Measuring Impulse-DSI and its Derivatives Via the Isometric Belt Squat: Assessing Relationships with Sprint Ability Among Division 1 Team Sport Athletes

The dynamic strength index (DSI) is a common athlete profiling tool that has traditionally been derived as the ratio between the peak force in the isometric mid-thigh pull (IMTP) and the countermovement jump (CMJ) to identify ballistic strength deficits. Recently, an impulse-based DSI (iDSI) has been proposed for the relevance of impulse metrics in explosive sports. Furthermore, the isometric belt squat (IBSq) may be a superior alternative to the IMTP for measuring lower-body force potential owing to its removal of the torso and upper limb involvement. This study assessed the reliability of two novel IBSq and CMJ 200ms impulse metrics. Furthermore, we assessed the individual relationships and predictive utility of the DSI derivatives and 40m sprint performance. 43 Division 1 team sport athletes (30 males and 13 females) were recruited. Reliability was assessed using the intraclass correlation coefficient (ICC) and the coefficient of variation (COV). The relationships between DSI and iDSI derivatives, 0-10m, 10-30m, and 30-40m split times were assessed using Pearson’s correlation coefficient and stepwise regression. The IBSq and CMJ impulses demonstrated acceptable reliability (ICC > .93, COV < 9.6%). IBSq and CMJ impulses explained up to 56% of the variance in 0-10m time (p <.002) and up to 49% of the variance in 30-40m time (p <.0001). The IBSq-derived iDSI score may be a reliable and accurate test for predicting sprint performance among Division 1 team sport athletes. Practitioners may monitor iDSI derivatives to enhance training decisions relevant to sprint speed.