Razor hamstring curl and Nordic hamstring exercise architectural adaptation: impact of exercise selection and intensity

Objectives: To investigate knee flexor strength and biceps femoris long head (BFlh) architectural adaptations following two different Nordic hamstring exercise (NHE) interventions and one razor hamstring curl (RHC) intervention. Methods: Thirty recreationally-active males performed a total of 128 reps of NHEbodyweight (n=10), NHEweighted (n=10) or RHCweighted training (n=10) across six weeks. Following the intervention, participants avoided any eccentric training for four weeks (detraining period). Strength results during the NHE and RHC were recorded pre and post intervention, as well as following detraining. Architectural characteristics of the BFlh were assessed weekly throughout the intervention and detraining periods. Results: For the NHEweighted group, NHE strength increased (+81N, p=0.044, d=0.90) and BFlh fascicles lengthened (+1.57cm, p<0.001, d=1.41) after six weeks of training. After one week of detraining, BFlh fascicle lengths shortened, with the largest reductions seen in the NHEweighted group (-0.96cm, p=0.021, d=-0.90). Comparatively, BFlh fascicle length and NHE strength responses were moderate in the NHEbodyweight group and negligible in the RHCweighted group. The greatest RHC strength changes (+82N, p=0.038, d=1.15) were seen in the RHCweighted group. Conclusions: NHEweighted interventions induce large BFlh fascicle lengthening responses and these adaptations decay after just one week of detraining. NHEbodyweight training has a moderate impact on BFlh architecture while the RHCweighted group has the least. Weighted NHE and RHC training promoted exercise-specific increases in strength. These findings suggest that exercise selection and intensity should be considered when prescribing exercises aiming to increase eccentric strength and BFlh fascicle length.