Fig. 10. Histogram showing the mass-fractions (open bars) and work-fractions (shaded bars, inter-individual mean ± S.E.M.) of muscle work estimated for the four muscle regions (SB: Ant, Mid, Post and TB; see Fig. 1) from which in vivo fascicle strains were sampled. Mass-fraction data were obtained for all six pigeons, whereas work-fractions were obtained for those animals and regions in which reliable work loops (Fig. 9) were obtained over multiple wingbeat cycles. Mass-weighted work fractions were calculated by multiplying the net work from each region by the fractional mass (Table 1), then dividing by the summed averages for all regions. The average summed work was 1.28 W and average duration was 0.12 s. Individual bird values were based on a minimum sample of 8 wingbeats, with an average sample size of 40 wingbeats (Table 4).