtalar joint axis. The magnitude of thesubtalar joint moments produced by muscular con-traction are dependent not only on the perpendiculardistance of the line of action of the muscular con-tractile force to the subtalar joint axis (ie, momentarm) but also on the magnitude of the muscular con-tractile force.14, 15

Since none of the intrinsic muscles of the plantarand dorsal foot directly cross the subtalar joint, theycannot directly cause either pronation or supinationmoments of the subtalar joint. However, in a fashionsimilar to the ligaments of the plantar and dorsal foot,the intrinsic muscles of the foot can resist deforma-tion of the joints of the foot under weightbearingloads.24 Therefore, both the intrinsic muscles and theligaments of the foot can indirectly affect the overallbalance of pronation and supination moments actingacross the subtalar joint axis by stabilizing the jointsof the foot during weightbearing activities.

Observations from Subtalar Joint AxisPalpation

During the subtalar joint axis palpation method pre-viously described, the examiner uses the thumb ofone hand to exert a force plantarly on the foot anduses the contralateral hand to sense any resultantsubtalar joint motion at the fifth metatarsal head. Ifthe thumb pushing on the plantar foot is medial tothe subtalar joint axis, then subtalar joint supinationoccurs, and if the thumb pushes lateral to the subta-lar joint axis, then subtalar joint pronation occurs. If,however, the examiner detects no motion when thethumb pushes on the plantar foot, then that point isthe point of no rotation and is marked on the plantarfoot (Fig. 5). Palpation of the plantar foot is startedat the plantar heel and is carried out distally at 1- to2-cm intervals to the metatarsal head level to deter-mine the points of no rotation for the plantar foot,which are hypothesized to be the plantar representa-tion of the subtalar joint axis.15, 17, 20

Early on in the performance of the subtalar jointaxis palpation method on many individuals of manyage groups and foot types, the author noticed a seem-ingly unusual finding that seemed contrary to popu-lar podiatric biomechanics theory of the time. Thetheory taught that when ground reaction force actedon the forefoot, its effect on foot function would besignificantly affected by the spatial location of theoblique midtarsal joint axis.3, 4, 25 Owing to this popu-lar theory, the author initially assumed that the line of

points of no rotation on the plantar foot wouldchange direction at the midtarsal joint level since therearfoot points of no rotation should represent thesubtalar joint axis location and the forefoot points ofno rotation should represent the oblique midtarsaljoint axis location. However, in the more than 2,000feet examined in the past 16 years with the subtalarjoint axis palpation method, it has been noted thatthe points of no rotation of the rearfoot and forefootalways can be represented as a single straight line,with no apparent change of direction in the points ofno rotation being at the midtarsal joint level (Fig.6).17, 26, 27

This consistent experimental observation thussupports the theory that any forces that act on theplantar foot while it is loaded to simulate weightbear-ing conditions will have a direct effect in producingsubtalar joint pronation and supination moments, re-gardless of whether that force is applied to the plan-tar rearfoot or plantar forefoot. Therefore, it seemslogical to assume that ground reaction force actingon any aspect of the plantar foot that is medial to thesubtalar joint axis will produce subtalar joint supina-tion moment and any ground reaction force acting onthe plantar foot lateral to the subtalar joint axis willproduce a subtalar joint pronation moment, regard-less of the orientation of the oblique midtarsal jointaxis.14-17, 20

During weightbearing conditions, the combinationof tensile loading of the plantar ligaments and com-pression loading of all the joints of the foot creates,in effect, a more tightly joined and stable architec-ture to the foot when compared with nonweightbear-ing conditions, where the forefoot is unloaded andmore mobile relative to the rearfoot. Therefore, dur-ing many weightbearing motions, the foot may ap-proximate a relatively rigid unit with all the bones ofthe foot rotating around the central pivot of the talusat the subtalar joint axis. If this concept is valid, thenduring many weightbearing motions, the foot can beeffectively modeled as a rigid body with the calca-neus, cuboid, and navicular all rotating as a singleunit around the talus at the subtalar joint axis andthe more flexible forefoot undergoing relativelysmall motions that aid in keeping the forefoot in aplantigrade position.

In regard to the idea of the foot being modeled asa rigid body, Nigg28 has noted that in the field of bio-mechanics, the rigid body concept may be validwhere the deformations of the parts of the structureof interest are insignificant relative to the motion ofthe structure as a whole. In addition, biomechanicsresearchers have effectively modeled the foot as arigid body for years, and this modeling technique is


an integral part of the determination of how forcesacting on the foot affect the biomechanics of thefoot, lower extremity, and body as a whole duringweightbearing activities.29

Constraint and Nonconstraint Mechanisms of the Foot

The idea that the foot may rotate in inversion and ev-ersion about the talus mostly at the subtalar joint andonly slightly at the oblique midtarsal joint duringweightbearing activities is not new or unique. Huson26

proposed that inversion and eversion of the foot oc-curs about the talus at the subtalar joint axis, ratherthan at separate subtalar joint and midtarsal jointaxes. Huson26 classified the foot as functioning withthree osteoligamentous mechanisms: 1) the tarsalmechanism, 2) the tarsometatarsal mechanism, and3) the metatarsophalangeal mechanism.

Huson26 noted that when multiple joints interact toproduce motions, which occurs in the foot, the typeof motion that is produced at the joints may be de-scribed as being either a “constraint” or “noncon-straint” mechanism. In a constraint mechanism, themovement pattern of the joints is imposed by the

shape of the articular surfaces, the insertion patternand length of the joint ligaments, the number of jointswithin the mechanism, and the relative position ofthese joints to each other. In other words, the mo-tions of the bones in a constraint mechanism arekinematically closely coupled. However, in a noncon-straint mechanism, either muscular or external forcesare necessary to determine the direction and magni-tude of motion at the joints. In other words, the mo-tions of the bones in a nonconstraint mechanism mayoccur independently of each other depending on theexternal forces acting on them and are kinematicallyonly loosely coupled to each other.26

Huson26 described the tarsal mechanism as a con-straint mechanism that consists of the calcaneus, nav-icular, and cuboid being kinematically closely cou-pled to each other, rotating on the more fixed talus ina defined and predictable manner as a unit duringfoot inversion and eversion. The tarsometatarsalmechanism was described as being a nonconstraintmechanism since the bones in the tarsometatarsalmechanism may move differently in relation to eachother and in relation to the rearfoot, depending on thedirection and magnitude of ground reaction force ormuscular forces acting on it. The tarsometatarsal

Figure 5. The plantar location of the subtalar joint (STJ) axis is determined by the examiner using one thumb topush on the plantar aspect of the foot and the opposite thumb to detect subtalar joint movements.20 When thethumb presses on the plantar foot medial to the subtalar joint axis, subtalar joint supination motion occurs (left, A).When the thumb presses on the foot lateral to the subtalar joint axis, subtalar joint pronation motion occurs (right,C). Once the thumb presses on a location where no subtalar joint rotational motion occurs, this point on the plantarfoot is marked as a point of no rotation (PNR) (center, B). Plotting the points of no rotation on the plantar foot rep-resents the plantar location of the subtalar joint axis.17

