structural differences between them - NCBI

A COMPARISON OF THE JOINTS OF THE ARMAND LEG AND THE SIGNIFICANCE OF THESTRUCTURAL DIFFERENCES BETWEEN THEM

By C. P. MARTINTrinity College, Dublin

THE work embodied in this paper commenced in an attempt to find anexplanation of the many differences in structure between the elbow- and knee-joints in Man. The arm and leg are built on a common plan and in mostrespects closely resemble each other. Consequently the difference in thestructure of the joints is the more remarkable.

The difference can be explained in part by the fact that the bones of thefore-arm have retained the movements of pronation and supination, while inthe leg the bones are almost immovably fixed together. A further part of thedifference can be explained by the fact that the ulna, or post-axial bone,preponderates at the elbow, but the tibia, or pre-axial bone, preponderates atthe knee. Parsons has shown (1) that in the Reptiles the great extensor muscleof the fore-limb is inserted into the ulna, but in the leg the corresponding muscleis inserted into the tibia. This apparently led to the difference in the boneswhich predominate at these two joints. In the Mammals the limbs havedeparted from the reptilian position and have been turned in under the body,the two limbs rotating in opposite directions during this process. The limbshave then become supporting as well as propelling organs. In the leg the tibiacarries the foot and is placed on the medial side of the limb, that is the sidebest fitted for supporting the weight of the body; in the fore-arm the radiuscarries the "hand," and by pronation of the "hand" is also placed on themedial side of the limb. Thus in many quadrupeds the tibia and radius cometo preponderate in the leg and fore-arm respectively; the fibula often disappearscompletely, but the upper end of the ulna into which the extensor muscle isinserted always persists, its persistence has been determined by the insertionof this muscle. This prominence of the ulna at the elbow-joint leads to con-siderable difference between that joint and the knee.

There is, however, one matter of contrast between the joints which cannotbe wholly explained by these facts; namely that the ulna at the elbow projectsas the olecranon above the level of the joint and interlocks with the trochleaof the humerus, whereas the tibia at the knee has no upward projection and inconsequence there is no interlocking of the bones. The ulna cannot be comparedmorphologically with the tibia, but, as stated above, the great extensor musclesof the two limbs are inserted into these bones and therefore they are com-parable functionally.

512 C. P. Martin

The presence of the olecranon is associated to some extent with the freedomof movement between the bones of the fore-arm, for it fixes the ulna as regardsrotation and therefore assists it to act as a base for the movements of pronationand supination of the hand. As, however, a well-developed olecranon is foundin animals which have lost the power of supinating the hand, it must servesome other function. It is this other function of the olecranon which has to bedefined.

The greatest development of the olecranon is found in the highly specialisedquadrupedal Mammals, the Carnivora and the Ungulata. In them the ole-cranon is one of several similar features in the limbs. The skeleton of each limbconsists of three segments, the adjacent segments meeting one another at anangle. In the fore-limb the humerus passes downwards and backwards; theradio-ulna passes downwards and slightly forwards, or sometimes, especiallyin the standing posture of the animal, straight downwards; and the carpus is

Fig. 1. Diagram of limbs of a typical quadruped. Arrow points in direction of animal's head.Note that segments meet each other at an angle and that the upper end of segments whichslope downwards and forwards is prolonged up above articulations in which they take part.

usually aligned with the radio-ulna or inclines slightly backwards in digitigradeanimals and straight forwards in plantigrade animals. In the hind-limb thefemur passes downwards and forwards, the tibia downwards and backwards,and the tarsus downwards and forwards (see fig. 1). In each limb, therefore,the bones pass downwards and forwards or downwards and backwards, andthe corresponding segments in the two limbs pass in different directions. Inboth limbs, with the exception of the carpus which is very short, the upper endsof the bones which pass downwards and forwards are prolonged above thelevel of the articulations in which they take part. Thus in the femur the greattrochanter is prolonged above the level of the hip; in the tarsus the calcaneus isprolonged above the ankle, and in the radio-ulna the olecranon projects abovethe level of the elbow. In these quadrupeds therefore the great trochanter atthe hip-joint and the posterior projection of the calcaneus at the ankle-jointare features similar to the projection of the olecranon at the elbow; and ifany functional significance can be ascribed to the great development of the

Joints of the Arm and Leg and the Structural Differences 513

olecranon in these animals then the same significance will apply to the upwardprojection of the great trochanter and the posterior end of the calcaneus.

