Effect of changing kick frequency on inter-joint coordination during undulatory underwater swimming

Undulatory underwater swimming (UUS) contributes to race performance in competitive swimming, yet how inter-joint coordination changes with kick frequency remains unclear. This study examined the effects of kick frequency on trunk-hip and hip-knee inter-joint coordination during UUS using the Modified Vector Coding Technique. Eight male swimmers performed UUS trials at 80-120% of their optimal kick frequency, with kinematics captured using three-dimensional underwater motion analysis. Swimming velocity increased with kick frequency but plateaued beyond 100%. The proportion of the hip-knee anti-phase with distal dominance pattern --hip and knee rotating in opposite directions with greater angular displacement at the knee-- increased with kick frequency, whereas the absolute durations of this pattern per cycle changed little, indicating that the time required for kick reversal remained nearly constant regardless of kick frequency. Under high-frequency conditions (110% and 120%), preserving the time for kick reversal within a shortened kick cycle was accompanied by constrained trunk undulation during the late downward kick. These findings suggest that when kick frequency is acutely increased beyond the optimal value, hip-knee reversal time is maintained but trunk undulation is impaired, likely contributing to the plateau in swimming velocity.