A B C

Sensingthumb

Thumb exertingplantar force

PNR


mechanism consists of the five metatarsal bones andtheir connections to the tarsus, which have variableamounts of stiffness to each other, with the secondmetatarsal acting as the stiffest metatarsal (whichHuson26 describes as a “fixed spoke”) during closed ki-netic chain inversion and eversion of the foot. Huson26

also considered the metatarsophalangeal mechanismto be a nonconstraint mechanism, consisting of themetatarsophalangeal joints since the bones in themetatarsophalangeal mechanism may move indepen-dently of each other, depending on the magnitude anddirection of ground reaction force and muscularforces acting on it.26

A recent study by Cornwall and McPoil27 of 153subjects that measured the 3-D kinematics of the cal-caneus, navicular, and first metatarsal during walk-ing has supported Huson’s theory of strong kinemat-ic tarsal coupling during subtalar joint pronation andsupination. The study demonstrated that the 3-D

movements of the calcaneus and navicular were near-ly identical to each other and that the first metatarsalmoved very similarly to the calcaneus and navicularbut had slightly more independent motion. The au-thors thought that the findings of their study support-ed Huson’s theory of a constraint mechanism of thetarsus and a nonconstraint mechanism of the tar-sometatarsal unit.

The study by Cornwall and McPoil,27 as well as therelatively sophisticated roentgen stereophotogram-metry studies done by Van Langelaan,9 Benink,10 andLundberg and Svensson,11 all indicate that the con-cept of a two-axis midtarsal joint that determines thedirection of motion of the forefoot to the rearfootduring weightbearing activities is probably a greatoversimplification of reality and may not be accurate.In support of this view, in a recent commentary onthe two-axis model of the midtarsal joint, Payne30 re-viewed the available research pertaining to the mid-tarsal joint and suggested that the two-axis model ofthe midtarsal joint needs to be improved upon sincerecent research findings do not support it.

Rotational Equilibrium Across theSubtalar Joint Axis

The basic concept of rotational equilibrium is verystraightforward. It states that for an object to meetthe conditions of rotational equilibrium, it eithermust be at rest (ie, not rotating) or must be rotatingat a constant velocity. In addition, for rotational equi-librium to exist across an axis of rotation, the sum ofthe moments acting in one direction must exactlyequal the sum of the moments acting in the oppositedirection across the axis of rotation (Fig. 7). There-fore, any object with an axis of rotation that is at restmust have a net moment acting across that axis ofrotation that is equal to zero since the moments act-ing in opposite directions exactly counterbalanceeach other.23, 31 Specifically, in regard to the foot, inorder to meet the conditions of rotational equilibri-um across the subtalar joint axis, the subtalar jointeither must not be rotating or must have a constantrotational velocity, and the sum of the magnitudes ofsubtalar joint pronation moments must exactly equalthe sum of the magnitudes of subtalar joint supina-tion moments (Fig. 8).14

During relaxed bipedal stance, the rotational posi-tion of the subtalar joint commonly is determined bymeasuring the position of the calcaneal bisection tothe ground, which is called the relaxed calcanealstance position.32 Feet that function most normallyhave a subtalar joint rotational position in relaxed bi-pedal stance that is approximately halfway between

Figure 6. When the points of no rotation (PNRs) arelocated on the plantar foot by means of the subtalarjoint axis (STJ) palpation technique,20 they approxi-mate the plantar representation of the subtalar jointaxis. The fact that the points of no rotation follow theline of the subtalar joint axis and not the line of theoblique midtarsal joint (OMTJ) axis indicates that re-action forces on both the rearfoot and forefoot act di-rectly on the subtalar joint axis, regardless of theorientation of the oblique midtarsal joint axis. This alsolends support to the theory that the foot may be effec-tively modeled as a type of rigid body rotating aboutthe talus and subtalar joint axis during closed kineticchain pronation and supination motion, as has beensuggested by other researchers.9-11, 26, 27

STJ axis

PNRs

OMTJ axis


mately halfway between the neutral position and themaximally pronated position while in relaxed bipedalstance, it is at this rotational position of the subtalarjoint that the magnitudes of subtalar joint pronationmoment and subtalar joint supination moment mustequal each other exactly. Viewed another way, thefoot will rotate about the subtalar joint in relaxed bi-pedal stance until it “finds” the rotational position ofthe subtalar joint where the sum of the magnitudesof subtalar joint pronation moments and the sum ofthe magnitudes of subtalar joint supination momentscan exactly counterbalance each other so that rota-tional equilibrium can be achieved and the subtalarjoint can come to rest. This mechanism is true notonly for feet that function normally, but also for feetthat function abnormally.

The concept of rotational equilibrium across thesubtalar joint axis can then be used to explain whythe spatial location of the subtalar joint axis is of crit-ical importance to the normal function of the foot.Any translation or rotation of the spatial location ofthe subtalar joint axis away from its normal positionwill cause a change in the lengths of the momentarms that muscular contractile forces and ground re-action force have available to produce either prona-tion or supination moments of the subtalar joint andwill produce a change in the prevailing balance ofsubtalar joint pronation and supination moments(Fig. 8).

Any medial or lateral deviation in the spatial loca-tion in the subtalar joint axis also can have signifi-cant effects on the kinematics and on the magnitudeand temporal patterns of loading forces acting on thestructural components of the foot and lower extremi-ty during weightbearing activities. As mentioned ear-lier, the author has described these foot types withabnormal subtalar joint axis location as having eithermedially or laterally deviated subtalar joint axes.14, 17, 20

It is likely, theoretically, that medial or lateral spatialmovements of the subtalar joint axis in relation tothe plantar foot of as little as 1 mm to 2 mm cancause abnormal foot function and can lead to footand/or lower-extremity pathology.33

It then becomes clear that with the determinationof the spatial location of the subtalar joint axis, alongwith the knowledge that the foot can be approximat-ed or modeled as a rigid body that rotates around thesubtalar joint axis, the concept of subtalar joint axisrotational equilibrium becomes a very effectivemethod of analyzing many of the interactions of theexternal and internal forces acting on the foot. Amore detailed analysis of the concept of rotationalequilibrium across the subtalar joint axis and its ef-fect on the loading forces on the structural compo-

Figure 7. A, When a weight of 200 N is positioned 3.0m from the axis of rotation in a seesaw, 600 Newton-meters (Nm) of moment is produced. Rotational equi-librium can be produced in the seesaw only when thecounterclockwise (CCW) moments equal the clockwise(CW) moments across the axis of rotation. B, If the axisof rotation is moved in the seesaw to a location that is2.0 m from the left end of the board and 4.0 m from theright end of the board, then the counterclockwise mo-ments (200 N × 2.0 m = 400 Nm) are of less magni-tude than the clockwise moments (200 N × 4.0 m =800 Nm). Clockwise rotation will result since rotationalequilibrium is not achieved. C, Rotational equilibriumwill again be achieved once the seesaw board rotates ina clockwise direction until its right end rests on theground and becomes motionless. Ground reaction force(GRF) acting on the right end of the board contributes acounterclockwise moment to the seesaw, which allowsrotational equilibrium to be achieved again.

