The swimmer gains power from the water by swimming and propels the water.At that time, the flow of water around it is called unsteady state (turbulent state), not a steady state at a constant speed like a car or an airplane.In recent studies, swimmers use the force of this unsteady fluid to propel, and it is said that they obtain forces that cannot be calculated from the size and muscle mass of the object.However, there is no means to directly measure this propulsive force, and even if the magnitude of the force is known with the conventional body-attached sensor, the direction and cause are unknown.

　The research focuses on underwater dolphin kicks (kick swimming) in swimming races.Using a circulating water tank, a motion capture system and a stereo PIV system were used to simultaneously measure the movement of the swimmer and the flow of surrounding water, achieving three-dimensional visualization of the flow.

　As a result, kick swimming included twisting motions (internal / external rotation) of the lower limbs in addition to the motion of swinging both legs up and down.A strong vortex is formed on the sole side of the foot during the lowering motion, and the twisting motion of the lower limbs enters at the finish phase of the kicking down, so that the toes approach each other and the vortices are gathered in the center to form a lump.A strong jet flow is emitted from the toes by forming a strong downward flow so that the mass of the vortex attracts the surrounding water.From this, it was found that this vortex mass maximizes the action of the force applied to the foot.

　With the three-dimensional visualization method of the flow developed this time, the magnitude of the thrust obtained by the swimmer can be calculated only by shooting with a camera, so it is expected to contribute to effective instruction in swimming.

Paper information:[Journal of Biomechanics] Aquasi three-dimensional visualization of unsteady wake flow inhumanundulatory swimming