Simulation and comparison of vortex generation and fluid force during underwater and surface breaststroke kicks using computational fluid dynamics

Swimmers perform breaststroke kicks to propel forward underwater and on the surface. We investigated the differences in propulsive and braking mechanisms between underwater and surface breaststroke kicks by simulating vortex generation and fluid force using computational fluid dynamics (CFD). One male breaststroke swimmer performed underwater and surface breaststroke kicks; kinematic data and the swimmer's digital model were collected. Vortex generation and fluid force were obtained using CFD for each kick type. Vortices were generated for the toe side during the recovery phase in both kick trials. During the out-sweep and in-sweep phases, vortex generation was observed on the dorsal side of the foot for both kick types. Clockwise and anticlockwise rotating vortices were observed on the toe side during the glide phase. Braking fluid force on the entire body was greater for the surface kick than for the underwater kick during the recovery (-173.5 vs. -134.0 N), in-sweep (-57.6 vs. -22.4 N), and glide phases (-68.0 vs. -24.3 N). During the out-sweep phase, a greater drag coefficient in the propulsive direction was observed for the foot during the surface kick (2.06) than during the underwater kick (1.29). Conversely, the drag coefficient in the braking direction applied to the swimmer's upper body was greater for the surface kick (-0.05) than for the underwater kick (-0.01). Slower forward velocity on the surface compared to the underwater breaststroke kick is mainly caused by the wave drag effect rather than by differences in vortex generation and the inability to produce a propulsive effect.