A COMPARISON of the JOINTS of the ARM and LEG and the SIGNIFICANCE of the STRUCTURAL DIFFERENCES BETWEEN THEM by C

Home , Arm, Olecranon, Tarsus (skeleton)

A COMPARISON OF THE JOINTS OF THE ARM AND LEG AND THE SIGNIFICANCE OF THE STRUCTURAL DIFFERENCES BETWEEN THEM By C. P. MARTIN Trinity College, Dublin THE work embodied in this paper commenced in an attempt to find an explanation 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 most respects closely resemble each other. Consequently the difference in the structure of the joints is the more remarkable. The difference can be explained in part by the fact that the bones of the fore-arm have retained the movements of pronation and supination, while in the leg the bones are almost immovably fixed together. A further part of the difference 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 at the knee. Parsons has shown (1) that in the Reptiles the great extensor muscle of the fore-limb is inserted into the ulna, but in the leg the corresponding muscle is inserted into the tibia. This apparently led to the difference in the bones which predominate at these two joints. In the Mammals the limbs have departed from the reptilian position and have been turned in under the body, the two limbs rotating in opposite directions during this process. The limbs have then become supporting as well as propelling organs. In the leg the tibia carries the foot and is placed on the medial side of the limb, that is the side best fitted for supporting the weight of the body; in the fore-arm the radius carries the "hand," and by pronation of the "hand" is also placed on the medial side of the limb. Thus in many quadrupeds the tibia and radius come to preponderate in the leg and fore-arm respectively; the fibula often disappears completely, but the upper end of the ulna into which the extensor muscle is inserted always persists, its persistence has been determined by the insertion of this muscle. This prominence of the ulna at the elbow-joint leads to considerable difference between that joint and the knee. There is, however, one matter of contrast between the joints which cannot be wholly explained by these facts; namely that the ulna at the elbow projects as the olecranon above the level of the joint and interlocks with the trochlea of the humerus, whereas the tibia at the knee has no upward projection and in consequence there is no interlocking ofthe bones. The ulna cannot be compared morphologically with the tibia, but, as stated above, the great extensor muscles of 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 freedom of movement between the bones of the fore-arm, for it fixes the ulna as regards rotation and therefore assists it to act as a base for the movements of pronation and supination of the hand. As, however, a well-developed olecranon is found in animals which have lost the power of supinating the hand, it must serve some other function. It is this other function of the olecranon which has to be defined. The greatest development of the olecranon is found in the highly specialised quadrupedal Mammals, the Carnivora and the Ungulata. In them the olecranon is one of several similar features in the limbs. The skeleton of each limb consists of three segments, the adjacent segments meeting one another at an angle. In the fore-limb the humerus passes downwards and backwards; the radio-ulna passes downwards and slightly forwards, or sometimes, especially in 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 which slope downwards and forwards is prolonged up above articulations in which they take part. usually aligned with the radio-ulna or inclines slightly backwards in digitigrade animals and straight forwards in plantigrade animals. In the hind-limb the femur 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, and the corresponding segments in the two limbs pass in different directions. In both limbs, with the exception of the carpus which is very short, the upper ends of the bones which pass downwards and forwards are prolonged above the level of the articulations in which they take part. Thus in the femur the great trochanter is prolonged above the level of the hip; in the tarsus the calcaneus is prolonged above the ankle, and in the radio-ulna the olecranon projects above the level of the elbow. In these quadrupeds therefore the great trochanter at the hip-joint and the posterior projection of the calcaneus at the ankle-joint are features similar to the projection of the olecranon at the elbow; and if any 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 upward projection of the great trochanter and the posterior end of the calcaneus. When such an animal moves forwards it does so by pushing backwards on the ground with its hind-feet, and first pulling and then pushing backwards with its fore-feet. These actions are, by the resistance of the ground, converted into a forward motion of the animal's body. The various segments of the limbs are levers by which the animal brings about these stresses and, as the feet are fixed 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 is not absolutely fixed, for it is not on the ground, but it is fixed relatively to the upper end. The weight bears on these levers at the centre of the articulations at 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 this fixed point will not produce any forward movement of the animal, or at most can only produce a movement downwards and slightly forwards. But at each step the animal requires a forward and upward impulse, the upward element being necessary to balance the force of gravity while the foot is off the ground and the limb is being brought forwards preparatory to another step. But in the case of those segments of the limbs which pass downwards and forwards, movement of their upper ends