Fig. 6. Muscle power output as a function of the phase of stimulation onset for muscles 177c and 179. (A) Muscle power for both muscles relative to the phase of stimulation onset. Muscle power varied as a function of stimulation onset phase for both muscles. The stimulation onset phase at which power peaks differed for the two muscles. The region within the vertical lines indicated by the double-headed arrow represents the range of stimulus onset phase within which muscle 177c performed positive work while muscle 179 absorbed mechanical energy. Within these lines, muscle 177c always functioned as a power generator when operating under preferred-speed running conditions (two muscle action potentials per cycle; 8 Hz cycle frequency). Muscle 179 always functioned as an energy absorber when operating under preferred-speed running conditions (three muscle action potentials per cycle; 8 Hz cycle frequency). The filled green (177c) and open red (179) symbols represent the measured mean onset phases during running, including standard deviations. (B) Normalized muscle strain plotted versus phase. Muscle length for each muscle, where 177c is represented by the green line and 179 is represented by the red line, was normalized relative to its maximum and minimum lengths.