145

'APPLICATION OF BIOMECHANICAL METHODS TO THE STUDY OFHUMAN LOCOMOTION AND ATHLETIC ACTIVITIES'

J. P. PAUL, B.Sc., Ph.D., A.R.C.S.T., C.Eng., F.I.Mech.E.

The major characteristic of human locomotion is thatonly in exceptional cases does it proceed along a straightline. The alterations in direction of movement and thealterations in speed of movement correspond toaccelerations to which all body parts are subject. Toproduce accelerations externally imposed forces arerequired and, in fact, a study of displacements without aknowledge of the forces or a study of forces without aknowledge of the displacements gives very little furtherknowledge about the mechanics of the system ofprogression. It is convenient in discussing bodymechanics to think in terms of the centre of gravity ofthe body which is that point at which the whole mass ofthe body may be considered to be concentrated inrespect of its response to external applied force.

During walking on a level surface the centre of gravityof the human, if he is normal, rises and falls twice percycle through a distance of approximately two inchesand sways from side to side once per walking cycle,again through an amplitude of approximately twoinches. Thus the centre of gravity is subject toaccelerations in the vertical direction and accelerationsin the medio-lateral direction and the patterns ofvariation of ground to foot force must correspond tothis.

A more subtle phenomenon is the variation in theforward speed of progression. For a patient walking in astraight line forwards at an average speed measured overa considerable distance of, say, S miles per hour, hefluctuates in forward speed from a maximum of 115% ofS to a minimum of 85% of S during each cycle oflocomotion. Thus opre would expect as well as verticaland medio-lateral forces between the ground and thefoot that there would also be anterior/posteriorcomponent -of ground to foot force. One might expectthat this would be necessary anyway in order tomaintain forward progression but, on a level surface, andin the absence of a contrary wind there is very littleresistance to forward movement and the front to background foot force is almost exclusively concerned withproducing the variations in forward velocity.

The walking cycle can best be described in terms ofthe recognisable events during it. Starting from, say, heelstrike of the left foot the next event for the left foot willbe when it leaves the ground at toe off, it will thenprogress through the swing phase until the next heelstrike of the left foot. This cycle from heel strike of leftfoot to next heel strike of the same foot is taken to be100% if full cycle. For a symmetrical gait, heel strike ofthe right foot would occur at 50% of this cycle and, to

talk in round numbers, it is found that the toe-off of theleft foot would occur at approximately 60% on thecycle, that is, the swing phase occupies 40%o of the cycle.For the right foot therefore, its toe-off would be 40obefore heel strike, that is 40% before 50% of the cyclebeing discussed - this is at the 10o mark measuredrelative to the events of the left foot. In fact, thesenumbers that I quote here are by no means constant andvary particularly with the speed of locomotion. Theslower the walking speed the longer the support phaseand the shorter the swing phase. It will be noticed thatthere are two phases of about 10%o each during whichthe body is supported by both feet and, as walking speedincreases, this overlap period of double supportdiminishes progressively and indeed, in race walking, oneof the difficult decisions to be made by the judges iswhether walking is in fact proceeding and there existsthis period of double support.

In the measurement of ground to foot force the mostusually adopted procedure is to construct adynamometer, or in fact several dynamometers, andinstall them in recesses in the floor surface so that theyare flush with the floor surface, the intention being thatsuccessive foot falls of the test subject should land onseparate dynamometers. Since many people whenwalking have a very narrow lateral tracking distancebetween feet, this means that it is impossible to arrangedynamometers which will cater one for each foot only,although such instruments have been constructed andare in use at some centres. The primary disadvantage isthat the constraint on the test subject to walk with hislegs slightly apart and this is, of course most abnormalfor the majority of walking subjects.

