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Purpose: The aim of this study was to evaluate the effect of external loading on mechanics of vertical jumping. We hypothesized that the muscular mechanical output could be higher under no-load conditions than in the presence of either positive or negative external loads. Methods: Fifteen physically active men performed maximal countermovement jumps (CMJ) on a force plate while a pulley system provided approximately constant vertical force acting in a way to either reduce or increase the body weight. As a result, the weight of the body approximately corresponded to the gravity acceleration from 0.70 to 1.30g (g = 9.81 m·s-2). Results: Regarding the jumping kinematics, we observed a significant (P < 0.001) load-associated decrease in both the peak velocity and lowering of the center of mass during the eccentric jump phase, but not in the duration of the subsequent concentric jump phase. Regarding the muscular mechanical output, both the mean power (P¯) and peak momentum (M) revealed significant (P < 0.001) changes associated with loading, and further post hoc analyses revealed significantly higher values (P < 0.05-0.001) of both P¯ and M for 1.00g compared with most of the other loading conditions applied. Conclusion: The results suggest that subject's own body provides the optimal load for producing maximum mechanical output in vertical jumping. If corroborated by the results of future studies performed on other rapid movement, our findings could support the hypothesis that the muscular system is designed for producing maximum mechanical output in rapid movements when loaded only with the weight and inertia of its own body. more...