BACKGROUND: Recent work has questioned the utility of resistance exercise in reducing the prevalence and severity of hamstring strains. Inter-limb differences in hamstring recruitment may partly explain why preemptive hamstring exercise does not prevent future hamstring strains. Therefore, we examined how movement strategy (bilateral vs. unilateral movement), eccentric (ECC) exercise tempo (fast: 2-second vs. slow: 4-second), and external load characteristics (coupled load vs. independent loads) affected mean electromyography (mEMG) of the biceps femoris (BF) and gluteus maximus (GMAX) during prone hamstring exercise (PHE). METHODS: Twenty college-aged students aged 18 - 33 (M = 23.4, SD = 3.12 years) with >3 months of targeted hamstring training (>1x per week) completed familiarization (lab visit 1) and experimental protocols (lab visit 2) during separate visits at least 72 hours apart. Active knee range of motion (aROM) and maximum voluntary contractions (MVC) were obtained prior to the experimental PHE variations. Separate linear mixed models examined mEMG data for interactions between exercise variation, tempo, and side (right vs. left) during the ECC and concentric (CON) phases of the PHE. RESULTS: Isokinetic strength and aROM did not differ between legs, ps > 0.174. Three-way interactions between variation, tempo, and side were not detected for GMAX or BF mEMG during the ECC (ps > 0.530) or CON (ps > 0.341) PHE. The main effect of side was detected for BF and GMAX mEMG during the ECC (ps < 0.001) and CON (ps < 0.006) PHE. The left BF was preferentially excited compared to the right BF during the ECC (∆M [right - left] = -5.70 %MVC), p < 0.001) and CON (∆M = -7.59 %MVC, p < 0.001) PHE. The contralateral right GMAX recruitment was greater than the left during the ECC (∆M = 2.21 %MVC, p < 0.001) and CON (∆M = 1.81 %MVC, p = 0.006) PHE. CONCLUSION: The PHE resulted in biased recruitment of the left BF and right GMAX despite no differences in pre-experimental knee aROM or isokinetic/isometric strength measurements. Further investigation is warranted to decipher whether this intra-exercise bias predicts hamstring strain risk. FUNDING: The project was supported by an internal grant awarded to the PI (NS) from the Faculty Research Committee - Research Seed Funding (FY2022 - FY2023) at Georgia Southern University.

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