A

B

C

200 N 200 N

3.0 m 3.0 m

2.0 m 4.0 m

2.0 m4.0 m

200 N

200 N

200 N

200 N

CCW moment = 600 Nm CW moment = 600 Nm

CCW moment = 400 Nm CW moment = 800 Nm

GRF = 100 N

Rotational equilibrium:CW moments = CCW moments

No rotational equilibrium:CCW moments < CW moments400 Nm (CCW) < 800 Nm (CW)Clockwise rotation occurs

Rotational equilibrium:CCW moments = CW moments(200 N)(2.0 m) + (100 N)(4.0 m) = (200 N)(4.0 m)800 Nm (CCW) = 800 Nm (CW)

the neutral and the maximally pronated position ofthe subtalar joint.33 Rotational equilibrium then dic-tates that since the subtalar joint of the normal footcomes to rest at a rotational position that is approxi-


nents of the foot has been described previously bythe author.14

Physical Characteristics of Feet withMedially Deviated Subtalar Joint Axes

Since the subtalar joint axis passes anteriorlythrough the talar neck, a foot that has a talar neckand head that are relatively medially positioned in re-lation to the plantar surface of the foot will also havea medially deviated subtalar joint axis.2, 5, 6, 8, 12 It isthis abnormal medial spatial location of the talarhead that causes the characteristic clinical appear-ance of the foot with medial deviation of the subtalarjoint axis (Figs. 9–11).

During relaxed bipedal stance, feet with mediallydeviated subtalar joint axes will demonstrate the fol-lowing three clinical signs that distinguish them fromother types of feet. First, when the foot is viewedfrom above, the medial border of the midfoot willdemonstrate a convex shape within the transverseplane. Second, also evident when viewed fromabove, the soft-tissue contour of the talar head andneck at the anterior ankle joint area will be more me-dially positioned and internally rotated in relation tothe calcaneus than normal (Fig. 10). Third, when thefoot is viewed from its posterior aspect in a standingposition, there will be increased convexity in the me-dial midfoot area just inferior and anterior to the me-dial malleolus (Fig. 11). All of these clinical signs aredirectly caused by the abnormal medial position ofthe talar head in relation to the calcaneus and rest ofthe foot.

Even though feet with medially deviated subtalarjoint axes typically have lower medial longitudinalarch height than normal, some feet with medially de-viated subtalar joint axes may also have a normalmedial longitudinal arch height during relaxed bi-pedal stance. In addition, the posterior bisection ofthe calcaneus may be either inverted, vertical, or ev-erted during relaxed bipedal stance in feet with me-dially deviated subtalar joint axes. Therefore, mea-surement of the relaxed calcaneal stance position isnot a reliable indicator of the position of the subtalarjoint axis or of the prevailing pronation and supina-tion moments acting across the subtalar joint axisduring weightbearing activities.17

Since the concept of subtalar joint axis spatial loca-tion dictates that the actions of ground reaction forceand muscular contractile force in feet with mediallydeviated subtalar joint axes will cause increased mag-nitudes of subtalar joint pronation moment and de-creased magnitudes of subtalar joint supination mo-ment when compared with normal, these feet nearly

Figure 8. The forces acting on either side of the sub-talar joint (STJ) axis may be modeled to be mechani-cally similar to the seesaw illustrated in Figure 7. Inthis model, counterclockwise rotation is supinationmotion, and clockwise rotation is pronation motion. A,Rotational equilibrium cannot be achieved until thesupination moments equal the pronation moments. B,If a medially deviated subtalar joint axis is present ina foot (modeled above as the left shift of the subtalarjoint axis), the relative moment arms for the forcescausing subtalar joint supination and pronation mo-ments are altered so that the supination moments areless than the pronation moments, which results inpronation motion. C, In a foot with a medially devi-ated subtalar joint axis, the resultant pronation mo-tion will stop only once rotational equilibrium is againachieved by another structure in the foot (eg, the floorof the sinus tarsi of the calcaneus), causing a supina-tion moment to counterbalance the excessive prona-tion moments caused by the medial subtalar joint axisdeviation.14

A

B

C

Force causing STJsupination moment

Force causing STJpronation moment


Supination moment Pronation moment



Medially deviatedSTJ axis



STJ axis

Supination moment Pronation moment

Medially deviated STJ axis

Rotational equilibrium across STJ axis:Supination moments = Pronation moments

No rotational equilibrium:Supination moments < Pronation momentsPronation motion occurs

Rotational equilibrium:Supination moments = Pronation moments


will always be maximally pronated at the subtalarjoint during relaxed bipedal stance.14-17, 34, 35 Owing tothe maximally pronated subtalar joint position, thesefeet will tend to have increased magnitudes of in-terosseous compression forces acting between theanterior aspect of the lateral process of the talus andthe floor of the sinus tarsi of the calcaneus. In gener-al, the greater the medial deviation of the subtalarjoint axis, the greater will be the interosseous com-pression force within the sinus tarsi, owing to the in-creased subtalar joint pronation moments acting onthese feet.14

Because of the increased subtalar joint pronationmoments acting on a foot with a medially deviatedsubtalar joint axis, this foot type will be much morelikely to resist supination motion when subtalar jointsupination moments act upon it. For example, footorthoses designed to increase subtalar joint supina-tion moment will be less likely to supinate feet withmedially deviated subtalar joint axes out of their max-imally pronated position when compared with feetwith more normal subtalar joint axis locations.14-17 Tosufficiently counteract the increased magnitudes ofsubtalar joint pronation moment acting on these feet,conservative treatment of pronation-related symp-toms of the foot and lower extremity should includefoot orthoses using antipronation techniques, such asthe Blake inverted orthosis36-39 and/or medial heelskive techniques.16, 17 It is logical to conclude thatboth the Blake inverted and medial heel skive orthot-ic techniques effectively increase the subtalar joint

Figure 9. Feet with medial deviation of the subtalarjoint (STJ) axis have an internally rotated and mediallytranslated talus that results in excessive subtalar jointpronation moments during weightbearing activities.

Figure 10. Two signs of medial deviation of the sub-talar joint axis, when the foot is viewed from above,are the abnormal medial convexity in the medial mid-foot area (black arrows) and the abnormally mediallypositioned and internally rotated soft-tissue contourof the talar head and neck (white lines and arrows).

