(a) AR and cross-bridge costs versus contraction cycling frequency, as a percentage of the cost of continuous contraction, from in situ studies; energy is normalized to muscle force (electronic supplementary material, text S3). (b) ECOT and VmuscCOT for running. Following equation (2.5), AR cost can be calculated from the slope of regression equation, 0.6VmuscCOT. Cross-bridge cost is given by the regression intercept, or as ECOT−AR cost. (c) Mass-specific metabolic cost, ECOT, and mechanical work, k, per metre versus body size and estimated stride cycles m−1; redrawn from [3].