Purpose: To examine the differences between performing Nordic hamstring exercises once or twice a week on hamstring eccentric strength and other muscle-strain risk factors in high-level football players. Methods: In this randomized trial, 32 football players (18-23 y old) completed an 8-week Nordic hamstring exercise training program in 1 of 2 experimental groups: group 1 (once a week; n=15) and group 2 (twice a week; n=17). Knee-flexor/extensor peak torques and biceps femoris long-head muscle architecture were assessed through isokinetic dynamometry and ultrasonography, respectively, before and after the training programs. Analysis of covariance, effect sizes (ESs), and t tests for percentage change were used to assess the effect of the 2 interventions on the outcome measures. Results: Group 2 demonstrated higher hamstring concentric peak torque than group 1 posttraining (155-164 vs 149-158 N·m; P=.043; ES=0.27), although there was also a statistical trend for higher hamstring eccentric peak torque (212-234 vs 198-221 N·m; P=.098; ES=0.37), hamstring-to-quadriceps conventional ratio (0.56-0.59 vs 0.54-0.57; P=.089; ES=0.31), and hamstring-to-quadriceps functional ratio (0.76-0.84 vs 0.71-0.79; P=.076; ES=0.50). No between-groups differences were found for muscle thickness (P=.864; ES=0.12), pennation angle (P=.289; ES?=?0.18), fascicle length (P=.406; ES?=?0.03), and quadriceps concentric peak torque (P=.340; ES=0.02). Conclusion: Only the Nordic hamstring exercise training program performed twice a week strengthened the hamstrings of high-level football players, while similar changes in muscle architecture occurred with both once- and twice-weekly sessions.
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