Figure 11. When the foot with medial deviation of thesubtalar joint axis is viewed from the posterior aspect,there is abnormal medial convexity in the medial midfootjust inferior to the medial malleolus (white arrows).

supination moments and decrease the subtalar jointpronation moments by redirecting the reactionforces acting on the plantar foot from a more lateralto a more medial location.14-17

Characteristic symptoms that the author has notedas occurring with increased frequency in individualswith medially deviated subtalar joint axes includeplantar fasciitis, hallux limitus, second metatarsopha-langeal joint capsulitis, abductor hallucis musclestrain, sinus tarsi syndrome, posterior tibial tendini-tis, posterior tibial tendon dysfunction, medial tibial

Internally rotated and medially translated talus

STJ axis


stress syndrome, chondromalacia patellae, and pesanserinus bursitis. Many other types of painful syn-dromes of the foot and/or lower extremity also mayoccur owing to the medial deviation of the subtalarjoint axis since the increased subtalar joint pronationmoment that results may cause abnormal magni-tudes of loading forces on the structural componentsof the foot and/or lower extremity. Effective treat-ment of these pathologies with the use of the con-cepts of subtalar joint axis location and rotationalequilibrium are beyond the scope of this paper andare discussed in detail elsewhere.17

Physical Characteristics of Feet withLaterally Deviated Subtalar Joint Axes

Feet that have laterally deviated subtalar joint axesare much less common than feet that have mediallydeviated subtalar joint axes. For a foot to have a lat-erally deviated subtalar joint axis, the talar head andneck must be more laterally positioned in relation tothe plantar foot than normal (Fig. 12). In general, thelateral deviation is more evident at the rearfoot thanat the forefoot when the subtalar joint axis palpationtechnique is used in these feet. The lateral deviationof the subtalar joint axis dictates that the actions ofthe ground reaction force and muscular contractileforce will cause increased magnitudes of subtalar jointsupination moment and decreased magnitudes ofsubtalar joint pronation moment relative to normal.14

During relaxed bipedal stance, feet with laterallydeviated subtalar joint axes will demonstrate the fol-lowing three clinical signs that are caused by the ab-normal lateral location of the talar head. First, whenthe foot is viewed from above, the medial border ofthe midfoot will demonstrate a concave shape withinthe transverse plane. Second, the soft-tissue contourof the talar head and neck at the anterior ankle areawill be more laterally positioned in relation to thecalcaneus than normal (Fig. 13). And third, when thefoot is viewed from posterior in standing, there willbe increased concavity in the medial midfoot areajust inferior and anterior to the medial malleolus(Fig. 14). Feet with laterally deviated subtalar jointaxes tend also to have higher than normal mediallongitudinal arch height and commonly demonstratea pes cavus deformity.

One of the most interesting aspects of this foottype is that when it is observed during relaxed bi-pedal stance, one or both of the peroneal tendonsmay be under tension (ie, bowstrung), indicatingtonic contractile activity in the peroneal muscles(Fig. 15). This clinical finding is evident only whenthe standing patient is observed from the lateral as-pect. Therefore, the examiner must be sure to direct-ly examine the peroneal tendons for tonic peronealcontraction during the standing examination of thepatient.

The increased tonic activity in the peroneus brevisand/or longus produces a subtalar joint pronationmoment that is necessary to counteract the exces-sive subtalar joint supination moments that are com-mon in the foot with a laterally deviated subtalar

Figure 12. Feet with lateral deviation of the subtalarjoint (STJ) axis have an externally rotated and laterallytranslated talus that results in excessive subtalar jointsupination moments during weightbearing activities.

Figure 13. Two signs of lateral deviation of the subta-lar joint axis, when the foot is viewed from above, arethe abnormal medial concavity in the medial midfootarea (black arrows) and the abnormally laterally posi-tioned and externally rotated soft-tissue contour ofthe talar head and neck (white lines and arrows).

Externally rotated and medially translated talus

STJ axis


joint axis. The author has repeatedly observed thatfeet with significant lateral deviation of the subtalarjoint axis will maximally supinate at the subtalarjoint (ie, the foot will invert until the patient is stand-ing on the lateral aspect of the foot) during relaxedbipedal stance if the patient is instructed or shownhow to relax the peroneal muscles during standing.Therefore, the tonic contractile activity of the per-oneal muscles that commonly occurs in feet with alaterally deviated subtalar joint axis is probably nec-essary to allow the foot to function in a plantigradeposition during weightbearing activities. Without ab-normal contractile activity of the peroneal muscles infeet with a moderate to severe lateral deviation ofthe subtalar joint axis, the individual would eitherwalk on the lateral aspects of the feet or repeatedlyexperience inversion ankle sprains.

Because of the increased supination moments act-ing on feet with laterally deviated subtalar joint axes,these feet characteristically also will be much moredifficult to pronate with foot orthoses than feet withmore normal subtalar joint axis locations.14, 17 To suf-ficiently counteract the increased magnitudes of sub-talar joint supination moment acting on these feet,conservative treatment of supination-related symp-toms of the foot and lower extremity should includethe judicious use of foot orthoses with everted heelcup shapes (ie, lateral heel skives), everted balancingpositions, and forefoot valgus wedges. These in-shoemodifications will redirect the reaction forces actingon the plantar foot from a more medial location to amore lateral location, thereby causing increased sub-talar joint pronation moments and a decreased ne-

cessity for the individual to activate the peronealmuscles to allow a more stable, plantigrade gait pat-tern.17 Even though these orthotic modifications areextremely useful in such feet, it also must be stronglyemphasized that attempting to excessively pronate apatient’s foot past the maximally pronated subtalarjoint position with everted orthoses and/or valguswedges must be done with extreme caution and careto avoid possible degenerative changes to the jointsof the foot in the future.

Symptoms that the author has noted as occurringwith increased frequency in individuals with laterallydeviated subtalar joint axes include inversion anklesprains and peroneal tendinitis. By clinical applicationof the above biomechanical theories, the author hassuccessfully and conservatively treated more than 100patients with chronic peroneal tendinitis who had pre-viously failed treatment with multiple other foot or-thosis designs. The methods of conservative treat-ment of pathologies caused by feet with laterallydeviated subtalar joint axes are beyond the scope ofthis paper and are discussed in detail elsewhere.17

Effects of Rearfoot Structure and Position on Subtalar Joint Moments

In a normal foot, since the subtalar joint axis is posi-tioned lateral to the plantar weightbearing area of thecalcaneus (ie, the medial calcaneal tubercle), ground

Figure 14. When the foot with lateral deviation of thesubtalar joint axis is viewed from the posterior as-pect, there is abnormal medial concavity in the me-dial midfoot just inferior to the medial malleolus(black arrows).