around their fixed lower ends will obviously produce a forward and upward impulse to the animal's body. Accordingly, Mivart(2) recognised that the tarsus is the lever mainly concerned in forward movement 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 downwards and forwards direction are the levers by which an animal moves forwards. As already stated all of these bones are prolonged up above the articulations at their upper ends, and the extensor muscle by which the power is applied to the-lever is inserted into the extreme upper end of the prolongation. The bones are therefore levers of the second order in which the weight is between the power and the fulcrum. The mechanical advantage of such a lever is obtained by dividing the total length of the lever from power to fulcrum by the distance from the weight to the fulcrum. It is evident, therefore, that the upward prolongation of these bones confers a mechanical advantage on the extensor muscles 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 if the tarsus and radio-ulna did not project up above these joints. In this last case if the feet were on the ground and the animal were propelling itself forwards the tarsus and radio-ulna would still be levers of the second order, but the points of application of the power and of the weight would approximately coincide and no mechanical advantage would accrue. For it should be noted that the lower end of the extensor muscles is composed of non-contractile tendon, and the point of application of the power is not at the insertion of the Anatomy Lxvmn 33 514 C. P. Martin muscle but at the place where the tendon passes round the upper end of either the tarsus or the radio-ulna. The mechanism of the lower limb in Man when he is moving forwards is very 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 the second order, and that the backward projection of the calcaneus adds to the

F Fig. 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 quadruped if 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 order if 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 races with a long calcaneus and slendercalf muscles, and others with a short calcaneus and therefore bulky calf muscles. If we now consider movement of the tarsus and radio-ulna when the feet are off the ground and the lower ends of these bones are free to move we will find that the upward projections also in this case add to the power of the extensor muscles but in a different manner to the former case. The weight is Joints of the Arm and Leg and the Structural Differences 515 now approximately at the lower end of the bones; the power is applied at the insertion ofthe 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 first order, and the mechanical advantage of such a lever is found by dividing the distance from the fulcrum to the power by the distance from the fulcrum to the weight. Obviously, as in this case, the former distance considerably exceeds the latter there is no mechanical gain from the lever; there is in fact a loss of power but a gain of speed. If the upper ends of these bones did not project, the muscle 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 and posterior surface of the bone. The point of application of the power would still be at the insertion, but, as this point is now between the weight and the fulcrum, the lever is one of the third order. In levers of the third order the mechanical advantage is obtained, as in the other orders, by dividing the distance from the power to the fulcrum by the distance from the weight to the fulcrum, and in this case again there would obviously be a loss of power but a gain 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 that the bones do project up above the joints. But in the second case, that is, when the bones would be acting as levers of the third order, the extensor tendon would lie nearly parallel to the axis of the bone. The force exerted by this tendon can be resolved into two components, one in the line of the axis of the bone which has no power to move it but can only compress it against the bone above, and the other at right angles to the axis which is the effective component in moving the bone. As the tendon would be almost parallel to the axis its component in the line of the axis would be much greater than its component at right angles to it, and the power of the muscle would be considerably wasted. But in the first case, that is, in the actual conditions which exist in the quadrupeds, the tendon lies almost at right angles to the axis of the bone, and consequently the whole of the power of the muscle is available to move it. The upward projections of the tarsus and radio-ulna therefore ensure that the tendons of the extensor muscles are inserted approximately at right angles to the axes of the bones and therefore increase the power of these muscles to move the bones on the articulations at their upper ends. The olecranon, the upward projection of the calcaneus and, to a lesser degree, the great trochanter of the femur therefore can be seen to have a considerable functional significance in the quadrupeds; they increase very considerably the power of the extensor muscles when the feet are on the ground, and these bones are used as levers of the second order to propel the animal forwards, and they also increase the power of the muscles when the feet 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 forward movement of the body are situated in the hind-limb, but only one in the fore- 33 -2 516 C. P. Martin limb. This accords with the known fact that it is the hind-limb which imparts the greater part of the impulse to propel the animal. It is also noteworthy that the upward projection of the femur is not so marked as the similar projections of the radio-ulna and tarsus. This