Fig. 12. The effect of varying stimulus phase or duration during in vitro crawling cycles. (A) Varying the timing of the 0.68 s stimulus by as little as 0.2 s, about 10% of the strain cycle duration, resulted in no positive work being done by the same muscle preparation as in Figs 2 and 11. (B) Varying stimulus duration from 0.28 to 1.08 s for a 3.6 mm VIL while holding the timing of the stimulus midpoint at the same value as in A and in Fig. 11 resulted in positive work being done for a portion of the strain cycle in both cases, as well as for intermediate stimulus duration values (not shown).