Figure 15. Feet with laterally deviated subtalar jointaxes often demonstrate tonic contraction of the per-oneus brevis and/or peroneus longus muscles duringrelaxed bipedal stance (black arrows), which is evi-dent in the visualization and/or palpation of the bow-stringing of the peroneal tendons on the lateralaspect of the foot. Tonic peroneal contraction is oftenpresent in feet with laterally deviated subtalar jointaxes to increase the pronation moment so that theforefoot can remain in a plantigrade position.


reaction force acting on the medial calcaneal tuberclewill cause a subtalar joint supination moment. Themagnitude of subtalar joint supination moment pro-duced is dependent not only on the length of the mo-ment arm from the subtalar joint axis to the plantaraspect of the medial calcaneal tubercle, but also onthe magnitude and direction of the ground reactionforce vector acting on the plantar calcaneus.13-17, 20, 34

The morphology of the articulating surfaces of thetalus and calcaneus determines the spatial locationof the subtalar joint axis in relation to the talus andcalcaneus at each rotational position of the subtalarjoint.9-11 However, morphological variations in thecalcaneus separate and distinct from the talar articu-lating facets also will cause an alteration in the spa-tial location of the subtalar joint axis in relation tothe plantar calcaneus. Therefore, the spatial locationof the subtalar joint axis relative to the plantar calca-neus will be determined not only by the morphologyof the articulating facets of the subtalar joint, butalso by the structure of the body of the calcaneusand by the rotational position of the subtalar joint.34

Sgarlato1 and Root et al 3 have described frontalplane deformities of the rearfoot, such as rearfootvarus and rearfoot valgus, and also have proposedtheories on how these structural deformities of thefoot affect the function of the foot and lower extrem-ity. The author proposes that the biomechanical ef-fects of the frontal plane rearfoot deformities can bemore easily analyzed mechanically and more com-pletely understood if they are instead approachedwith the use of the subtalar joint axis location/rota-tional equilibrium theory.

Rearfoot varus and rearfoot valgus deformitiesshould be considered as having their effect on footfunction not by altering the frontal plane structure ofthe posterior calcaneus but rather by altering the po-sition of the medial calcaneal tubercle in relation tothe subtalar joint axis, which, in turn, effectively al-ters the length of the subtalar joint moment arm tothe medial calcaneal tubercle (Figs. 16 and 17). Rear-foot varus deformity may then be viewed as havingan effect on the subtalar joint moments similar tothat of surgically shifting the plantar aspect of thecalcaneus to a more medial position in relation to thedorsal aspect of the calcaneus since rearfoot varusdeformity, in effect, causes the medial calcaneal tu-bercle to be positioned relatively more medial to thesubtalar joint axis. According to the same logic, rear-foot valgus deformity may be viewed as having an ef-fect on subtalar joint moments similar to that of sur-gically shifting the plantar aspect of the calcaneus toa more lateral position in relation to the dorsal as-pect of the calcaneus since rearfoot valgus deformi-

ty, in effect, causes the medial calcaneal tubercle tobe positioned relatively less medial (or relativelymore lateral) to the subtalar joint axis.17, 18, 20, 34 Inother words, the spatial location of the subtalar jointaxis in relation to the medial calcaneal tubercle is af-fected by variations in the frontal plane structure ofthe calcaneus.

To further clarify these ideas, it is helpful to ana-lyze these rearfoot deformities in more specific de-tail. A foot with a rearfoot varus deformity, whichmay be caused by a varus structural alignment of ei-ther the tibia or the calcaneus, has a calcaneus that isinverted in relation to the ground while in neutralcalcaneal stance position.1 The inverted position ofthe body of the calcaneus positions the medial cal-caneal tubercle more medial to the subtalar joint axisthan would be present in a foot without a rearfootvarus deformity (Figs. 16 and 17). An increase inrearfoot varus deformity will then cause an increasein subtalar joint supination moment due to the in-creased length of moment arm that ground reactionforce has available to produce subtalar joint supina-tion moment. A decrease in rearfoot varus deformitywill cause a decrease in the subtalar joint supinationmoment due to the decreased moment arm thatground reaction force has available to produce subta-lar joint supination moment.17, 40

Therefore, the common clinical observation thatfeet with high degrees of rearfoot varus deformityhave increased tendency toward supination instabili-ty of the rearfoot (ie, inversion ankle sprains) is ex-plained easily by the fact that increased rearfootvarus deformity increases the net subtalar joint supi-nation moment because of the increased length ofsubtalar joint moment arm to the medial calcanealtubercle. However, the common clinical observationthat lesser degrees of rearfoot varus deformity tendto cause pronation of the foot from the subtalar jointneutral position is explained not by the inverted posi-tion of the calcaneus to the ground, but rather by theeffect that rearfoot varus deformity has on the posi-tion of the forefoot to the ground (Fig. 18).40

Rearfoot varus deformity causes the forefoot to beinverted to the ground while the subtalar joint is inneutral position, assuming the foot has no frontalplane forefoot deformity.1 The inverted position ofthe forefoot to the ground causes an increase inground reaction force on the lateral aspect of theforefoot, which causes an increased subtalar jointpronation moment since the lateral forefoot is lateralto the subtalar joint axis (Fig. 18). The increase insubtalar joint pronation moment tends to causepronation motion away from the subtalar joint neu-tral position. Therefore, the final rotational position


Figure 16. The posterior aspect of the right foot is modeled with the calcaneus resting on the medial calcaneal tu-bercle connected via the subtalar joint (STJ) axis to the tibia and talus, which are joined together as a talotibialunit. A, Ground reaction force on the calcaneus (GRFC) in the normal foot causes a subtalar joint supination mo-ment since it acts medial to the subtalar joint axis. B, In a foot with a rearfoot varus deformity, the inverted positionof the rearfoot effectively increases the subtalar joint supination moment by moving the medial calcaneal tuberclemore medial to the subtalar joint axis. C, In a foot with a rearfoot valgus deformity, the everted position of the rear-foot effectively either decreases the subtalar joint supination moment or increases the subtalar joint pronation mo-ment by moving the medial calcaneal tubercle more lateral in relation to the subtalar joint axis.

Figure 17. A, In a normal foot, the medial calcaneal tubercle is medial to the subtalar joint (STJ) axis. B, In a footwith a rearfoot varus deformity, the medial calcaneal tubercle is effectively shifted more medial to the subtalar jointaxis, which increases the subtalar joint supination moment when ground reaction force acts on the calcaneus. C, Ina foot with a rearfoot valgus deformity, the medial calcaneal tubercle is effectively shifted more lateral in relation tothe subtalar joint axis, which either decreases the subtalar joint supination moment or increases the subtalar jointpronation moment.

A

A B C

B CTalotibial

unit

Medial

Calcaneus

Lateral

STJ axis

GRFC GRFC GRFC

Normal Rearfoot varus Rearfoot valgus

Normal Rearfoot varus Rearfoot valgus

Metatarsalheads Sesamoids

Medialcalcanealtubercle

STJ axis STJ axisSTJ axis

STJ axis


that the subtalar joint assumes during relaxed bipedalstance occurs only when the subtalar joint hasreached a rotational position in which an exact coun-terbalancing of subtalar joint pronation moments andsubtalar joint supination moments has been attained(ie, subtalar joint rotational equilibrium).14, 17, 35, 40