apparently is due to the fact that the femur is very close to the body wall and therefore muscles can pass from the pelvis to the shaft or lower end of the bone and be inserted at a favourable angle. These muscles can exert a great force on the femur, and the need for an upward projection of the bone to give the muscles a favourable angle of insertion is not so urgent. But as the radio-ulna and calcaneus are at a distance from the body and as the limbs cannot become too bulky the muscles which operate on these bones are compelled to pass down close to and almost parallel to the axis of the limb. They therefore would have to be inserted at a very unfavourable angle if the radio-ulna and calcaneus were not prolonged upwards. If an animal moves backwards it does so by using the segments of the limbs which pass in the opposite direction, that is, downwards and backwards, as levers of the second order in the same manner as it used the other segments to obtain forward movement. But animals rarely move backwards, and the ability to do so rapidly and forcibly is not of much advantage to them. So we find that the upper ends of the levers concerned in backward movement are not modified in the same way that is found in those concerned in forward movement. Thus the upper ends of the humerus and tibia-fibula are not prolonged upwards'. In the quadrupeds the olecranon and upward projection of the calcaneus are adaptations for a definite purpose. What has happened to these processes in animals which have abandoned a quadrupedal gait? As regards the upward projection of the calcaneus, Hooton(5) gives the following 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 the length 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 bone above the shoulder-joint. Owing to the rough ground they inhabit such animals have often to raise the front end of the body vertically upwards in order to surmount some obstacle, and this has often to be done from a standing position. The animal accomplishes this movement by pushing downwards and slightly forwards with its fore-legs. The humerus is therefore used as a 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. Joints of the Arm and Leg and the Structural Differences 517 in other words the more a brachiating habit of life is adopted the greater is the retrogression of the calcaneus. Other animals which have ceased to use the hind-limb as an organ of terrestrial propulsion furnish similar evidence. Thus in the bats the posterior end of the calcaneus does not project backwards but is turned downwards. These animals when resting suspend themselves by their hind-limbs, and the downward turned tuberosity of the calcaneus is admirably suited to flex the soles of the feet towards each other when the creature is hanging by its feet from a branch. It thus may be regarded as an adaptation for this peculiar mode of resting. Among the sloths the tuberosity varies. In Choloepus it is short, flattened from side to side, and its lower edge projects downwards, thus approaching the condition found in the bats. In Bradypus it is long, but the flattening from side to side and the turning down of its lower edge are still evident. From the above facts it seems clear that in those animals which have ceased to use the hind-limb as an organ of propulsion the posterior projection of the calcaneus has been modified to a greater or less degree. In the opposite direction we may notice the great development of this projection in the saltatory animals. It seems evident therefore that this projection is a lever developed in terrestrial quadrupeds or bipeds and adapted for propelling the animal 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 have ceased to use the fore-limbs as organs of propulsion it has almost invariably been modified. It is absent in the bats. In the kangaroos, wallabies and sloths it is small. In the jerboas it is rather large, but these animals are possibly quadrupeds when feeding or traversing short distances. In the brachiating apes it is small. Man occupies a position between the lower apes and the anthropoids. Martin(6) gives the following figures for the height of the upper surface of the olecranon in the Primates: Gorilla 0-8 Australian 1-8 Gibbon 1.0 Negro 1.9 Orang-utan 11 Fuegian 2-5 Chimpanzee 1-4 Neanderthal 4*7 European 17 Lower apes 6-4 Melanesian 17 Lemurs 8*8 The above figures are obtained by making a true outline of the upper end of the ulna at right angles to the plane of the central ridge of the greater sigmoid cavity. The axis of the upper end of the bone is drawn on this outline, and a perpendicular to the axis is drawn from the tip of the olecranon. The maximum distance from the upper surface of the olecranon to this perpendicular is then measured 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 anthropoids it has already disappeared to a greater extent than in Man. Thirdly, among the anthropoids the extent to which the olecranon has diminished is, with the exception of the gorilla, directly in proportion to the extent to which brachiating habits have been adopted. In face of these facts it seems difficult to avoid the conclusion that the olecranon was first developed to serve as a lever for the extensor muscles of the fore-limb in quadrupedal animals, and therefore to increase the efficiency of these muscles in propelling the animal forwards. Subsequently, in those animals that have abandoned quadrupedal methods of progression the olecranon has retrogressed. If the above views are correct it would appear that the extent to which the upper ends of the femur, tarsus and ulna project in Man should furnish some evidence as to his ancestry, and as to the particular line of development by which his ancestors were gradually modified. For these projections are all adaptation for the use of