When such an animal moves forwards it does so by pushing backwards onthe ground with its hind-feet, and first pulling and then pushing backwardswith its fore-feet. These actions are, by the resistance of the ground, convertedinto a forward motion of the animal's body. The various segments of the limbsare levers by which the animal brings about these stresses and, as the feet arefixed on the ground, the lower ends of the segments are the fulcra of the levers.In the case of the upper segments of each limb the lower end of the segment isnot absolutely fixed, for it is not on the ground, but it is fixed relatively to theupper end. The weight bears on these levers at the centre of the articulationsat their upper ends.

In the case of those segments which pass downwards and backwards,fixation of their lower ends and movement of their upper ends around thisfixed point will not produce any forward movement of the animal, or at mostcan only produce a movement downwards and slightly forwards. But at eachstep the animal requires a forward and upward impulse, the upward elementbeing necessary to balance the force of gravity while the foot is off the groundand the limb is being brought forwards preparatory to another step. But inthe case of those segments of the limbs which pass downwards and forwards,movement of their upper ends around their fixed lower ends will obviouslyproduce a forward and upward impulse to the animal's body. Accordingly,Mivart(2) recognised that the tarsus is the lever mainly concerned in forwardmovement in the cat, and Huxley (3) recognised the same fact as regards Man.It appears therefore that the bones in the limbs which are aligned in a down-wards and forwards direction are the levers by which an animal moves forwards.

As already stated all of these bones are prolonged up above the articulationsat their upper ends, and the extensor muscle by which the power is applied tothe-lever is inserted into the extreme upper end of the prolongation. The bonesare therefore levers of the second order in which the weight is between thepower and the fulcrum. The mechanical advantage of such a lever is obtainedby dividing the total length of the lever from power to fulcrum by the distancefrom the weight to the fulcrum. It is evident, therefore, that the upwardprolongation of these bones confers a mechanical advantage on the extensormuscles when an animal is propelling itself forwards.

Figs. 2 and 3 show the actual conditions found at the ankle- and elbow-joints in quadrupeds. Figs. 4 and 5 show the conditions as they would be ifthe tarsus and radio-ulna did not project up above these joints. In this lastcase if the feet were on the ground and the animal were propelling itself forwardsthe tarsus and radio-ulna would still be levers of the second order, but thepoints of application of the power and of the weight would approximatelycoincide and no mechanical advantage would accrue. For it should be notedthat the lower end of the extensor muscles is composed of non-contractiletendon, and the point of application of the power is not at the insertion of the

Anatomy Lxvmn 33

muscle but at the place where the tendon passes round the upper end of eitherthe tarsus or the radio-ulna.

The mechanism of the lower limb in Man when he is moving forwards isvery similar to that of the hind-limb in quadrupeds during the same action.Keith(4), Hooton(5) and Huxley(3) all recognise that the foot is a lever of thesecond order, and that the backward projection of the calcaneus adds to the

FFig. 2. Fig. 3.

Fig. 2. Plan of condition which is actually found at elbow- and ankle-joints of a quadruped.Fig. 3. Plan to show that radio-ulna and tarsus are levers of the second order when the animal's

feet are on the ground and these bones are used to propel the animal forwards.

Pand ,W

Fig. 4. Fig. 5.

Fig. 4. Plan showing conditions as they would be at the elbow- and ankle-joints of a quadrupedif the radio-ulna and tarsus did not project upwards above these joints.

Fig. 5. Plan showing that conditions shown in fig. 4 would still be a lever of the second orderif the feet are on the ground and the limbs are used to propel the animal forwards.

power of the muscles of the calf. Keith (4) points out that there are some raceswith a long calcaneus and slender calf muscles, and others with a short calcaneusand therefore bulky calf muscles.

If we now consider movement of the tarsus and radio-ulna when the feetare off the ground and the lower ends of these bones are free to move we willfind that the upward projections also in this case add to the power of theextensor muscles but in a different manner to the former case. The weight is