Through an analysis similar to the one describedabove, a foot with a rearfoot valgus deformity hasthe medial calcaneal tubercle either less medially po-sitioned or more laterally positioned in relation tothe subtalar joint axis than normal since a rearfootvalgus has an everted calcaneus-to-ground relation-ship (Figs. 16 and 17). In addition, many feet thatfunction similarly to a foot with rearfoot valgus de-formity may not demonstrate an everted frontalplane deformity, but may show apparent lateral dis-placement of the calcaneus to the tibia (John Weed,DPM, personal communication, 1981). Either a rear-foot valgus deformity or a laterally displaced calca-neus will cause the medial calcaneal tubercle to beeither less medially positioned or more laterally posi-tioned than normal, which will cause a decrease in

the moment arm that ground reaction force has toproduce subtalar joint supination moment by its ac-tion on the plantar calcaneus. Severe rearfoot valgusdeformity or severe lateral displacement of the calca-neus may cause such a large lateral relative move-ment of the medial calcaneal tubercle to the subtalarjoint axis that the medial tubercle becomes posi-tioned lateral to the subtalar joint axis. When thislevel of deformity occurs, ground reaction force act-ing on the plantar calcaneus causes a subtalar jointpronation moment rather than the normal subtalarjoint supination moment.32

Therefore, the common clinical finding that feetwith rearfoot valgus deformity and/or a laterally dis-placed calcaneus are maximally pronated at the sub-talar joint during relaxed bipedal stance (and duringother weightbearing activities) is explained easily bythe relative reduction in the moment arm that groundreaction force has available to produce subtalar jointsupination by its actions on the medial calcaneal tu-bercle. Without an adequate subtalar joint supinationmoment acting across the subtalar joint axis to coun-

Figure 18. A, In a rearfoot varus deformity standing in subtalar joint (STJ) neutral position, both the calcaneus andforefoot are inverted to the ground. B, Ground reaction force acting on the calcaneus (GRFC) causes a relativelysmall supination moment when compared with the pronation moment that results from ground reaction force act-ing on the fifth metatarsal head (GRF5). As a result of the larger magnitude of subtalar joint pronation moment,pronation motion occurs until there is ground reaction force acting on the first metatarsal head (GRF1), which alsocauses a decrease in the ground reaction force on the fifth metatarsal head. C, Therefore, subtalar joint pronationmotion will continue in the rearfoot varus foot until a subtalar joint rotational position occurs in which the supina-tion moments counterbalance exactly the pronation moments (right). This mechanical analysis demonstrates thatpronation occurs in a rearfoot varus foot specifically owing to its inverted forefoot position, and not owing to its in-verted rearfoot position.

A B CTalotibial

unit medial

Calcaneus

lateral

STJ axis

GRFC GRF5

GRF1

GRFCGRF5

GRFC

GRF5GRF1

Rearfoot varus: STJ neutral Rearfoot varus: STJ pronating Rearfoot varus: STJ rotational equilibrium


terbalance the pronation moments being caused bythe effects of ground reaction force on the lateral fore-foot and midfoot, the foot is likely to be maximallypronated since the position of subtalar joint rotationalequilibrium for these feet in standing is the maximallypronated subtalar joint position. In effect, feet withrearfoot valgus deformity or a laterally displaced cal-caneus also have medially deviated subtalar joint axesbecause of the relative medial position of the subtalarjoint axis to the plantar calcaneus.14, 15, 17, 20, 33-35

As mentioned earlier, rotational motion of the sub-talar joint during weightbearing activities also altersthe relative position of the medial calcaneal tubercleto the subtalar joint axis (Fig. 19). Closed kineticchain subtalar joint pronation causes the calcaneusto evert in relation to the talus, which causes thetalus and subtalar joint axis to translate medially andinternally rotate in relation to the medial calcaneal tu-bercle. Closed kinetic chain subtalar joint supinationcauses the calcaneus to invert in relation to the talus,which causes the talus and subtalar joint axis totranslate laterally and to rotate externally in relationto the medial calcaneal tubercle.13-19, 34

Therefore, because of these relative spatial transla-tions and rotations of the subtalar joint axis in rela-tion to the plantar foot during closed kinetic chainsubtalar joint pronation and supination, the subtalarjoint moment arms to the medial calcaneal tuberclealso are altered. Subtalar joint pronation causes eithera decrease in the subtalar joint supination momentarm or an increase in the subtalar joint pronation mo-ment arm that ground reaction force has available byits actions on the medial calcaneal tubercle. Subtalarjoint supination causes either an increase in the subta-lar joint supination moment arm or a decrease in thesubtalar joint pronation moment arm that ground re-action force can produce by its actions on the medialcalcaneal tubercle. In effect, pronation and supinationmotion of the subtalar joint can be thought to influ-ence the relative lengths of the supination and prona-tion moment arms to the medial calcaneal tubercle,similar to the way in which structural deformities ofthe rearfoot also influence the moment arm lengths tothe medial calcaneal tubercle.17, 18, 34

The above analysis points to one of the most im-portant aspects of the subtalar joint axis location/ro-

Figure 19. When a normal foot is standing slightly pronated from the subtalar joint (STJ) neutral position, the sub-talar joint axis passes just lateral to the medial calcaneal tubercle posteriorly and passes over the first inter-metatarsal space anteriorly (center, B). Subtalar joint pronation causes the subtalar joint axis to internally rotateand medially translate in relation to the plantar foot so that it becomes positioned well medial to the first metatarsalhead and more medial to the medial calcaneal tubercle (left, A). Subtalar joint supination causes the subtalar jointaxis to externally rotate and laterally translate so that it becomes positioned more laterally over the metatarsalheads and more lateral to the medial calcaneal tubercle (right, C).

A B C

STJ pronated STJ slightly pronated from neutral STJ supinated

Metatarsalheads


STJ axis

Sesamoids

STJ axis


tational equilibrium theory. By focusing on the deter-mination of the relative lengths of the moment armsfrom the medial calcaneal tubercle to the subtalarjoint axis, it allows both rotational position of thesubtalar joint axis and congenital structure of therearfoot complex to be biomechanically quantifiedby means of the same parameters. Combining thisanalysis with the other spatial movements of impor-tant anatomical landmarks in the rearfoot (eg, theposterior calcaneus for determining the moment armlength for the Achilles tendon/gastrocnemius-soleuscomplex) will allow future researchers to more effec-tively quantify the kinetics of the rearfoot complexduring weightbearing activities.

Possibly more importantly, the subtalar joint axislocation/rotational equilibrium theory readily ex-plains the biomechanical effects of rearfoot surg-eries, such as the posterior calcaneal displacementosteotomy, which commonly are used in the treat-ment of acquired flatfoot disorders, such as posteriortibial tendon dysfunction.41-48 Medial displacement ofthe posterior fragment in a posterior calcaneal dis-placement osteotomy alters the relative structural lo-cations of the posterior-plantar calcaneus to themore superiorly located talar articulating facets (Fig.

