Fig. 10. (A) Relationship between instantaneous roll acceleration, measured at the midpoint of the first half of downstroke, and the difference in wrist velocity in the body coordinate system, measured at the same time. (B) Relationship between inter-wingbeat roll acceleration and the difference in world coordinate system wrist velocity at mid-downstroke. Note that the y-axis scale in A is an order of magnitude larger than that in B. In general, within-wingbeat roll accelerations were due to inertial effects and therefore related to movements in the body coordinate system. Inter-wingbeat roll accelerations include an aerodynamic component and therefore should be related to velocities in the world coordinate system. The instantaneous and inter-wingbeat roll accelerations were not correlated with one another, nor were the wrist velocity differences measured in the two coordinate systems.