Fig. 3. Timing of various stages of the wing and foot stroke cycle. Wing (black circles) and foot (white circles) stokes are divided into three stages, which are illustrated in Fig. 2 and described in the text. Thrust from foot propulsion ubiquitously corresponded with the transition between the upstroke and downstroke of the wings, when drag is probably greatest because of the large angle of attack of the wings. Correspondence in timing between power and recover phases of wing and foot propulsion could therefore be important in maintaining steady speed, minimizing the cost of drag during diving.