Hamstring muscle architecture and microstructure changes following Nordic hamstring exercise training and detraining

This is the authors' abstract. We don't add key points for this paper.

• Nine weeks of Nordic hamstring exercise training substantially increased hamstring muscle volume as well as fiber length and cross-section. • Nordic hamstring exercise induced non-uniform adaptations across the 4 hamstring muscles; the hypertrophy and fiber length elongation were greater in the semitendinosus and biceps femoris short head compared to the semimembranosus and biceps femoris long head. • Upon 3 weeks of detraining, gains in fiber length underwent rapid reversal, but gains in volume and fiber cross-section were mostly retained, which underscores the need for consistent training to sustain all the protective benefits imparted by the exercise. Background While Nordic hamstring exercise (NHE) training has been shown to reduce hamstring strains, the muscle-specific adaptations to NHE across the 4 hamstrings remain unclear. This study investigates architectural and microstructural adaptations of the biceps femoris short head (BFsh), biceps femoris long head (BFlh), semitendinosus (ST), and semimembranosus (SM) in response to an NHE intervention. Methods Eleven subjects completed 9 weeks of supervised NHE training followed by 3 weeks of detraining. Magnetic resonance imaging was performed at pre-training, post-training, and detraining to assess architectural (volume, fiber tract length, and fiber tract angle) and microstructural (axial (AD), mean (MD), radial (RD) diffusivities, and fractional anisotropy (FA)) parameters of the 4 hamstrings. Results NHE training induced significant but non-uniform hamstring muscle hypertrophy (BFsh: 22%, BFlh: 9%, ST: 26%, SM: 6%) and fiber tract length increase (BFsh: 11%, BFlh: 7%, ST: 18%, SM: 10%). AD (5%), MD (4%), and RD (5%) showed significant increases, but fiber tract angle and FA remained unchanged. After detraining, only ST showed a significant reduction (8%) in volume, which remained higher than the pre-training value. While fiber tract lengths returned to baseline, AD, MD, and RD remained higher than pre-training levels for all hamstrings. Conclusion The 9-week NHE training substantially increased hamstring muscle volume with greater hypertrophy in ST and BFsh. Hypertrophy was accompanied by increases in fiber tract lengths and cross-sections (increased RD). After 3 weeks of detraining, fiber tract length gains across all hamstrings declined, emphasizing the importance of sustained training to maintain all the protective adaptations.