Fig. 6. Positive and negative external work durations. The times during which positive external work is done at each step during the push (open circles and red bars in Fig. 1) and negative external work is done during the brake (filled circles and blue 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 clearly greater than the duration of negative external work up to a speed of 14 km h^–1 (asterisks indicate P<0.05: see Table 1). This suggests that, with increasing speed, the work contribution by the contractile machinery is progressively substituted by elastic storage and recovery by tendons (see text). Statistics as in Fig. 2.