Fig. 8. A partial regression component model of the factors underlying pectoralis power output among individual birds as a function of flight speed with a least-squares multiple linear regression of power against selected factors. The arrows indicate the proposed relationships between variables, together with their partial regression coefficients, which indicate the relative strength of the relationship. Pectoralis power output as a function of flight speed is determined by the combination of work per wingbeat and wingbeat frequency, but work per wingbeat exerts the strongest effect. Pectoralis work per wingbeat is influenced by several factors, but the two most important are muscle force (r²=0.77) and muscle length change (r²=0.45; also see Fig. 6). The double-headed arrow between muscle length change and work loop shape factor indicates that increases in length change are correlated with decreases in the shape factor, and vice versa. The correlation also influences the total effect of the variables; for example, the total effect of muscle length change is 0.80-(0.63x0.40), or 0.55. There were no significant correlations between model variables aside from those indicated in the model via arrows. Statistical tests were conducted based on a set of individual means across speeds (5 individuals x 7 speeds: N=35).

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