Fig. 5. Muscle stress plotted against fascicle strain, forming in vivo mass-specific work loops, shown for three level speeds (0.7, 1.3 and 2.0 m s^-1, left to right) and two 16° incline speeds (0.7 and 1.3 m s^-1, broken lines) for the lateral gastrocnemius (LG, top) and the digital flexor-4 (DF-IV, bottom) from one individual that was nearest the means for all birds (bird 4). The area enclosed in the loop is the net work performed per unit muscle mass per stride. Arrows indicate the direction of the work loop. Counter-clockwise loops indicate net positive energy, and clockwise loops indicate net energy absorption. Color indicates timing of electromyographic (EMG) activity relative to force and length change.

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