This study aimed to investigate the effect of 2 different Nordic hamstring exercise (NHE) training programs (constant workload versus progressive workload) on multiple risk factors for hamstring strain injury in high competitive level athletes. Twenty-one premier league rugby union players (approximately 26 years) were randomized into 1 of 2 experimental groups and completed an 8-week NHE training program: constant training group (CTG) or progressive training group (PTG). Knee flexor/extensor strength and biceps femoris long head (BFLH) muscle architecture were assessed through isokinetic dynamometry and ultrasonography, respectively, before and after the training programs. Increased hamstring concentric (delta 8%, p = 0.004, d = 0.42) and eccentric (delta 7%, p = 0.041, d = 0.39) peak torques, as well as H:Q conventional (delta 7%, p = 0.015, d = 0.54) and functional (delta 6%, p = 0.026, d = 0.62) ratios were observed in the follow-up compared with baseline for the PTG group only. Nordic hamstring exercise training significantly increased BFLH muscle thickness (CTG delta 7%, p = 0.001, d = 0.60; PTG delta 7%, p < 0.001, d = 0.54) and fascicle length (CTG delta 8%, p < 0.001, d = 0.29; PTG delta 10%, p < 0.001, d = 0.35) in both groups. Training adaptations on hamstring peak torque (concentric and eccentric) and H:Q ratio (conventional and functional) "likely favor" PTG, whereas changes in BFLH fascicle length "possibly favor" PTG. In conclusion, only PTG enhanced hamstring strength; both NHE training programs increased BFLH muscle thickness and fascicle length, but progressive training generated greater changes on fascicle length than constant training. Medical/coaching staff should be aware that NHE with adequate training periodization with workload progression to maximize its benefits.

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