20). The net mechanical effect of this osteotomy is tocause an overall increase in subtalar joint supinationmoment since both the moment arm that ground re-action force has available to cause subtalar joint su-pination moment and the moment arm that gastroc-nemius and/or soleus contraction has available tocause subtalar joint supination moment are in-creased. By analyzing the relative location of the sub-talar joint axis in relation to the plantar calcaneusprior to rearfoot surgery, the surgeon may be betterable to predict the correct amount of calcaneal dis-placement necessary to achieve optimal postsurgicalresults.13

Effects of Forefoot Structure and Position on Subtalar Joint Moments

Since the plantar aspect of the forefoot is subjectedto ground reaction force in nearly all weightbearingactivities, the relative position of the weightbearingstructures of the forefoot to the spatial location ofthe subtalar joint axis has an important effect on thebiomechanics of the foot and lower extremity as awhole. Closed kinetic chain pronation of the subtalarjoint causes medial translation and internal rotation

Figure 20. A, In a foot with a normal subtalar joint (STJ) axis location, ground reaction force acting under the cal-caneus (GRFC) causes a subtalar joint supination moment. B, In a foot with a severely medially deviated subtalarjoint axis, such as that seen in posterior tibial dysfunction, ground reaction force under the calcaneus actually maycause a subtalar joint pronation moment. C, Posterior calcaneal displacement osteotomies exert their mechanicaleffect on the feet with severely medially deviated subtalar joint axes by altering the relative location of the plantarand posterior calcaneus so that ground reaction forces and Achilles tendon tensile forces acting on the calcaneusare relatively more medially positioned (B) than preoperatively (C).

A B CTalotibial

unit

Calcaneus

Normal STJaxis location

MediallydeviatedSTJ axis

Posteriorcalcanealdisplacementosteotomy

GRFC GRFC GRFC

Normal STJ axis location Medially deviated STJ axis Posterior calcanealdisplacement osteotomy


of the subtalar joint axis in relation to the stationaryforefoot. Conversely, closed kinetic chain subtalarjoint supination causes lateral translation and exter-nal rotation of the subtalar joint axis in relation tothe forefoot (Figs. 2 and 19). Because of the relativetranslations and rotations of the subtalar joint axisabove the weightbearing portions of the forefoot dur-ing closed kinetic chain pronation and supination,these motions will have a direct effect on the relativemoment arm lengths that ground reaction force hasavailable to produce either pronation or supinationmoment of the subtalar joint during weightbearingactivities.13-19

The theory that the ground reaction force actingon the metatarsal heads may, in fact, cause direct pro-duction of subtalar joint supination and/or pronationmoments is based on the premise, which is often usedin biomechanical studies of the lower extremity, thatthe foot may be modeled effectively as a rigid bodyrotating about the talus during weightbearing activi-ties.28, 29 In support of this theory, the author hasnoted earlier that during the subtalar joint axis palpa-tion examination technique on more than 2,000 feet,the points of no rotation on the plantar rearfoot andplantar forefoot represent a single line, indicating thatthe rearfoot and forefoot are basically “locked” into arelatively rigid structure that rotates primarily aboutthe subtalar joint axis (Fig. 6). In addition, Fuller49 re-cently described how the location of the subtalarjoint axis in relation to the center of pressure on ei-ther the plantar forefoot or the plantar rearfoot canexplain the mechanical basis for certain pedal pathol-ogy. Finally, Huson’s theory on the constraint mecha-nism of the human tarsus,26 which has been support-ed by the tarsal kinematic research of Cornwall andMcPoil,27 and the roentgen stereophotogrammetrystudies done by Van Langelaan,9 Benink,10 and Lund-berg and Svensson11 are all consistent with the cur-rent author’s theory that the subtalar joint axis is theprimary inversion/eversion axis of the foot.

To further expand on this topic, an example of anormal foot in which the subtalar joint axis passessuperior to the first intermetatarsal space area of thefoot will be explored. In this foot, ground reactionforce acting on the first metatarsal head will producea small subtalar joint supination moment since thefirst metatarsal head lies medial to the subtalar jointaxis (Figs. 2 and 19). If the foot then pronates so thatthe subtalar joint axis internally rotates and trans-lates medially, the spatial location of the subtalarjoint axis will then be medial to the first metatarsalhead. In this pronated foot, ground reaction forceacting plantar to the first metatarsal head will nowcause a subtalar joint pronation moment since the

first metatarsal head is lateral to the subtalar jointaxis. If the same foot then supinates so that the sub-talar joint axis externally rotates and laterally trans-lates until it is positioned over the third metatarsalhead, then any ground reaction force under the firstmetatarsal head will have a longer subtalar joint supi-nation moment arm than in the initial resting posi-tion. Therefore, the relative spatial location of thesubtalar joint axis to the metatarsal heads is alteredby closed kinetic chain pronation and supination mo-tions that, in turn, will determine both the magnitudeand the direction of moment that is produced acrossthe subtalar joint axis by the actions of ground reac-tion force on any specific weightbearing area of themetatarsal heads or forefoot.13-17, 20

In a similar fashion to closed kinetic chain subtalarjoint pronation and supination, variances in the trans-verse plane structural relationships of the forefoot tothe rearfoot also may alter the relative spatial loca-tion of the subtalar joint axis to the forefoot. Weiss-man50 claims that the ideal forefoot adductus angle is8°. Through radiographic analysis, forefoot rectusfoot types have been defined as having a forefoot ad-ductus angle of less than 12° to 14°, and forefoot ad-ductus foot types have been defined as having a fore-foot adductus angle of greater than 12° to 14°.1, 32, 50

Therefore, the large variances in the transverse planerelationship of the forefoot relative to the rearfoot inthe human population indicates that each foot pos-sesses a unique transverse plane structural angularalignment of the forefoot to the rearfoot, which, inturn, affects the relative alignment of the subtalarjoint axis to the plantar forefoot.

The importance of transverse plane structure ofthe foot on the balance of subtalar joint pronationand supination moments during weightbearing activi-ties can be explained more easily by examining therelative spatial location of the subtalar joint axis tothe forefoot in three feet with identical rearfootstructure but variances in forefoot-to-rearfoot trans-verse plane structure (Fig. 21). Foot A has a relative-ly normal forefoot adductus angle of 10°. Foot B hasan increased forefoot adductus angle of 20°, whichwould be considered a forefoot adductus foot type.Foot C has a forefoot adductus angle of 0°, whichwould be considered a forefoot rectus foot type.

In Foot A, with a 10° forefoot adductus angle, thesubtalar joint axis passes anteriorly over the first in-termetatarsal space. In Foot B, the increased fore-foot adductus angle of 20° causes the forefoot to berelatively more medially positioned in relation to thesubtalar joint axis. The ground reaction force actingon the first metatarsal head of Foot B now has alonger moment arm to produce subtalar joint supina-


tion moment, and ground reaction force acting onthe fifth metatarsal head now has a shorter momentarm to produce subtalar joint pronation moment. Be-cause of this relative medial shift of the forefoot tothe subtalar joint axis, which is caused by the 10° in-crease in forefoot adductus angle, ground reactionforce acting on the plantar forefoot of Foot B nowcauses a net increase in subtalar joint supination mo-ment, when compared with Foot A.

According to the same logic, the decrease in fore-foot adductus angle in Foot C causes a relative later-al shift in the forefoot to the subtalar joint axis, whencompared with Foot A. Ground reaction force actingon the forefoot of Foot C will cause increased subta-lar joint pronation moment since the fifth metatarsalhead now has a greater pronation moment arm andthe first metatarsal head has switched from having asupination moment arm to a pronation moment arm.As a result of this relative lateral shift of the forefootto the subtalar joint axis, which is caused by the 10°decrease in forefoot adductus angle, ground reactionforce acting on the plantar forefoot of Foot C nowcauses a net increase in subtalar joint pronation mo-ment, when compared with Foot A.

