Background Muscular responses to training programs with the Nordic hamstring exercise (NHE) have been widely investigated to understand its preventive effects on muscle strain injury. Conversely, little is known about muscular responses to the stiff-leg deadlift (SLD), an exercise that preferably activate the biceps femoris long head, precisely the knee flexor muscle most commonly injured. Aims The aim of the current study was to verify the effect of training programs using the NHE or the SLD on muscle architecture, strength, and functional performance of well-trained rugby players. Methods Twenty-three male rugby union players completed a 5-week training program using either the NHE (n = 12) or the SLD (n = 11). Biceps femoris long head muscle architecture, hamstring and quadriceps isokinetic peak torques, and countermovement jump performance were assessed before and after the training program. Results There was no significant group-by-time interaction for any outcome (p > 0.05). NHE group experienced higher percent increases than SLD group for biceps femoris fascicle length (~ 14% vs. ~ 6%; p = 0.015). Within-group analysis indicated that NHE group presented greater effect sizes for muscle architecture outcomes, hamstring eccentric peak torque and hamstring-to-quadriceps functional ratio; conversely, SLD group presented greater effect sizes for hamstring concentric peak torque and countermovement jump height. Conclusions The NHE should preferably be used to enhance the hamstring eccentric strength and the biceps femoris fascicle length (our primary outcomes, supported by previous studies as risk factors for hamstring strain injury), while the SLD favor improvements in hamstring concentric strength and jump performance.
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