Joint-level determinants of the dynamic strength index: implications for testing and monitoring

The aims of this study were to determine the associations and differences between the traditional dynamic strength index (DSI_GRF, calculated from ground reaction force [GRF] data) and joint-level dynamic strength indices (DSI_NJM, calculated from net joint moment [NJM] data). Eight female NCAA Division I lacrosse players performed three maximal effort isometric mid-thigh pulls (IMTP) and countermovement jumps (CMJ). GRF and motion capture data were recorded and used to calculate hip, knee, and ankle NJM. The ratio between peak forces during the IMTP and CMJ tests was used to calculate the DSI_GRF and the ratio of peak NJM was used to calculate ankle, knee, and hip DSI_NJM. Associations and differences between DSI_GRF and DSI_NJM were examined with Pearson's correlation coefficients, paired t-tests and Hedge's g effect sizes. The results showed that only the hip DSI_NJM was correlated with DSI_GRF (hip: p = 0.037; r = 0.738). In addition, athletes exhibited significantly greater DSI_NJM than DSI_GRF. Although these results suggest DSI_GRF are associated with hip DSI_NJM, the fact that joint-level DSI_NJM were generally greater than DSI_GRF suggests the ratio between peak NJM during the CMJ and IMTP may not provide intuitive or useful data for strength and conditioning coaches or sport scientists.