The peculiarities which have been described in station, locomotion, and the act of rising to a vertical position nevertheless all depend on such anatomical lesions of the muscles of the back and lower extremities as render the adequate performance of their functions impossible. Thus, the widening of the base of support by straddling the legs is necessitated by weakness in all the extensor muscles of the limbs—the glutæi, quadriceps, and gastrocnemii—which by their tension should normally provide solid columns for the support of the trunk. The lordosis begins with the first difficulty experienced in steadying the heels, but is increased when the gluteals become incapable of extending the pelvis on the femurs and when the sacro-lumbales are unable to extend the vertebral column on the pelvis. The backward projection of the shoulders, effected by the extensors of the upper portion of the spine, is an instinctive compensation for the lordosis, to prevent the trunk from falling altogether forward in front of the base of support.
The lateral oscillations of the trunk have been variously explained. Duchenne attributed them to weakness of the gluteus medius. This muscle, he asserted, is normally designed to restrain the tendency of the pelvis at each step to incline toward the leg which is off the ground.30 But, in reality, during the act of walking, the pelvis, and the trunk with it, are inclined toward the leg which is fixed, rotating upon the head of the femur on that side, and being slightly elevated on the opposite side, where the leg is being swung forward. This elevation assists in enabling the swinging leg to clear the ground (Ross, Hueter). The rotation is accomplished by the gluteal abductors on the active or fixed side, the femoral extremity of these muscles being fixed. Weakness of the gluteals must interfere with this rotation, and should therefore diminish lateral oscillation did this depend on the rotary movement.
30 Archives gén., 1868, p. 28.
In a case examined by Ross, in which the lateral oscillation was much marked, contractions of the gluteus medius were distinctly perceptible to the hand placed just above the great trochanter. In another case, where the gluteals were entirely destroyed, the oscillation, on the contrary, was barely perceptible. Ross himself explains the phenomenon more plausibly as a simple exaggeration of what occurs in normal locomotion. In this the centre of gravity is necessarily shifted at each step from the movable to the fixed leg by the inclination of the trunk and shoulders to the side of the latter. When the legs are placed far apart the body must incline farther in order to bring the weight in the same relative position. Moreover, from the weakness of the anterior tibial muscles the dorsal flexion of the foot, which should take place at the moment the leg is lifted off the ground, is impeded or rendered impossible; and the inclination of the pelvis on one side, which necessitates its increased elevation on the other, thus favors the swinging of the leg by leaving more room between the trunk and the ground (Ross).
The curious manner in which pseudo-hypertrophic patients rise from a sitting or recumbent position has been carefully studied by Gowers, and minutely analyzed by Ross in an adult case. The act to be accomplished demands a series of extensions of the leg and pelvis on the thigh and of the vertebral column on the pelvis. As the extensor muscles are all paretic, this can only be effected by means of the muscles of the upper extremities and of the weight of the body, which the arms compel to serve as a motor force. Thus, from a recumbent position the patient rolls upon his hands and knees: then, grasping the knee, he lifts the leg upright with the foot planted on the ground. The thighs remain strongly flexed, the trunk bent forward over the thighs. The action of grasping the thighs above the knees, which is so characteristic, serves to extend them by a double mechanism. In the first place, the knee-joints are pressed slowly but directly backward. In the second place, by the intermediary of the arms the weight of the body is transferred from the upper end of the femur, above the power of the quadriceps extensor, to the lower end of the lever, near the fulcrum at the knee. Thus a lever of the third order, with the power between the fulcrum and the weight, is partly transformed into a lever of the second order, with the weight between the fulcrum and the power; and thus the enfeebled quadriceps is able to act to more advantage. Moreover, when the body inclines so far forward that the centre of gravity is carried in front of the knees, it then becomes a force applied to the upper end of the femur capable of extending the knees without any action of the quadriceps.
When extension of the knee-joints is nearly complete, extension of the pelvis on the femurs is effected by grasping the thighs alternately higher and higher. By this manoeuvre the femur is pushed back and the trunk is pushed up; and thus is compensated the incapacity of the glutæi to perform their normal action of pulling up the pelvis flexed on the femurs. Enough power remains in these muscles, however, for a long time to complete the extension when, by the pushing movement, this has been nearly effected.
During these actions the patient constantly oscillates the trunk from side to side as he transfers the centre of gravity from one foot to the other. In this, the second stage of the disease, and where the same functional disturbances may arise with very various combinations of hypertrophy and atrophy in the muscles of the lower extremities, a third set of symptoms appears—certain deformities, namely, depending on muscular shrinkage. The earliest, and often the most marked, of these is talipes equinus. The patient becomes unable to plant his heels firmly on the ground, and these are gradually drawn up higher and higher, the patient resting first on the toes, then on the anterior surfaces of the phalanges; ultimately is unable to stand at all, the foot being drawn into a line with the leg, and the astragalus not unfrequently luxated. Some authors explain this deformity by the preponderating action of the gastrocnemius. The paralysis of this muscle, which coincides with its hypertrophy, even when not quite proportioned to it, renders such an explanation highly improbable. The elevation of the heel is due to the gradual shrinkage of the muscular tissue which accompanies the pseudo-hypertrophy; and on this account the talipes is at every stage of its development irreducible.
The other possible deformities in the lower extremities are permanent flexions at the knee- or hip-joints. Both existed in the case recently described by Pekelharing.31 Before the disease has reached its maximum degree of development in the lower extremities, its progress has usually been marked in another manner—namely, by the invasion of the trunk and arms. In cases 19-22 of Gowers's remarkable series, where four boys out of a family of ten children were affected by the disease, the hypertrophy first involved all the muscles of the lower extremities, and then passed to the trunk and arms.32
31 Loc. cit.
32 Three other boys in this family, and three girls, remained healthy.