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Fig. 4. Mechanical power output is plotted against the phase of muscle activation at four mean experimental lengths, and three strain amplitudes. The data shown are from one muscle preparation (Moth 1 in Tables 1, 2). (A–C) The muscle was subjected to length oscillations at three experimental strain amplitudes with mean values of 0.105±0.0053L_op (A), 0.078±0.0058L_op (B), and 0.049±0.0143L_op (C) (N=76 for each amplitude). At each amplitude setting, we imposed muscle length changes symmetrically around four experimental lengths: 0.98L_op (red), 1.02L_op(blue), 1.05L_op(green), and 1.12L_op(yellow). We measured mechanical power output at each combination of amplitude and experimental length as we varied the phase of activation through the strain cycle in 19 evenly spaced increments, expressed as a fractions of the cycle period. (D) Combined data from A–C. Limitations of our feedback controller resulted in some variation in strain amplitude within any one sweep of phase values. The effects of changes in strain amplitude and experimental length were small compared to the variation in mechanical power output with changes in the phase of activation. At each combination of strain amplitude and experimental length, power varied through a single maximum and minimum as we changed the phase of activation from 0 to 1. Power was positive between phase values of 0.2 and 0.6, and maximal between 0.3 and 0.4. With increasing amplitude, both the magnitude of the peak positive power output as well as the peak rate of energy dissipation (negative power) increased. Positive power output was consistently lowest at the experimental lengths (0.98L_op (red), and 1.02L_op (blue)) that were closest to the in vivo operating length.

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