Kinematic changes in the undulatory kicking during underwater swimming.

The contribution to total race distances of underwater undulatory swimming (UUS) is increasing at the elite level. However, little is known about the technical modifications during underwater swimming. In the present research, the aim was to compare the kinematic characteristics of competitive swimmers between the first and last kick of UUS. Fifty-four national level swimmers (26 males and 28 females) performed 25 m maximal efforts from a push start, and two sequential video cameras captured the underwater segment. Kicking parameters and segmental kinematics were calculated by means of two-dimensional direct linear transformation algorithms. Dolphin kick performance showed a clear impairment in velocity (η 2 : 0.65), but changes on kicking parameters depended on the swimmer's gender, with males decreasing kicking amplitude (η 2 : 0.25) and females decreasing kicking frequency (η 2 : 0.18) in the last kick. Decline in kicking performance seemed to be more related to the swimmers' body configuration when approaching the water surface (greater trunk inclination and maximal body amplitude in sagittal plane) than to technical modifications in the dolphin kick movement (no changes in the joints range of movement except the hip). Swimmers should control their vertical body amplitude at the end of underwater sections to minimise the decrease in kicking performance.