Walking is accomplished by poising the weight on one foot and then tilting the body forward with the other, which is then swung in front and placed on the ground to prevent falling. These acts are performed alternately by each leg, so that the "swinging limb" becomes the "supporting limb" of the next step. The swinging leg is described as having two phases, (1) active, while pushing off from the ground, and (2) passive, while swinging forward like a pendulum. In starting, one foot is placed behind the other, so that the line of gravity lies between the two, the hindmost limb having the ankle and knee a little bent. By suddenly straightening these joints it gives a "push off" with the toes and propels the body forward, so as to move it around the axis of motion of the fixed, or supporting ankle joint. At the end of the swing, the pendulous leg comes to the ground, and leaves the other limb in the attitude ready for the push off. Thus, on level ground walking is carried on. with but small muscular exercise; but in ascending an incline or going up stairs, the supporting limb has to elevate the body at each step by extending the knee and ankle joints by the thigh extensors and the calf muscles.
Running is distinguished from walking by the fact that, while in the latter both feet rest on the ground for the greater part of each pace, in the former the time that either foot rests on the ground is reduced to a minimum, and the body can never be said to be balanced on either leg, so that, in fact, there is no longer a "support limb." The legs are kept in a semiflexed position, ready for the push off or spring, which is so forcibly carried out that the body is propelled through the air without any support between each step, and has a constant tendency to fall forward. Thus, an interval of greater or less duration, according to the pace, exists during which both the feet are off the ground, because, the moment either foot comes to the ground, it at once executes a new spring without waiting for the swing of the other.