The influence of countermovement depth on joint coordination and ground reaction force waveform in countermovement jump

The ground reaction force (GRF) waveform during countermovement jumps (CMJs) is considered to reflect neuromuscular coordination strategies; yet the biomechanical mechanisms distinguishing unimodal from bimodal patterns remain unclear. This study investigated the influence of countermovement depth and velocity on GRF waveform shape and examined their relationship with joint moments and work. Twenty-six healthy young women (age: 22.1 ± 1.1 years; height: 160.8 ± 4.0 cm; body weight: 53.5 ± 5.6 kg) performed CMJs, and GRF waveforms were categorised as unimodal and bimodal patterns. Jump-related variables, joint moments, and work were analysed. Analysis of covariance (ANCOVA) was conducted using countermovement depth as a covariate, and correlation analyses examined the associations between depth and biomechanical parameters. Compared with the unimodal group, the bimodal group exhibited a significantly greater countermovement depth, with no significant difference in countermovement velocity. Before adjustment, knee joint work and ankle joint moments differed significantly between groups; however, these differences were no longer evident after adjusting for countermovement depth, indicating that it was a confounding factor. Correlation analyses demonstrated that a greater countermovement was associated with increased hip and knee joint work and reduced ankle joint contribution. These findings indicate that GRF waveform shape in CMJ is determined primarily by countermovement depth rather than velocity. The unimodal pattern reflected ankle-dominant simultaneous output, whereas the bimodal pattern reflected proximal joint-dominant sequential output. This study highlights the role of joint-specific coordination strategies and offers insight for developing individualised training and rehabilitation approaches.