Fig. 6. Representative work loops from the SUPRA and PECT muscles of a pigeon (Columba livia) engaged in (A,B) level, (C,D) ascending and (E,F) descending flight. Arrows indicate direction of contraction; bold lines indicate EMG activity. The hatched areas feature artificial negative stress (_m) due to compression of the SUPRA strain gauge by cross-talk from PECT force that remained even after a correction factor was applied; these areas were not included in the analysis.