The effects of skill practice on underwater fly kick development

Underwater fly kick is a key skill in swimming, contributing to increased performance in competition. Although its importance is recognised, the development of key aspects of underwater fly kick is not well explored or understood. The purpose of this study was to determine the effects of two commonly used interventions on the underwater fly kick performance of trained swimmers. Eleven skilled male athletes (FINA point score 679 ± 57) performed maximal underwater fly kick efforts before and after vertical kicking and a resisted training intervention. Ten reflective markers were placed on key bony landmarks, and a six-camera Qualisys motion capture system (100 Hz) recorded kinematics in a calibrated three-dimensional volume 6.9 metres in length. The vertical kicking intervention did not improve horizontal swimming velocity, but athletes demonstrated more symmetrical kicking techniques. Resisted training elicited a significant increase in horizontal swimming velocity following the intervention, without clear causational changes in technique. Therefore, this was potentially due to post-activation performance enhancement effects. While both interventions elicited changes such as increased knee flexion and reduced kick symmetry, these may reflect exploratory learning processes, allowing athletes to experiment with movement variability as they refine more efficient and effective patterns for long-term performance improvements.