Fig. 10. Schematic illustration of the effect of phase (between length and force) and strain pattern on digital flexor-4 (DF-IV) muscle work. The center panel shows the relationship between phase and work for the DF-IV (Fig. 8Cii). If the muscle undergoes a stretch-shorten cycle (large velocity) and muscle force and length are symmetrical, no net work results (A). However, if peak force precedes peak length, the muscle absorbs energy (B), and if peak force lags behind peak length, it produces energy (C). However, if the muscle contracts with constant velocity, whether positive, negative or isometric (as shown in D), phase has no impact on work. This explains why the interaction term, phase xvelocity, is a larger factor underlying DF-IV work than phase alone (Table 5); a change in velocity during force production (velocity) is required for phase to be an important factor in work. Vertical lines in A-D indicate peak force.