Background: The architectural and morphological adaptations of the hamstrings in response to training with different exercises have not been explored.

Purpose: To evaluate changes in biceps femoris long head ( BFLH ) fascicle length and hamstring muscle size following 10-weeks of Nordic hamstring exercise ( NHE ) or hip extension ( HE ) training.

Methods: 30 recreationally active male athletes ( age, 22.0±3.6 years; height, 180.4±7 cm; weight, 80.8±11.1 kg ) were allocated to 1 of 3 groups: ( 1 ) HE training ( n=10 ), NHE training ( n=10 ), or no training ( control, CON ) ( n=10 ). BFLH fascicle length was assessed before, during ( Week 5 ) and after the intervention with a two-dimensional ultrasound. Hamstring muscle size was determined before and after training via MRI.

Results: Compared with baseline, BFLH fascicles were lengthened in the NHE and HE groups at mid-training ( d=1.12–1.39, p < 0.001 ) and post-training ( d=1.77–2.17, p < 0.001 ) and these changes did not differ significantly between exercises ( d=0.49–0.80, p=0.279–0.976 ). BFLHvolume increased more for the HE than the NHE ( d=1.03, p=0.037 ) and CON ( d=2.24, p < 0.001 ) groups. Compared with the CON group, both exercises induced significant increases in semitendinosus volume ( d=2.16–2.50, ≤0.002 ) and these increases were not significantly different ( d=0.69, p=0.239 ).

Conclusion: NHE and HE training both stimulate significant increases in BFLH fascicle length; however, HE training may be more effective for promoting hypertrophy in the BFLH.