Purpose: To analyze the differences in the force-velocity (F-v) profile assessed under unconstrained (ie, using free weights) and constrained (ie, on a Smith machine) vertical jumps, as well as to determine the between-day reliability. Methods: A total of 23 trained participants (18 [1] y) performed an incremental load squat jump test (with ~35%, 45%, 60%, and 70% of the subjects` body mass) on 2 different days using free weights and a Smith machine. Nine of these participants repeated the tests on 2 other days for an exploratory analysis of between-day reliability. F-v variables (ie, maximum theoretical force [F0], velocity [v0], and power, and the imbalance between the actual and the theoretically optimal F-v profile) were computed from jump height. Results: A poor agreement was observed between the F-v variables assessed under constrained and unconstrained conditions (intraclass correlation coefficient [ICC] < .50 for all). The height attained during each single jump performed under both constrained and unconstrained conditions showed an acceptable reliability (coefficient of variation < 10%, ICC > .70). The F-v variables computed under constrained conditions showed an overall good agreement (ICC = .75-.95 for all variables) and no significant differences between days (P > .05), but a high variability for v0, the imbalance between the actual and the theoretically optimal F-v profile, and maximal theoretical power (coefficient of variation = 17.0%-27.4%). No between-day differences were observed for any F-v variable assessed under unconstrained conditions (P > .05), but all of the variables presented a low between-day reliability (coefficient of variation > 10% and ICC < .70 for all). Conclusions: F-v variables differed meaningfully when obtained from constrained and unconstrained loaded jumps, and most importantly seemed to present a low between-day reliability.
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