Impulse-Based Dynamic Strength Index: Considering Time-Dependent Force Expression

Abstract Haischer, MH, Krzyszkowski, J, Roche, S, and Kipp, K. Impulse-based dynamic strength index: considering time-dependent force expression. J Strength Cond Res 35(5): 1177–1181, 2021—The dynamic strength index (DSI) is a useful tool to assess an athlete's capacity to effectively use maximum strength during dynamic tasks. Although DSI is traditionally calculated based on peak forces, the ability to express force over time (i.e., impulse) is a better predictor of dynamic performance. The purpose of this study was to investigate the association between DSI calculated based on peak force (fDSI) and impulse (iDSI). Nineteen female collegiate lacrosse players performed countermovement jumps (CMJs) and isometric midthigh pulls (IMTPs). Peak force and impulse were extracted from CMJ and IMTP force-time data. Countermovement jump impulse was calculated by integrating force over the concentric movement time, whereas IMTP impulse was calculated by integrating force over the CMJ-matched movement time. Ratios between CMJ and IMTP peak force and impulse were used to calculate fDSI and iDSI, respectively. A moderate positive correlation existed between iDSI and fDSI (ρ = 0.644 [0.283–0.840], p = 0.003). Based on thresholds established in the literature, the 2 indices suggest conflicting training recommendations for 37% of athletes. Because impulse is a better predictor of dynamic performance, iDSI may represent a more valid method for assessing an athlete's capacity to effectively use maximum strength during dynamic tasks. Practitioners and researchers may want to consider augmenting current training and research practices with an impulse-based DSI.