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ABSTRACT Effect of Warm-Up Activity on Vertical Ground Reaction Forces in Basketball Players During Drop Jump Landings Jacob Hinkel-Lipsker Purpose: The purpose of this study was to examine the effect of certain warm-up activities on vertical ground reaction forces (GRFv) during a drop jump landing. Methods: Eight women and twelve men (mean age 22.6 ± 1.82 years) completed three warm-up activities on three separate days in a counterbalanced order: a passive stretching warm-up, a dynamic warm-up, and a no warm-up control. After completing each activity, participants were asked to perform eight drop jump landings on a force platform. GRFv data was sampled at 1000 Hz during each landing, and the variables analyzed were: peak forefoot and rearfoot magnitude, forefoot and rearfoot rate of loading, and impulse. Results: The dynamic warm-up had significantly greater values (p < 0.05) for forefoot peak magnitude, rearfoot peak magnitude, and forefoot rate of loading compared to the passive stretching and control conditions. Also, there were no significant differences among all activities for rearfoot rate of loading and impulse. Conclusions: The significantly greater values for forefoot peak magnitude, rearfoot peak magnitude, and forefoot rate of loading that the dynamic warm-up produced indicates that this activity may be effective in increasing muscular stiffness in the lower limbs. The failure of the passive stretching warm-up to reach significance indicates that this activity may not be effective in decreasing lower extremity muscular stiffness.