Fig. 2. (A) Contour plot showing the range of duty cycles and phases of activation that can achieve 99%, 80%, 60% and 40% of maximum power output (solid lines) and efficiency (broken lines). The colour bar represents the absolute values for power output (P_oL_o) and efficiency. The values for achieving optimum power (x) and efficiency (o) are also shown. The frequency of oscillation was 1.25 Hz. (B) Work loops (force vs length) for optimal power output (Bi) and optimal efficiency (Bii). The work loops of the contractile element (CE, green), the series elastic element (SEE, red) and the muscle tendon unit (MTU, blue) are shown. Positive length changes indicate stretch and the force is the same in the SEE, CE and MTU at any point in time. The duty cycle and phase for optimal power were found to be 0.368 and –5.11, respectively, compared to 0.098 and 5.91 for optimal efficiency. The thick lines indicate the duration of activation.