Everything You Need to Know About Squatting

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The Kinematics of Squatting

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<ul><li><p>SQUATTING KINEMATICS AND KINETICS AND THEIRAPPLICATION TO EXERCISE PERFORMANCEBRAD J. SCHOENFELD</p><p>Global Fitness Services, Scarsdale, New York</p><p>ABSTRACT</p><p>Schoenfeld, BJ. Squatting kinematics and kinetics and their</p><p>application to exercise performance. J Strength Cond Res</p><p>24(12): 34973506, 2010The squat is one of the most</p><p>frequently used exercises in the field of strength and</p><p>conditioning. Considering the complexity of the exercise and</p><p>the many variables related to performance, understanding squat</p><p>biomechanics is of great importance for both achieving optimal</p><p>muscular development as well as reducing the prospect of</p><p>a training-related injury. Therefore, the purpose of this article is</p><p>2-fold: first, to examine kinematics and kinetics of the dynamic</p><p>squat with respect to the ankle, knee, hip and spinal joints and,</p><p>second, to provide recommendations based on these bio-</p><p>mechanical factors for optimizing exercise performance.</p><p>KEY WORDS squat, biomechanics, kinetics, kinematics</p><p>INTRODUCTION</p><p>The squat is one of the most frequently usedexercises in the field of strength and conditioning.It has biomechanical and neuromuscular similar-ities to a wide range of athletic movements and</p><p>thus is included as a core exercise in many sports routinesdesigned to enhance athletic performance (20,62). It also is anintegral component in the sports of competitive weightliftingand powerlifting and is widely regarded as a supreme test oflower-body strength (17,18).Benefits associated with squat performance are not limited</p><p>to the athletic population. Given that most activities of dailyliving necessitate the simultaneous coordinated interactionof numerous muscle groups, the squat is considered one ofthe best exercises for improving quality of life because of itsability to recruit multiple muscle groups in a single maneuver(22). The squatting movement has close specificity to manyeveryday tasks (such as lifting packages and picking upchildren), as well as having an indirect correlation to count-less other chores and hobbies.</p><p>The squat also is becoming increasingly popular in clinicalsettings as a means to strengthen lower-body muscles andconnective tissue after joint-related injury. It has been usedextensively for therapeutic treatment of ligament lesions,patellofemoral dysfunctions, total joint replacement, andankle instability (14,56). Moreover, the closed-chain stancerequired for performance reduces anterior cruciate ligament(ACL) strain (64), making it superior to the knee extensionfor rehabilitation of ACL injury (21,65).Performance of the dynamic squat begins with the lifter in</p><p>an upright position, knees and hips fully extended. The lifterthen squats down by flexing at the hip, knee, and ankle joints.When the desired squat depth is achieved, the lifter reversesdirection and ascends back to the upright position. Thisdynamically recruits most of the lower-body musculature,including the quadriceps femoris, hip extensors, hip adduc-tors, hip abductors, and triceps surae (51). In addition,significant isometric activity is required by a wide range ofsupporting muscles (including the abdominals, erector spinae,trapezius, rhomboids, and many others) to facilitate posturalstabilization of the trunk. In all, it is estimated that over200 muscles are activated during squat performance (66).Squats can be performed at a variety of depths, generally</p><p>measured by the degree of flexion at the knee. Strengthcoaches often categorize squats into 3 basic groupings: partialsquats (40 knee angle), half squats (70 to 100), and deepsquats (greater than 100). However, no standardizedmeasures of quantification have been universally recognized,and terminology can differ between researchers. Othermodifying facts associated with the squat involve varyingintensity of load, foot placement, speed of movement, level offatigue, and position of load.When performed properly, squat-related injuries are</p><p>uncommon (75). However, poor technique or inappropriateexercise prescription can lead to a wide range of maladies,especially in combination with the use of heavy weights.Documented injuries from squatting include muscle andligamentous sprains, ruptured intervertebral discs, spondy-lolysis, and spondylolisthesis (72).Considering the complexity of the exercise and the many</p><p>variables related to performance, understanding squat bio-mechanics is of great importance both for achieving optimalmuscular development as well as reducing the prospect ofa training-related injury. Therefore, the purpose of this article</p><p>BRIEF REVIEW</p><p>Address correspondence to Brad Schoenfeld, brad@lookgreatnaked.com.</p><p>24(12)/34973506</p><p>Journal of Strength and Conditioning Research 2010 National Strength and Conditioning Association</p><p>VOLUME 24 | NUMBER 12 | DECEMBER 2010 | 3497</p><p>Copyright National Strength and Conditioning Association Unauthorized reproduction of this article is prohibited.