Computational Fluid Dynamics Investigation of Vortex Formation, Shedding Patterns, and Resulting Hydrodynamic Forces in Breaststroke Leg Kicks Performed Fully Submerged Versus at the Water Surface

Breaststroke swimmers generate propulsion through a distinctive whip-like kick, which performs differently when executed fully submerged versus at the surface. To elucidate the underlying hydrodynamic mechanisms responsible for these differences, we employed computational fluid dynamics (CFD) to simulate vortex formation and the resulting fluid forces during both underwater and surface breaststroke kicks. A single highly skilled male breaststroke swimmer performed one complete underwater kick and one surface kick under controlled conditions. Three-dimensional kinematic data were recorded and used to create an accurate digital body model of the swimmer. These motion data then drove high-fidelity CFD simulations to quantify instantaneous vortex structures and body- segment fluid forces throughout the kick cycle. Vortex generation patterns were largely similar between the two conditions. During leg recovery, vortices formed along the toe side of the foot in both kicks. In the out-sweep and in- sweep phases, prominent vortices detached from the dorsal surface of the foot regardless of whether the kick was performed underwater or at the surface. During the subsequent glide phase, pairs of counter-rotating vortices (clockwise and anticlockwise) were consistently observed near the toes in both trials. Despite the qualitative similarity in vortex dynamics, substantial quantitative differences emerged in the resulting fluid forces. Braking (rearward) forces acting on the entire body were markedly higher during the surface kick than the underwater kick in three key phases: recovery 2 (Cd = 1.29). Conversely, the upper body endured greater rearward drag coefficients at the surface (−0.05) than underwater (−0.01), largely due to wave formation. These findings indicate that the slower average forward velocity observed in surface breaststroke kicking, compared with underwater kicking, arises primarily from increased wave drag and elevated braking forces on the torso and head, rather than from fundamental differences in lower-limb vortex generation or an inherent inability of the kick to produce propulsion at the surface.