514 C. P. Martin


now approximately at the lower end of the bones; the power is applied at theinsertion of the muscles, and the fulcrum is at the articulation at the upper end.As the fulcrum is between the power and the weight the lever is one of the firstorder, and the mechanical advantage of such a lever is found by dividing thedistance from the fulcrum to the power by the distance from the fulcrum to theweight. Obviously, as in this case, the former distance considerably exceeds thelatter there is no mechanical gain from the lever; there is in fact a loss ofpower but a gain of speed. If the upper ends of these bones did not project, themuscle tendon would have to be placed somewhat as is shown in fig. 4, that is,it would pass round the back of the joint and be inserted into the upper andposterior surface of the bone. The point of application of the power would stillbe at the insertion, but, as this point is now between the weight and thefulcrum, the lever is one of the third order. In levers of the third order themechanical advantage is obtained, as in the other orders, by dividing thedistance from the power to the fulcrum by the distance from the weight to thefulcrum, and in this case again there would obviously be a loss of power but again in speed. This, of course, is always the case with levers of the third order.As far as leverage is concerned, therefore, the advantages of these two arrange-ments would appear to be about equal, and no benefit arises from the fact thatthe bones do project up above the joints. But in the second case, that is, whenthe bones would be acting as levers of the third order, the extensor tendonwould lie nearly parallel to the axis of the bone. The force exerted by thistendon can be resolved into two components, one in the line of the axis of thebone which has no power to move it but can only compress it against the boneabove, and the other at right angles to the axis which is the effective com-ponent in moving the bone. As the tendon would be almost parallel to theaxis its component in the line of the axis would be much greater than itscomponent at right angles to it, and the power of the muscle would be con-siderably wasted. But in the first case, that is, in the actual conditions whichexist in the quadrupeds, the tendon lies almost at right angles to the axis ofthe bone, and consequently the whole of the power of the muscle is availableto move it. The upward projections of the tarsus and radio-ulna thereforeensure that the tendons of the extensor muscles are inserted approximately atright angles to the axes of the bones and therefore increase the power of thesemuscles to move the bones on the articulations at their upper ends.

The olecranon, the upward projection of the calcaneus and, to a lesserdegree, the great trochanter of the femur therefore can be seen to have aconsiderable functional significance in the quadrupeds; they increase veryconsiderably the power of the extensor muscles when the feet are on theground, and these bones are used as levers of the second order to propel theanimal forwards, and they also increase the power of the muscles when thefeet are free and the bones are moved on the articulations at their upper ends.

It should be remarked that two of the levers which produce forwardmovement of the body are situated in the hind-limb, but only one in the fore-

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516 C. P. Martin

limb. This accords with the known fact that it is the hind-limb which impartsthe greater part of the impulse to propel the animal. It is also noteworthy thatthe upward projection of the femur is not so marked as the similar projectionsof the radio-ulna and tarsus. This apparently is due to the fact that the femuris very close to the body wall and therefore muscles can pass from the pelvis tothe shaft or lower end of the bone and be inserted at a favourable angle. Thesemuscles can exert a great force on the femur, and the need for an upwardprojection of the bone to give the muscles a favourable angle of insertion is notso urgent. But as the radio-ulna and calcaneus are at a distance from the bodyand as the limbs cannot become too bulky the muscles which operate on thesebones are compelled to pass down close to and almost parallel to the axis ofthe limb. They therefore would have to be inserted at a very unfavourableangle if the radio-ulna and calcaneus were not prolonged upwards.

If an animal moves backwards it does so by using the segments of the limbswhich pass in the opposite direction, that is, downwards and backwards, aslevers of the second order in the same manner as it used the other segments toobtain forward movement. But animals rarely move backwards, and theability to do so rapidly and forcibly is not of much advantage to them. So wefind that the upper ends of the levers concerned in backward movement are notmodified in the same way that is found in those concerned in forward move-ment. Thus the upper ends of the humerus and tibia-fibula are not prolongedupwards'.

In the quadrupeds the olecranon and upward projection of the calcaneusare adaptations for a definite purpose. What has happened to these processesin animals which have abandoned a quadrupedal gait?

As regards the upward projection of the calcaneus, Hooton(5) gives thefollowing observations for the Primates:

Man Large (A terrestrial biped)Baboon Large (A terrestrial quadruped)Gorilla Medium (A brachiating ape but lives largely on

the ground)Chimpanzee Short (A brachiating ape)Orang-utan Shorter (A more specialised brachiating ape)Gibbon Shortest (The most highly specialised brachiating

form)The remarks in brackets in this table are mine. It will be noticed that thelength of the calcaneus varies inversely as the arm-length to body-length ratio,

1 In some animals, especially those inhabiting rough and broken ground, e.g. goats, sheep,and deer, the greater tuberosity of the humerus does form an upward projection of this boneabove the shoulder-joint. Owing to the rough ground they inhabit such animals have often toraise the front end of the body vertically upwards in order to surmount some obstacle, and thishas often to be done from a standing position. The animal accomplishes this movement bypushing downwards and slightly forwards with its fore-legs. The humerus is therefore used asa lever of the second order to propel the front end of the body upwards and slightly backwards.Hence the increased leverage of this bone furnished by the projection.


in other words the more a brachiating habit of life is adopted the greater is theretrogression of the calcaneus.

