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Fig. 4. Positive and negative external work durations. (A) Old subjects, (B) young subjects, with the data from A (gray) superposed for comparison. The time during which positive external work is done at each step during the push (open circles and red horizontal bars in Fig. 1), and negative external work is done during the brake (filled circles and blue horizontal bars in Fig. 1) are plotted as a function of the running speed. It can be seen that the duration of positive external work is greater than the duration of negative external work up to a speed of 13 km h^–1 (Table 1) indicating an average force during the brake greater than during the push, which is qualitatively consistent with the force–velocity relation of muscle. This suggests that work done by muscle within the muscle-tendon units at low speeds is progressively substituted, with increasing speed, with elastic energy storage and recovery by tendons (see text). On average, the ratio t_push/t_brake is greater in the old subjects indicating a less elastic behavior (Table 2). Asterisks denote statistically significant difference (P<0.05). Other indications as in Fig. 2.

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