The swinging movement of the posterior leg during the periods which, with Dalla Rosa, I have named the third and first has hitherto been considered essentially a pendulum movement in accordance with the explanation of the brothers Weber, and this idea has been almost universally carried too far.

I consider there are three factors producing the forward movement of the swinging leg during the third period - the muscle work of the flexors of the hip, especially ilio-psoas, rectus, femoris, and tensor fasciae latae, the weight of the swinging leg acting behind the point of fixation, and, lastly, the rotation of the pelvis round the transverse and vertical axes of the hip of the supporting leg, the muscle work in rotation on the vertical axis being assisted by the acquired momentum. I consider, further, that the proportion of work performed by these different forces varies considerably in different ways of walking, but am inclined to think that in all cases muscular action is the most important.

A pendulum movement of the arms often takes place, especially in quick walking, forward and backward in the opposite direction to the rotation of the pelvis, i.e., the arm swings backward on the side on which the pelvis rotates forward, and vice versa. This is done to assist the rotation of the trunk in the opposite direction to that of the pelvis, and, in my opinion, is the result of muscular activity.

There are few people nowadays who do not consider muscular activity to be the essential force in all kinds of walking. As certainly as walking downhill is helped by gravity, so certainly is walking uphill hindered by gravity. If in walking we have some help from the vis inertiae of the mass movements during the period which, with Dalla Rosa, we have called the second, we certainly also use muscular power to raise the centre of gravity, for the muscles of the lower leg fix the ankle in a position of dorsal flexion, while the powerful quadriceps femoris extends the knee. This raising of the centre of gravity varies in different walks, occasionally occurring at the end of the second but generally at the beginning of the third period. One may regard this as a storing up of the force of gravity by means of muscular power. As in a machine for altering the direction of movements, so in the first period, when we allow the centre of gravity which had just been carried obliquely upward and forward to one side to fall obliquely downward and forward through the sagittal plane to the other side, the force of gravity which had been raised and stored up by means of muscular power is set free as momentum.

When the foot of the swinging leg touches the ground this momentum is converted into heat.

The momentum, when once acquired by muscular force after walking for a short time, naturally helps, especially in quick walking.

In quick walking we increase the number of steps taken in a second and lengthen each step; the latter is done by flexing the moving leg more at the hip. Since this leg can be brought to the ground further forward than before, the posterior leg before leaving the ground is also inclined more than before by means of a stronger dorsal flexion at the ankle. Both knees are then fully extended, the anterior is then raised more from the ground than before, and by stronger plantar flexion at the ankle of the posterior leg the body is powerfully driven forward. If this last movement is done forcibly, the posterior leg may leave the ground before the anterior one has reached it. This means that we have changed from walking to running, at least as long as the body is propelled forward in an almost horizontal direction by means of the strong plantar flexion. If, on the other hand, the body is propelled directly upward, which is often helped by flinging the arms upward too, the movement then becomes jumping. In both cases the character of the movement has been changed, so that the period during which both feet touch the ground has been exchanged for one in which neither foot touches the ground.

If we want to take one good run (maximum length) or one good jump (maximum height), we prepare for it by taking a start, so producing a rapid momentum and great force. We then land with both feet on the ground, and with knees bent, and prevent a jar by bending them still more. This flexion in gymnastic language is termed a combined eccentric flexion. Quadriceps femoris resists this flexion and is thereby lengthened.

From the above we see that the muscle work of walking and running is not confined to the 56 per cent. of our muscle mass which is concerned with the movements of the lower extremities. The extensors of the spine help as well as the lateral flexors of the trunk situated on its dorsal and ventral aspects, and obviously also the muscles which rotate the trunk on its vertical axis. The flexors and extensors of the arm at the shoulder also act, producing the pendulum movement already described. Lastly, to add to all these important facts, we must remember that the whole respiratory apparatus, and especially the respiratory muscles, work more quickly and strongly during walking than during rest. In walking we thus use the greater part of our muscular system more completely and harmoniously than in ordinary gymnastic movements, and obtain in the best possible way the physiological effects of exercise on circula-lation, respiration, and metabolism, to which are due its great hygienic and therapeutic value. Of all "Medical Gymnastic Systems" the most powerful and the most far-reaching is walking.