Increasing Braking and Amortization Forces during the Countermovement Jump Does Not Necessarily Improve Jump Height

This study aimed to investigate the acute effects of altering the braking rate of force development (B-RFD) and amortization force (Amf) during countermovement jumps (CMJs) on CMJ height. Nineteen healthy men and women with training experience participated, performing six CMJ variations at different velocities (preferred and fast) and depths (knee angles: 60°, 90°, and 120°). The measured variables included CMJ height, B-RFD, Amf, and impulses during the early and latter halves of the concentric phase (EI and LI, respectively). A two-way analysis of variance was employed, along with a correlational analysis of the rates of change for each variable. Significant velocity and depth effects were observed for B-RFD and Amf (p < 0.05). However, there was no significant velocity effect on CMJ height. No significant correlations were observed between the rates of change in B-RFD and Amf with CMJ height. Additionally, a high or a very high correlation (r ≥ 0.67) was observed between the rate of change in B-RFD and Amf with the rate of change in EI, while a moderate negative correlation (r = -0.43 to -0.53) was found between the rate of change in EI and LI. These findings suggest that improvements in B-RFD and Amf were associated with improvements in EI, while improvements in EI led to a reduction in LI, and consequently, improvements in B-RFD and Amf were not associated with an increase in CMJ height. In other words, improvements in B-RFD and Amf did not necessarily contribute to improvements in CMJ height.