The author proposes that the characteristic clini-cal changes that occur in feet with structural vari-ances in the transverse plane relationship of the

forefoot to the rearfoot are due to the relative medialor lateral shift of the forefoot to the subtalar jointaxis and the mechanical effect this has on the bal-ance of subtalar joint moments, which result notonly from ground reaction force but also from mus-cular tensile forces and ligamentous tensile forcesduring weightbearing activities. Even though thespatial location of the subtalar joint axis remainsconstant to the rearfoot as long as the subtalar jointrotational position does not change, the changes intransverse plane structural relationship of the fore-foot to the rearfoot significantly affects the balanceof subtalar joint supination and pronation moments.In turn, any alteration in the balance of subtalar jointpronation and supination moments affects the rota-tional equilibrium across the subtalar joint axis,which will change the mechanical response of thefoot to both externally and internally generatedforces acting upon it.14, 34, 35, 49

The above analysis of the effect of structural vari-ances of transverse plane forefoot-to-rearfoot relation-ship also readily explains the biomechanical effects ofrearfoot and/or forefoot surgeries that alter the trans-verse plane structural position of the forefoot rela-tive to the rearfoot. A classic example of this is theEvans calcaneal osteotomy, in which the anteriorcalcaneus is structurally lengthened by means of an

Figure 21. In Foot A, there is 10° of forefoot adductus and the subtalar joint (STJ) axis is positioned just lateral tothe medial calcaneal tubercle and over the first intermetatarsal space of the forefoot. In Foot B, 10° of forefoot ad-ductus has been added to Foot A so that 20° of forefoot adductus results. The medial shift in the forefoot in rela-tion to the subtalar joint axis in Foot B results in a net increase in subtalar joint supination moment. In Foot C, theforefoot adductus of Foot A has been changed to a forefoot rectus (0° forefoot adductus). The lateral shift in theforefoot in relation to the subtalar joint axis in Foot C results in a net increase in subtalar joint pronation moment.

A B C

Normal (10° forefoot adductus)

Forefoot adductus (20° forefoot adductus)

Forefoot rectus (0° forefoot adductus)

Sesamoids


STJ axis

Metatarsalheads

STJ axis STJ axis


osteotomy and bone graft (Fig. 22). Since the Evanscalcaneal osteotomy structurally lengthens the lateralcolumn of the foot, leaving the medial column thesame length, the forefoot is directly forced into amore adducted structural position in relation to therearfoot.51-56

The increased structural forefoot adductus thatthe Evans procedure creates results in a more medialposition of the forefoot relative to the subtalar jointaxis that, in effect, increases the subtalar joint supi-nation moment by a mechanism similar to that de-scribed above for a forefoot adductus foot type. Thenet result of the Evans procedure, therefore, is to in-crease the subtalar joint supination moments and de-crease the subtalar joint pronation moments actingon the foot; this, in turn, helps eliminate the commonsymptoms seen in flexible pes valgus deformitiesthat are caused by excessive subtalar joint pronationmoments. The above analysis leads to the obviousconclusion that one of the possible practical benefitsof the subtalar joint axis location/rotational equilibri-um theory would be to allow for more accurate pre-operative prediction of the amount of transverseplane correction necessary in a surgery, such as the

Evans calcaneal osteotomy, to achieve optimal post-surgical results.

Conclusion

The subtalar joint axis location/rotational equilibriumtheory of foot function attempts to explain and clarifythe many experimental and clinical observations ofthe mechanical behavior of the foot that researchersand clinicians have made over the years. The theoryalso was developed to explain and clarify the au-thor’s experimental and clinical observations relatingto the mechanical behavior of the foot.13-18, 20, 33-35, 40 Inaddition, the theory was developed to offer an addi-tional theory of foot function that may improve on thetraditional podiatric biomechanics theories, whichare based largely on the works of Root et al.7, 32

The central premise of the theory is that the spatiallocation of the subtalar joint axis in relation to the os-seous components of the foot, which may be alteredby both subtalar joint rotational position and footstructure, directly affects the magnitude and relativebalance of pronation and supination moments actingacross the subtalar joint axis during weightbearing

Figure 22. A, The plantar aspect of a right foot with a severely medially deviated subtalar joint axis (STJ). B and C,The mechanical effect of an Evans calcaneal osteotomy on a foot with a severely medially deviated subtalar jointaxis is to lengthen the lateral column of the foot by interposing a bone graft into the anterior calcaneus so that theforefoot becomes structurally more adducted in relation to the rearfoot and the subtalar joint axis. The Evans cal-caneal osteotomy therefore produces its biomechanical effect by effectively increasing the supination momentacting across the subtalar joint axis in much the same way that an increase in the forefoot adductus angle also in-creases the subtalar joint supination moment.

A B C

STJ axis

STJ axis

Bone graftin place

Bone graft

Calcanealosteotomy

Medially deviated STJ axis Evans calcaneal osteotomywith bone graft

Lateral column lengthening withSTJ axis in more normal position

STJ axis


activities. In addition, alterations in the spatial loca-tion of the subtalar joint axis change the mechanicaleffect that both external forces (eg, ground reactionforce) and internal forces (ie, ligamentous tensileforces, muscular tensile forces, and joint compres-sion forces) have on the structural components of thefoot and lower extremity. The net balance of subtalarjoint pronation and supination moments then affectsnot only the rotational position of the subtalar jointbut also the direction, angular velocity, and angularacceleration of movement of the foot about the subta-lar joint axis, and the other pedal joint axes, duringweightbearing activities.

It is vitally important to point out that the simplifi-cation of the foot into this one model of foot functiondoes not explain all of the complex interactions be-tween the external and internal forces that act on themultiple joint axes of rotation that exist within thehuman foot and lower extremity. Specifically, thejoints of the midfoot, including the midtarsal joints,have been purposely omitted in order to simplify thediscussion of the mechanical characteristics of thesubtalar joint. When this was done, however, manyimportant factors that affect the mechanics of thefoot and that also are critical in determining the me-chanical characteristics of the foot and lower ex-tremity may have been omitted. Hopefully, the theoryof foot function presented here will serve as a usefulbasis for the production in the future of a more com-plete theory of foot function that will, likewise, bebased on scientific research and clinical observation.

The development of better theory to explain theexperimental results of researchers and observationsof clinicians is of paramount importance if a higherlevel of knowledge of the mechanical nature of thefoot and lower extremity is desired. Better theoryalso allows more coherent synthesis of new researchand clinical findings and provides important direc-tion for future research. Finally, and possibly mostcritically, better theory allows for a more completeunderstanding of the complex interactions of thefoot and lower extremity with their surrounding envi-ronment so that more effective methods of treatmentof painful conditions of the foot and lower extremitymay be developed.

Acknowledgment. Special thanks to Eric Lee forhis comments regarding this manuscript.

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stj axis location rotational equilibrium theory foot function

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