The alternative then is to make each dynamometersufficiently small to accommodate one foot only and yetsufficiently large to make sure that the full period offoot contact takes place while the foot is on thedynamometer. Obviously one cannot train all the testsubjects so that the foot lands in appropriate and correctplaces. Most centres use two plates arranged so thatsuccessive foot falls of left and right foot are measured.Another important factor here is that the human is verysensitive to changes in the supporting surface and thedynamometer therefore must be sufficiently rigid not todeflect more than one thousandth of an inch in anydirection when the foot lands on it and, of course, to besufficiently robust to withstand the forces exerted on it.Another difficulty of any measuring system is theadequacy of its response to dynamic signals. This isgenerally best judged by the speed at which the

copyright. on O

ctober 7, 2021 by guest. Protected by

http://bjsm.bm

j.com/

Br J S

ports Med: first published as 10.1136/bjsm

.6.3-4.145 on 1 Decem

ber 1972. Dow

nloaded from

146

instrument vibrates by itself if set into vibration. It isgenerally conceded that the ideal minimum frequency ofvibration should be five hundred cycles per second,although very few of the force plates currently availablecan equal this ideal performance.

The record obtained for the vertical component ofground to foot force has a characteristic double peakedshape extending from zero at heel strike up to of coursezero again at toe-off. The two maxima correspond veryclosely to the times when the contra-lateral foot is itselfleaving the ground and regaining it. The values of thesemaxima are approximately 35 to 40% greater than bodyweight and between them is a trough of some 60 to 70%of body weight corresponding to the period duringwhich the body is accelerating towards the groundhaving passed its highest point. The front to backcomponent of force is generally initially directed on thefoot in a direction opposite to the progression directionand it remains of this sense up to approximately themiddle of stance, when it changes over to becomeforwards directed on foot. In the overlap during doublesupport there is simultaneously forwards force on therear foot, together with a backward force on the forwardfoot, so that these two components are working againsteach other for part of the cycle. The sideways force bythe ground on the foot is much smaller than the othersin magnitude being at maximum approximately 7% ofbody weight and is for most of the cycle directedmedially.

From the records of displacement of the limbsegments giving instantaneous position at various stagesin the locomotion cycle and the known values of theforces, one can infer the position of the resultant groundforce relative to areas of interest, particularly the joints,and one can therefore infer the forces transmitted byligamentous structures and by muscle. Further, fromviewing successive instants in time, one can recognise themode of action of the muscles, whether they are loadedisometrically or are exerting force and contracting orexerting force and extending, that is absorbing energy.This interpretation however is open to question in theassumptions it makes in respect of the muscles which arecontributing to any load action at any time, in that mostjoint movements correspond to the simultaneous actionof several muscles whose resultant effect is very muchthe same, and the allocation of the share of load toindividual muscles is generally not possible. Certainlymyo electric signals can be measured and the presence orabsence of stimulation signal on the recorder can be usedas an indication whether a muscle is in fact participatingin a particular movement. As far as the gross muscles ofthe leg are concerned, in most normal activity allsynergistic muscles are stimulated to some degree at thesame time and the art of electro-myography has not yetprogressed far enough to be able to allocate a share ofload to the individual muscles corresponding to the

intensity of the EMG signal. The interpretation of theEMG is of course particularly complicated since a givenlevel of signal can correspond to quite different forcesdepending on the velocity of contraction of the muscle,its instantaneous length relative to a standard positionand, also, on whether it is at that instant lengthening orshortening, as well as on other metabolic factors such asfatigue and so on. Even the interpretation of periods ofmuscular activity in the respect of EMG records isfraught with a difficulty which is not always recognisedin that there is a time delay between the onset of EMGactivity and the initiation of force and, correspondingly,a similar delay between the termination of EMG activityand the diminution of muscle force to zero.

