Figure 3.

(a) Old subjects. (b) Young subjects. (a(i),b(i)) The external work done per unit distance (filled circles) is plotted with its two components: the work done against gravity (open circles) and to sustain the forward velocity changes (open squares). Note that the external work is lower in old than in the young subjects due to a lower work done against gravity with a similar work to sustain the forward velocity changes. (a(ii),b(ii)) The total mechanical work done per unit distance (dashed line) is given as the sum of the external work (solid line) and the internal work to accelerate the limbs relative to the centre of mass (dotted line). Note the sharp increase with speed of the total work done by the old subjects due to a greater internal work resulting from their greater step frequency (figure 2). Statistics and other indications are the same as in figure 1.