</p></li><li><p>is 2-fold: first, to examine kinematics and kinetics of thedynamic squat with respect to the ankle, knee, hip. andspinal joints and. second, to provide recommendations basedon these biomechanical factors for optimizing exerciseperformance.</p><p>JOINT KINETICS AND KINEMATICS DURING THE SQUAT</p><p>Ankle Complex</p><p>The ankle complex is comprised of the talocrural and subtalarjoints. Actions taking place at these joints include dorsiflex-ion/plantar flexion and eversion/inversion, as well as a smallamount of abduction and adduction (63,73). During squatperformance, the talocrural joint (articulation of the tibiaand fibula with the talus) facilitates movement throughthe actions of dorsiflexion and plantar flexion, whereas theprimary action at the subtalar joint is to maintain posturalstability and limit eversion/inversion at the foot. Normaltalocrural range of motion is 20 dorsiflexion and 50 plantarflexion, whereas range of motion about the subtalar jointis approximately 5 each for eversion and inversion withoutforefoot movement (13). The gastrocnemius and soleus,collectively referred to as the triceps surae, are the primarymusculature responsible for carrying out dynamic ankle jointmovement, contracting concentrically during plantar flexionand eccentrically during dorsiflexion (54,63).The ankle complex contributes significant support and</p><p>aids in power generation during squat performance (28).However, kinetic data of the ankle during squatting is limitedbecause most studies have focused on the biomechanicsof the knee, hip, and spine. Peak ankle moments of 50 to300 Nm have been reported during the squat, which are farbelow those seen at the knee and hip (18).Dionisio et al. (15) evaluated the ankle complex as part of</p><p>a comprehensive analysis of joint activity while squatting.They found that, in the upright position before squatting, thecenter of pressure (COP) was projected at approximately themid-foot, with ankle joint torque directed toward plantarflexion. During the acceleration phase, COP shifted towardthe heel, with plantar flexion torque decreasing. Finally, thedeceleration phase was characterized by displacement of COPto the toes with a correspondingly large increase in plantarflexion torque about the ankle. Specific quantification of jointtorque, however, was not reported in this study. What is more,subjects were instructed to squat as fast as possible, making therelevance of these findings to slower cadences questionable.The gastrocnemius has been the primary ankle joint muscle</p><p>studied in squat performance. It is believed that the medialhead of the gastrocnemius acts as dynamic knee stabilizerduring squatting, helping to offset knee valgus moments aswell as limiting posterior tibial translation (7,56). Thegastrocnemius shows only moderate levels of activationduring squatting, with activity tending to progressivelyincrease as the knees flex and decrease as the knees extend(16). This is consistent with the fact that its force arm peaks ator near maximal knee flexion (18).</p><p>Toutoungi et al. (70) reported that the soleus was moreactive than the gastrocnemius when squatting at high degreesof flexion. Considering the respective anatomic configura-tions of the 2 muscles, this appears logical. The soleus isa pure plantar flexor with proximal attachments at the tibiaand fibula and distal attachments at the calcaneous. Thegastrocnemius, on the other hand, is a bi-articular muscle thatcarries out plantar flexion as well as assisting in knee flexion.Given these dual roles, the gastrocnemius functions mostlyisometrically during a squat, with little or no change in fiberlength throughout performance.Dionisio et al. (15) reported a co-activation of the gastro-</p><p>cnemius and tibialis anterior during the mid-eccentric phaseof a squat, presumably to provide stability to the ankle.As previously noted, however, these results were achievedat a high speed of movement. Thus, its application would bespecific to protocols designed to achieve speed-strength,including lower-body plyometrics.Weakness of the ankle musculature has been implicated</p><p>in the genesis of faulty movement patterns during the squat.Bell et al. (7) found that a lack of strength in the medialgastrocnemius, tibialis anterior, or tibialis posterior maydecrease an individuals ability to control knee valgus andfoot pronation motions, as well as contributing to excessivemedial knee displacement (MKD) and dynamic valgus.A high degree of mobility at the ankle is required to</p><p>facilitate balance and control in both the ascent and descent ofthe squat.When ankle joint flexibility is compromised, there isa tendency for the heels to rise off the floor at higher degreesof flexion. This can result in compensatory joint momentsat the ankle, knees, hips, and spine, potentially leading toinjury when squatting under external load. Toutoungi et al.