Nuzzo, JL and McBride, JM. The effect of loading and unloading on muscle activity during the jump squat. J Strength Cond Res 27(7): 1758–1764, 2013—The purpose of this study was to explore the effect of loading and unloading on lower-body muscle activity during the jump squat (JS). Thirteen strength-power trained male subjects completed JS with loads less than (10, 20, and 30% maximal dynamic strength [MDS]), equal to (35% MDS), and greater than (40, 50, and 60% MDS) their body mass. Loads less than body mass were accomplished with a custom-designed unloading apparatus, and loads greater than body mass were accomplished with a barbell and weights. Surface electromyography was used to measure eccentric and concentric phase muscle activity of the vastus lateralis, vastus medialis, and biceps femoris during the JS. Data were analyzed using a 2-way repeated measures analysis of variance with 2 within-subjects factors (load condition and jump phase). A significant (p < 0.05) load condition × jump phase interaction was found for vastus lateralis, vastus medialis, and biceps femoris muscle activity. Post hoc pairwise comparisons revealed that eccentric phase activity for all muscles was significantly reduced during the 10 and 20% MDS load conditions (unloading) when compared with the 35% MDS condition (body mass). However, there were no significant changes in concentric phase activity for all muscles when compared with the body mass condition. These findings demonstrate that unloading during the JS causes significant reductions in eccentric phase muscle activity. Alterations in eccentric phase muscle activity during unloading may then have negative consequences on concentric phase JS kinetic and kinematic variables. Thus, assisted jump training with unloading is not recommended.