Other animals which have ceased to use the hind-limb as an organ ofterrestrial propulsion furnish similar evidence. Thus in the bats the posteriorend of the calcaneus does not project backwards but is turned downwards.These animals when resting suspend themselves by their hind-limbs, and thedownward turned tuberosity of the calcaneus is admirably suited to flex thesoles of the feet towards each other when the creature is hanging by its feetfrom a branch. It thus may be regarded as an adaptation for this peculiarmode of resting. Among the sloths the tuberosity varies. In Choloepus it isshort, flattened from side to side, and its lower edge projects downwards, thusapproaching the condition found in the bats. In Bradypus it is long, but theflattening from side to side and the turning down of its lower edge are stillevident.

From the above facts it seems clear that in those animals which haveceased to use the hind-limb as an organ of propulsion the posterior projectionof the calcaneus has been modified to a greater or less degree. In the oppositedirection we may notice the great development of this projection in thesaltatory animals. It seems evident therefore that this projection is a leverdeveloped in terrestrial quadrupeds or bipeds and adapted for propelling theanimal forwards.

Similar evidence seems to apply in the case of the olecranon of the ulna.In the specialised quadrupeds it is always large. In those animals which haveceased to use the fore-limbs as organs of propulsion it has almost invariablybeen modified. It is absent in the bats. In the kangaroos, wallabies and slothsit is small. In the jerboas it is rather large, but these animals are possiblyquadrupeds when feeding or traversing short distances. In the brachiatingapes it is small. Man occupies a position between the lower apes and theanthropoids. Martin(6) gives the following figures for the height of the uppersurface of the olecranon in the Primates:

Gorilla 0-8 Australian 1-8Gibbon 1.0 Negro 1.9Orang-utan 11 Fuegian 2-5Chimpanzee 1-4 Neanderthal 4*7European 17 Lower apes 6-4Melanesian 17 Lemurs 8*8

The above figures are obtained by making a true outline of the upper end of theulna at right angles to the plane of the central ridge of the greater sigmoidcavity. The axis of the upper end of the bone is drawn on this outline, and aperpendicular to the axis is drawn from the tip of the olecranon. The maximumdistance from the upper surface of the olecranon to this perpendicular is thenmeasured and expressed as a percentage of the physiological length of the ulna.

From this table certain facts are evident. First, that in the higher races of

518 0. P. Martin

Man the olecranon is gradually disappearing. Secondly, that in the anthropoidsit has already disappeared to a greater extent than in Man. Thirdly, amongthe anthropoids the extent to which the olecranon has diminished is, with theexception of the gorilla, directly in proportion to the extent to which brachi-ating habits have been adopted. In face of these facts it seems difficult to avoidthe conclusion that the olecranon was first developed to serve as a lever for theextensor muscles of the fore-limb in quadrupedal animals, and therefore toincrease the efficiency of these muscles in propelling the animal forwards.Subsequently, in those animals that have abandoned quadrupedal methods ofprogression the olecranon has retrogressed.

If the above views are correct it would appear that the extent to which theupper ends of the femur, tarsus and ulna project in Man should furnish someevidence as to his ancestry, and as to the particular line of development bywhich his ancestors were gradually modified. For these projections are alladaptation for the use of the limbs as terrestrial propelling organs. Man stilluses his legs for this purpose but he has ceased so to use his arms. It is easytherefore to account for these features in his lower limbs. For when his ancestorsdescended from the trees and became terrestrial bipeds the posterior projectionof the calcaneus still served as the main lever for propelling his body forwardsand therefore would be conserved or even increased, and as regards the greattrochanter, though with Man's upright attitude it lost its usefulness as a leverfor propelling the body forwards, yet it became a very convenient insertionpoint for the abductors and rotators of the thigh. The size of Man's calcaneuswould suggest, however, that his ancestors descended from the trees before thedevelopment of brachiating habits had led to its retrogression, otherwise itmust have secondarily reincreased.

The relatively large olecranon in Man, on the other hand, is not so easilyexplained unless it is accepted as a heritage from his quadrupedal, or semi-quadrupedal ancestors. It should be noted that all the lower apes are semi-quadrupedal in gait even in their arboreal habits. Morton (7) uses the expression" pronograde apes " to distinguish them from the orthograde brachiatinganthropoids. The size of Man's olecranon then suggests that his ancestorscame from a race of semi-quadrupedal apes with well-developed olecrana, andthey must have become terrestrial before brachiating aboreal habits led to itsretrogression to such a marked extent as we see in the anthropoids.