There is another instrument which appears to havepossibilities in respect of the recording of locomotioncharacteristics and that is the accelerometer, which ofcourse measures the acceleration, that is the rate ofchange of velocity, along the sensitive axis of theinstrument. These instruments are now commerciallyavailable of suitable sensitivity and suitably small bulkand mass. As with all improvements of this type inmeasurement, there are disadvantages however, in thatthe accelerometer gives only the signal corresponding toacceleration in the direction in which it is pointing, andsince all limb segments rotate about some axis duringactivity, it is very difficult to get exactly the accelerationone wants. Furthermore, the accelerometer gives anothersignal corresponding to the inclination of its sensitiveaxis to the vertical. This is due to the effect of gravityand this cannot be cancelled out. Probably the majordisadvantage of accelerometers is the fact that they haveto be secured to the body. One would probably hope tobe able to measure the accelerations of the skeletalsystem but the trouble is that there is always interveningtissue between the skeleton and the skin surface and it isparticularly difficult to immobilise an accelerometerrelative to the skeletal structure without applying someconstraint to the user.

As was mentioned in respect of force plates it isimportant to consider the dynamic behaviour ofaccelerometers in that they are an instrument withmechanical parts which can vibrate and therefore theirnatural frequency of vibration will control to a largeextent their usefulness in dynamic recording. Generally,the more sensitive an instrument, the lower will be itsnatural frequency, but one would want a naturalfrequency of vibration of an accelerometer to be at least500 cycles per second, even though you are measuringlocomotion, which has a basic speed of approximatelyone cycle per second. The other difficulty withaccelerometers is that each instrument will measure onlythe quantity for the point to which it is attached, andthere are so many segments of the body and directionsin which they can move that to measure all byaccelerometers would mean attaching an impossible loadto the performer.

copyright. on O

ctober 7, 2021 by guest. Protected by

http://bjsm.bm

j.com/

Br J S

ports Med: first published as 10.1136/bjsm

.6.3-4.145 on 1 Decem

ber 1972. Dow

nloaded from

147

To summarise my philosophy of this type of research,I must say that I would think it essential to recordspatial configuration of the trunk and limbs, the forcestransmitted externally to the body and the phasicactivity of the muscles as measured by EMG. In the

absence of information on all these quantities I feel thatonly an incomplete analysis could be made, and, in myopinion, the acquisition of information less detailed thatthis can lead only too easily to erroneous conclusions.

CHANGES IN HEART MOVEMENTS DURING CONTROLLEDPHYSICAL ACTIVITY

R. VAS

Any significant physical activities immediatelyaccompanied by an increase in both heart rate andstroke - volume. The more severe the exercise the greaterthe changes are observed but the fitter the individual themore difficult it is to take consistent results at low levelsof loading and the more difficult it is to extractquantitative information about the functioning of theheart.

This paper is concerned with a new method ofstudying heart action during exercise which involves thedetection and recording of changes in the movement ofthe heart particularly at the apex which is proving to bea very useful parameter.

The principle of the method lies in measuring changesin magnetic inductance using a probe coil which is partof an oscillator and is placed over the apex. Tissuedisplacement under the probe, which need not be incontact with the skin, are converted to electrical signalswhich can be transmitted to any standard recorder. Thedisplacements observed- are complex in origin anddetailed features of the record can be associated withother known cardiological events.

RESULTS OBSER VEDATREST

Fig. 1 shows a typical apex movement as recordedabove the torso of a normal subject in basic metabolicrate (approx. 9OKcal/hr.). The 'a' wave represents the

IN

LV~~~~~~~~~~~~~~~~~~~~0-1See:~ ~~il

apex recoil during atrial contraction. It is said that theascending part of the 'a' wave represents the contractionof the right atrium and the descending part of the 'a'wave stands for the left atrial contraction. Theisovolumetric contraction (some call it the pre-systolicwave) is shown on the heart movement as the ascendingpart of the E wave. The E point itself marks the openingof the aortic valve and therefore the beginning of theejection phase of the heart. In this period the heartejects the blood in the aorta. The 0 point represents theopening AV valves i.e. the beginning of the diastolewhich in turn ends at the onset of the 'a' wave. Two

01 see.

Fig. 2

copyright. on O

ctober 7, 2021 by guest. Protected by

http://bjsm.bm

j.com/

Br J S

ports Med: first published as 10.1136/bjsm

.6.3-4.145 on 1 Decem

ber 1972. Dow

nloaded from