Abstract
Conflicting methodologies are used to define certain countermovement vertical jump (CMVJ) phases, which limits the identification of performance-enhancing factors (e.g., rate of force development). Purpose We (a) utilized a joint power approach to define CMVJ phases that accurately describe body weight unloading (i.e., unweighting) and eccentric (i.e., braking) actions, which were combined with the robustly defined concentric (i.e., propulsion) phase, and (b) determined whether the phases can be identified using only ground reaction force (GRF) data. Methods Twenty-one men performed eight maximal CMVJs while kinematic and GRF data were obtained. Hip, knee, and ankle joint powers were calculated by multiplying net joint moments (obtained using inverse dynamics) by joint angular velocities. The net sum of the joint powers (JPSUM) was calculated to define phases by the preeminence of negative (i.e., net eccentric actions) or positive (i.e., net concentric actions) power where appropriate. Unloading, eccentric, and concentric phases were identified using JPSUM and linked to GRF and center of mass velocity features. Results Bland and Altman plots of the bias and 95% confidence intervals for the limits of agreement (LOA), intraclass correlation coefficients (ICC), and coefficients of variation (CV) indicated precise agreement for detecting the unloading (bias, 0.060 s; LOA,-0.110 to 0.229 s) and eccentric (bias, 0.012 s; LOA,-0.010 to 0.040 s) phases with moderate (ICC, 0.578; CV, 40.72%) and excellent (ICC, 0.993; CV, 2.18%) reliability, respectively. The eccentric phase should be divided into yielding (eccentric actions while accelerating downward) and braking (eccentric actions while decelerating downward) subphases for detailed assessments. Conclusion CMVJ phases defined by combining joint and center of mass mechanics can be detected using only force platform data, enabling functionally relevant CMVJ assessments using instrumentation commonly available to practitioners.