Fig. 2. Representative strain, total stress and power when muscles 178 and 179 operated under in vivo strain and stimulation conditions. The shaded area represents the shortening phase of the oscillatory cycle. The stimulation pattern, represented on the graphs by the yellow squares, was also determined during running at the animal's preferred speed (3 pulses at 100 Hz) (Full et al., 1998; Ahn and Full, 2002). (A) The strain pattern as determined during preferred speed running. Since muscle 178 is slightly shorter in length, 178 experiences slightly longer strain amplitudes (18.5% for muscle 178, 16.4% for muscle 179). (B) In situ muscle stress during imposed running conditions. The shaded areas represent the shortening (or stance) phase of the oscillatory cycle. (C) Instantaneous muscle power during running. Muscle 178 absorbs energy during lengthening, while generating power during the shortening phase. Muscle 179 also absorbs energy during lengthening, but did not generate power during the shortening phase. (D) Work loops for muscles 178 and 179 under in vivo conditions. Work loops show that muscle 178 generated no net mechanical power over the cycle, while muscle 179 absorbed net mechanical power over the cycle. The arrows indicate the directions of the loops. The dominant clockwise work loop for muscle 179 illustrates that this muscle generated higher forces during lengthening than during shortening, resulting in negative work or mechanical energy absorption when operating under the conditions experienced during preferred speed running. These data for muscle 179 were previously reported (Full et al., 1998; Ahn and Full, 2002), and are presented to allow direct comparison to the data of muscle 178.