Reactive Countermovement Jump Mechanics Across Basketball Experience Levels

Countermovement jumps are usually tested as self-initiated maximal efforts, although many sport actions require athletes to react to an external event and still jump as high as possible. Background and Objectives: We examined how this rapid-initiation requirement was associated with CMJ mechanics, whether the pattern was similar across levels of basketball-specific experience, and whether arm swing altered the response. Methods: Forty-four men (15 physically active non-basketball controls, 15 mid-level basketball players, and 14 high-level basketball players) performed countermovement jumps with and without arm swing in four fixed-order blocks. Trial-level linear mixed-effects models included 564 valid attempts. Results: In the signal-initiated blocks, movement time was 0.091 s shorter (95% CI -0.103 to -0.078) and jump height was 1.63 cm lower (95% CI -2.04 to -1.21). Unloading (-0.069 s), braking (-0.011 s), and propulsion (-0.010 s) were all shorter; countermovement depth decreased, peak downward speed increased, and net propulsive impulse fell by 0.052 N s/kg despite a small increase in mean net propulsive force. Movement time decreased in all six group-by-jump cells. The jump-height reduction was statistically clear at the group level but small and of uncertain practical relevance for individuals. Basketball-specific experience did not clearly moderate the main condition effects. Arm swing reduced the movement-time difference by 0.043 s but did not clearly reduce the jump-height difference. Conclusion: The rapid-initiation condition was associated with a broad reorganization of movement timing and force-time strategy, with a smaller change in endpoint performance. Because the conditions were always performed in the same order, these findings should be interpreted as fixed-order condition differences and confirmed in counterbalanced designs.