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Higuchi, T, Nagami, T, Mizuguchi, N, and Anderson, T. The acute and chronic effects of isometric contraction conditioning on baseball bat velocity. J Strength Cond Res 27(1): 216–222, 2013—The purpose of this study was to examine (a) the acute change in bat velocity (BV) following three types of warm-up procedures for baseball hitting (experiment 1), and (b) the effect of an 8-week training program of isometric contraction conditioning (ISO) on BV (experiment 2). In experiment 1, the BV of 24 collegiate baseball players was measured before and after one of the three warm-up procedures; five standard bat (mass = 850.5 g) dry swings (SBS), five weighted bat (mass = 850.5 g + 680.4 g) dry swings (WBS), and four sets of 5-second maximal voluntary isometric contractions mimicking the bat swing movement pattern (ISO). BV was measured just before ball–bat impact. Experiment 2 followed experiment 1 and used only the ISO warm-up. Twelve of the 24 subjects formed the experimental group and underwent an 8-week training program (3 days per week) of ISO training. Results of experiment 1 indicated (a) no significant change in post-SBS BV (−0.33 m·s−1), (b) a significant decrease in post-WBS BV (−0.89 m·s−1; p < 0.05), and (c) a significant increase in post-ISO BV (+0.39 m·s−1; p < 0.05). In experiment 2, there was a significant increase in baseline BV after the 8-week training period (30.21 ± 1.83 m·s−1 to 31.15 ± 1.57 m·s−1). A comparison of BV before and after ISO warm-up revealed that change was significantly greater after the training period (100.17 ± 3.18% vs. 103.75 ± 1.91%). Our results suggest that warm-up with WBS does not increase BV and that using the ISO has both acute and chronic positive effects on BV as a warm-up procedure to improve BV.