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 6 weeks. Following the intervention, participants avoided any eccentric training for 4 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 (+1.57 cm, P < 0.001, d = 1.41) after 6 weeks of training. After 1 week of detraining, BFlh fascicle lengths shortened, with the largest reductions seen in the NHEweighted group (-0.96 cm, 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 1 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.