Numerical simulation of flow around the legs during walking in water with implications for hydrotherapeutic exercise

One of the common hydrotherapeutic exercises is walking in water because buoyancy reduces joint loading and increases mobility for a patient. The fluid drag forces (the forces that act on the person from the fluid in the direction opposing the direction of motion) cause changes in muscle activations, as walking in water changes the forces that act on the leg compared with overground walking. Here, through a series of numerical simulations, we quantify how the flow forces that act on the leg due to its motion in water change over a walking gait cycle. We show that besides drag forces that act on the walking legs and peak when the leg is accelerated forward, relatively large lateral forces (in the direction perpendicular to the direction of motion) also act on the leg. These forces are caused by the rapid acceleration of the opposite leg when the two legs are close, creating an asymmetric pressure distribution around the leg. These results are unexpected and could have significant implications for designing hydrotherapeutic plans for patients by considering the lateral forces besides the drag forces that act on the body while walking in water.