the limbs as terrestrial propelling organs. Man still uses his legs for this purpose but he has ceased so to use his arms. It is easy therefore to account for these features in his lower limbs. For when his ancestors descended from the trees and became terrestrial bipeds the posterior projection of the calcaneus still served as the main lever for propelling his body forwards and therefore would be conserved or even increased, and as regards the great trochanter, though with Man's upright attitude it lost its usefulness as a lever for propelling the body forwards, yet it became a very convenient insertion point for the abductors and rotators of the thigh. The size of Man's calcaneus would suggest, however, that his ancestors descended from the trees before the development of brachiating habits had led to its retrogression, otherwise it must have secondarily reincreased. The relatively large olecranon in Man, on the other hand, is not so easily explained 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 brachiating anthropoids. The size of Man's olecranon then suggests that his ancestors came from a race of semi-quadrupedal apes with well-developed olecrana, and they must have become terrestrial before brachiating aboreal habits led to its retrogression to such a marked extent as we see in the anthropoids. But once Man's ancestors had adopted a terrestrial mode of life they became implement users, and then certain factors would tend to conserve the olecranon. For in such actions as striking downwards with a hammer, club or sword, or hurling a javelin, rapid and forcible extension of the fore-arm is essential, and earlier in this paper it was pointed out that the presence of the olecranon not only added to the power of the extensor muscles when the arm was used as a propelling limb but also increases the power of these muscles when the ulna is extended on the humerus. The olecranon therefore would be useful in the performance of these acts and its atrophy would Joints of the Arm and Leg and the Structural Differences 519 thereby be at least delayed. This perhaps explains the large olecranon of Neanderthal 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 the question of Man's ancestry and concludes that he and the modern anthropoids had a common ancestor, but that the pre-human stock diverged from the pre- anthropoid one before the latter developed marked specialisations for a brachiating and arboreal life. The evidence furnished by the olecranon appears to confirm his conclusions and constitutes an additional and independent piece of evidence which has not hitherto been adduced. The reason why the human knee-joint is so unlike the elbow-joint is that the two limbs rotated in opposite directions when the transition from the reptilian to the mammalian stage was taking place. This led to the corresponding segments in the two limbs sloping in different directions, for the uppermost segment of each had to slope towards the other so as to bring the limb as nearly as possible under the centre of gravity of the body. Those segments which sloped downwards and forwards became the levers for propelling the animal forwards, and owing to the importance of this movement their upper ends became prolonged upwards to give increased efficiency in this movement. The other segments of the limbs remained unmodified. In the quadrupeds therefore the elbow-joint is mechanically equivalent to the ankle-joint, and the knee to the wrist. The two former are at the upper end of a lever concerned in forward progression; the two latter are not so situated. Therefore the knee- and wrist- joints are in many respects very similar in structure, and the elbow- and ankle-joints also closely resemble each other. Some other points of contrast between the two limbs in Man can be explained if the above facts are borne in mind. Thus the tarsus is rather long and the separate bones are firmly fixed together, while the carpus is short and the 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 composed of three bones. But as the tarsus is one of the levers concerned in forward progression, it is obviously advantageous to make it unyielding, to give it a reasonable length in order to acquire speed, and to make the point where the weight bears on it-that is at the ankle-joint-as rigid as possible. But these influences have not operated at the carpus, for it did not become a lever much concerned with forward progression, even in the plantigrade animals where it passes forwards, apparently because it is situated at the lower end of a bone which slopes downwards and forwards in the quadrupeds, and therefore it had a poor purchase for pushing the body weight forwards. Further, before the common ancestors of Man and the anthropoids had taken to an arboreal life, the two limbs had already rotated. For such a life it is necessary that the palms of the hands and the soles of the feet should be able to turn inwards so as to face each other. In the case of the hands this ability is 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 the power to pronate would result in turning the soles of the feet outwards. So we 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, or partial supination of the hands.

CONCLUSIONS Many of the structural differences which exist between the human knee- and elbow-joint, and between the ankle- and wrist-joints are due to the fact that Man 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 became terrestrial bipeds before the brachiating specialisations which are so evident in the anthropoids began to develop. The evidence from these joints therefore confirms the conclusions drawn by Morton and other workers from evidence supplied 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 in the 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.