But once Man's ancestors had adopted a terrestrial mode of life theybecame implement users, and then certain factors would tend to conserve theolecranon. For in such actions as striking downwards with a hammer, club orsword, or hurling a javelin, rapid and forcible extension of the fore-arm isessential, and earlier in this paper it was pointed out that the presence of theolecranon not only added to the power of the extensor muscles when thearm was used as a propelling limb but also increases the power of thesemuscles when the ulna is extended on the humerus. The olecranon there-fore would be useful in the performance of these acts and its atrophy would


thereby be at least delayed. This perhaps explains the large olecranon ofNeanderthal Man as he is generally depicted wielding an enormous club.But in modern races with their universal machinery it is slowly disappearing.

Morton (7) gives a summary of several different lines of evidence on thequestion of Man's ancestry and concludes that he and the modern anthropoidshad a common ancestor, but that the pre-human stock diverged from the pre-anthropoid one before the latter developed marked specialisations for abrachiating and arboreal life. The evidence furnished by the olecranon appearsto confirm his conclusions and constitutes an additional and independent pieceof evidence which has not hitherto been adduced.

The reason why the human knee-joint is so unlike the elbow-joint is thatthe two limbs rotated in opposite directions when the transition from thereptilian to the mammalian stage was taking place. This led to the correspond-ing segments in the two limbs sloping in different directions, for the uppermostsegment of each had to slope towards the other so as to bring the limb as nearlyas possible under the centre of gravity of the body. Those segments whichsloped downwards and forwards became the levers for propelling the animalforwards, and owing to the importance of this movement their upper endsbecame prolonged upwards to give increased efficiency in this movement. Theother segments of the limbs remained unmodified. In the quadrupeds thereforethe elbow-joint is mechanically equivalent to the ankle-joint, and the knee tothe wrist. The two former are at the upper end of a lever concerned in forwardprogression; the two latter are not so situated. Therefore the knee- and wrist-joints are in many respects very similar in structure, and the elbow- andankle-joints also closely resemble each other.

Some other points of contrast between the two limbs in Man can beexplained if the above facts are borne in mind. Thus the tarsus is rather longand the separate bones are firmly fixed together, while the carpus is short andthe bones loosely attached. Again, the distal articular surface in the ankle-joint is composed of a single bone, but the similar surface at the wrist is com-posed of three bones. But as the tarsus is one of the levers concerned in forwardprogression, it is obviously advantageous to make it unyielding, to give it areasonable length in order to acquire speed, and to make the point where theweight bears on it-that is at the ankle-joint-as rigid as possible. But theseinfluences have not operated at the carpus, for it did not become a lever muchconcerned with forward progression, even in the plantigrade animals where itpasses forwards, apparently because it is situated at the lower end of a bonewhich slopes downwards and forwards in the quadrupeds, and therefore it hada poor purchase for pushing the body weight forwards.

Further, before the common ancestors of Man and the anthropoids hadtaken to an arboreal life, the two limbs had already rotated. For such a life itis necessary that the palms of the hands and the soles of the feet should be ableto turn inwards so as to face each other. In the case of the hands this abilityis obtained by retaining the power of supination, but in the case of the feet as

520 C. P. Martin

the legs had rotated in the opposite direction to the arms, retention of thepower to pronate would result in turning the soles of the feet outwards. Sowe find that the ability to pronate the hind-limb has been lost, but at the mid-tarsal joint the power to invert the foot has been developed. In the Primates,then, inversion of the feet becomes the mechanical equivalent of supination, orpartial supination of the hands.

CONCLUSIONS

Many of the structural differences which exist between the human knee- andelbow-joint, and between the ankle- and wrist-joints are due to the fact thatMan is descended from a quadrupedal, or semi-quadrupedal arboreal type.The olecranon especially bears witness to Man's semi-quadruped ancestors.The evidence suggests that Man's ancestors abandoned the trees and becameterrestrial bipeds before the brachiating specialisations which are so evident inthe anthropoids began to develop. The evidence from these joints thereforeconfirms the conclusions drawn by Morton and other workers from evidencesupplied by other anatomical features.

I desire to express my thanks to Prof. Walmsley of Queen's University,Belfast, for the loan of much literature, and for many helpful suggestions inthe writing of this paper.

REFERENCES

(1) PARSONS, F. G. (1908). "Further remarks on traction epiphyses." J. Anat. and Phys.vol. XLII, p. 388.

(2) MIVART, ST G. (1881). The Cat. London.(3) HUXLEY, T. H. (1870). Lessons in Elementary Physiology. London.(4) KEITH, A. (1925). Engines of the Human Body, p. 59. London.(5) HOOTON, E. A. (1931). Up from the Ape. New York.(6) MARTIN, R. (1928). Lehrbuch der Anthropologie. Jena.(7) MORTON, D. J. (1927). "Human origin." Amer. J. Physical Anthropology, vol. x, p. 173.

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