The mechanical power output of the pectoralis muscle of blue-breasted quail (Coturnix chinensis): the in vivo length cycle and its implications for muscle performance

Sonomicrometry and electromyographic (EMG) recordings were made for the pec toralis muscle of blue-breasted quail (Coturnix chinensis) during take-off and horizontal flight. In both modes of flight, the pectoralis strain traje ctory was asymmetrical, with 70 % of the total cycle time spent shortening. EMG activity was found to start just before mid-upstroke and continued int o the downstroke. The wingbeat frequency was 23Hz, and the total strain was 23 % of the mean resting length. Bundles of fibres were dissected from the pectoralis and subjected in vitro to the in vivo length and activity patterns, whilst measuring force. The n et power output was only 80 W kg(-1) because of a large artefact in the for ce record during lengthening. For more realistic estimates of the pectorali s power output, we ignored the power absorbed by the muscle bundles during lengthening. The net power output during shortening averaged over the entir e cycle was approximately 350 W kg(-1), and in several preparations over 40 0W kg(-1). Sawtooth cycles were also examined for comparison with the simul ation cycles, which were identical in all respects apart from the velocity profile. The power output during these cycles was found to be 14 % lower th an during the in vivo strain trajectory. This difference was due to a highe r velocity of stretch, which resulted in greater activation and higher powe r output throughout the later part of shortening, and the increase in short ening velocity towards the end of shortening, which facilitated deactivatio n. The muscle was found to operate at a mean length shorter than the plateau o f the length/force relationship, which resulted in the isometric stress mea sured at the mean resting length being lower than is typically reported for striated muscle.