Fig. 7. Muscle strain, contraction velocity, contraction duration, and in situ force generation for muscles 177c and 179 as a function of time. (A) Muscle 179 (dashed lines) underwent larger strain amplitudes because it is shorter than 177c (solid lines). (B) Because both muscles cycled at (8 Hz) during running, 179 experienced faster shortening and lengthening velocities than the longer 177c. (C) Contraction duration is represented by the time to 50 % relaxation. During running, muscle 179 was stimulated for longer (three muscle action potentials at 100 pulses s–1) than 177c (two muscle action potentials at 100 pulses s–1) and thus 179 generated force for longer. (D) Muscle 177c generated force during muscle shortening and relaxed before the muscle began to lengthen. Muscle 179, however, generated lower levels of force during shortening, possibly because it shortened at very high velocities. Muscle 179 began generating force as the contraction velocity slowed and approached zero. Muscle 179 was still active as the muscle lengthened, when it generated its highest forces, resulting in net negative work during the running cycle. The shaded areas indicate the stance phase of running when the muscles shorten. Yellow squares represent the timing of muscle action potentials.
