Fig. A1. (Appendix). Conceptual diagram of mathematical cancellation between different contributors to external mechanical power. Simultaneous positive and negative external mechanical power (broken line) is produced during double support due to propulsion and collision, respectively. But collision power in humans also extends beyond double support, and propulsion power also precedes double support. External power due to leg motion (dotted line) overlaps and mathematically cancels these quantities in measurements of external power during single support, even though they do not physically interact. The result is that total external mechanical power (solid line) tends to underestimate the positive and negative power associated with both step-to-step transitions and leg motion.