(70) reported that ACL forces were significantly increasedwhensquatting was performed with feet flat versus heels elevatedduring both the descent (26 6 31 N vs. 95 6 40 N) and theascent (28 6 36 N vs. 49 6 57 N) of the move. Hemmerichet al. (26) found that a dorsiflexion angle of 38.5 6 5.9 wasnecessary to keep the heels down during a full squat.It has been shown that those exhibiting reduced range of</p><p>motion at the ankle joint have a predisposition to MKD. In astudy by Bell et al. (7), those with MKD displayed a clinicallymeaningful 20% reduced range of motion in dorsiflexionwhile squattinga finding that was attributed in part to tight-ness of the soleus. In addition, tightness of the lateral anklemusculature was shown to contribute to tibial abduction andexternal rotation facilitating excessive MKD and dynamicknee valgus alignment (7). This has implications for injurybecause knee valgus is believed to increase stress on the ACL,especially in combination with internal tibial rotation (42).</p><p>Knee Complex</p><p>The knee joint consists of the tibiofemoral, which carries outsagittal plane movement throughout a range of motion of 0 toapproximately 160 of flexion (36,63,73). The tibiofemoraljoint can be classified as a modified hinge joint that comprises</p><p>3498 Journal of Strength and Conditioning Researchthe TM</p><p>Squatting Kinematics and Kinetics</p><p>Copyright National Strength and Conditioning Association Unauthorized reproduction of this article is prohibited.</p></li><li><p>the articulation of the tibia and femur. A small amount ofaxial rotation is also present at the joint during dynamicmovement, with the femur rotating laterally during flexionand medially during extension with respect to the tibia. Thiscauses the instant center of rotation at the knee to shiftslightly throughout performance of the squat.Assisting the tibiofemoral joint is the patellofemoral joint,</p><p>a gliding joint in which the patella slides over the trochlearsurfaces of the femur during flexion and extension of the knee.This provides additional mechanical leverage in extensionbecause of a greater force arm, as well as reducing wear on thequadriceps and patellar tendons from friction against theintercondylar groove.The knee is supported by an array of ligaments and</p><p>cartilage. Of these structures, the ACL is often considered themost important stabilizer of the joint. Its primary role is toprevent anterior tibial translation at the knee, particularly atlow flexion angles (33). It also plays a role in limiting internaland external rotation of the knee and inhibiting varus/valgusmotion. The posterior cruciate ligament (PCL) can be con-sidered the counterpart to the ACL. Its primary function isto restrain posterior tibial translation (37). The medial andlateral collateral ligaments stabilize the knee in the frontalplane, helping to provide resistance to varus/valgus moments.Whereas the knee ligaments are the main static stabilizers</p><p>of the joint, the knee musculature assumes a dominant rolein dynamic joint stabilization (60). During the squat, theprimary muscles acting about the knee are the quadricepsfemoris (vastus lateralis, vastus medialis, vastus intermedius,and rectus femoris), which carry out concentric knee exten-sion, as well as eccentrically resisting knee flexion. Thequadriceps tendon and patellar tendon facilitate action of theknee extensors, allowing for optimal pull on the tibia duringdynamic movement.The hamstrings (biceps femoris, semitendinosus, semi-</p><p>membranosus) are technically antagonists of the quadriceps,opposing knee extensor moments. In closed-chain exercise,however, they behave paradoxically and cocontract with thequadriceps. This synergistic action has important implica-tions for enhancing the integrity of the knee joint in squatperformance. Specifically, the hamstrings exert a counter-regulatory pull on the tibia, helping to neutralize the anteriortibiofemoral shear imparted by the quadriceps and thusalleviating stress on the ACL (17,56).Forces at the knee during squat performance have been</p><p>extensively studied, with most focusing on 3 major areas ofinterest: a) tibiofemoral compression and shear and patello-femoral compression, b) muscle activity of the quadriceps andhamstrings, and c) anteroposterior and mediolateral kneestability (16).Both tibiofemoral and patellofemoral compression has</p><p>been shown to increase with increasing knee angle (23,49,68).It is theorized that these forces provide a protective functionat the knee by initiating a cocontraction between the quadsand hams. Specifically, the hamstrings exert a counter-</p><p>regulatory force on the tibia by pulling it posteriorly, therebyattenuating anterior tibial translation and counteracting shear(26,56,77).The highest recorded peak tibiofemoral compression</p><p>forces were obtained in a study of powerlifters lifting 2.5times bodyweight. Maximum values reached approximately8,000 N at 130 of knee flexion and were consistent withmaximal forces at the quadriceps tendon (49). These forcesslowly declined to 5,500 N at 60 of flexion. At 30 of kneeflexion, compressive force amounted to roughly 3,500 N,whereas quadriceps tendon force was reduced to approxi-mately 2,000 N. Patellar tendon force was approximately6,000 N at 130 and slowly decreased to approximately 2,000 Nat 30. It is...</p></li></ul>