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This study examined concurrent validity of countermovement vertical jump reactive strength index modified and force–time characteristics recorded using a 1-dimensional portable and laboratory force plate system. Twenty-eight men performed bilateral countermovement vertical jumps on 2 portable force plates placed on top of 2 in-ground force plates, both recording vertical ground reaction force at 1000 Hz. Time to takeoff; jump height; reactive strength index modified; and braking and propulsion impulse, mean net force, and duration were calculated from the vertical force from both force plate systems. Results from both systems were highly correlated (r ≥ .99). There were small (d < 0.12) but significant differences between their respective braking impulse, braking mean net force, propulsion impulse, and propulsion mean net force (P < .001). However, limits of agreement yielded a mean value of 1.7% relative to the laboratory force plate system (95% confidence limits, 0.9%–2.5%), indicating very good agreement across all of the dependent variables. The largest limits of agreement were for jump height (2.1%), time to takeoff (3.4%), and reactive strength index modified (3.8%). The portable force plate system provides a valid method of obtaining reactive strength measures, and several underpinning force–time variables, from unloaded countermovement vertical jump. Thus, practitioners can use both force plates interchangeably.
Lake is with the Department of Sport and Exercise Sciences, University of Chichester, Chichester, United Kingdom. Mundy is with the Centre for Sport, Exercise and Life Sciences, Coventry University, Coventry, United Kingdom. Comfort and McMahon are with the Department of Sport, Exercise and Physiotherapy, University of Salford, Salford, United Kingdom. Suchomel is with the Department of Human Movement Sciences, Carroll University, Waukesha, WI, USA. Carden is with the College of Life and Environmental Sciences, University of Exeter, Exeter, United Kingdom.