Early Changes of Hamstrings Morphology and Contractile Properties During 10 Days of Complete Inactivity

Purpose: The hamstrings (HS) muscle group plays a fundamental role in maintaining knee stability, thus contributing to the prevention and rehabilitation of lower limb musculoskeletal injuries. However, little is known about HS structural and functional adaptations after periods of prolonged inactivity. Our purpose was to investigate the HS morphological and contractile properties changes during 10 days of bed rest (BR). Methods: Ten young healthy males underwent a 10-day BR. HS cross-sectional area (CSA) (at 30%, 50%, and 70% of femur length), biceps femoris long head (BFlh) architecture were assessed by ultrasound imaging after 0 (BR0), 2 (BR2), 4 (BR4), 6 (BR6) and 10 (BR10) days of BR, while BFlh contractile properties (radial twitch displacement (Dm); contraction time (Tc)) were evaluated at the same time points by tensiomyography. HS muscle volume was assessed by magnetic resonance imaging at BR0 and BR10. Results: A reduction in muscle volume was observed in BFlh (p = 0.002; Δ = -3.53%), biceps femoris short head (p = 0.002; Δ = -3.54%), semitendinosus (p = 0.002; Δ = -2.63%), semimembranosus (p = 0.002; Δ = -2.01%) and HS pooled together (p < 0.001; Δ = -2.78%). Early changes in CSA were detected at 30% femur length already at BR6 for BFlh (p = 0.009; Δ = -2.66%) and BFsh (p = 0.049; Δ = -1.96%). We also found a reduction in fascicle length (Lf) at BR6 (p = 0.035; Δ = -2.44%) and BR10 (p < 0.001; Δ = -2.84%). Dm and Tc increased at BR2 (p = 0.010; Δ = 30.0%) and B10 (p = 0.019; Δ = 19.7%), respectively. Conclusions: Despite being a non-postural muscle group, HS exhibited a moderate reduction in muscle dimensions in response to a short unloading period. Small changes in BFlh Lf were also observed, accompanied by alterations in BFLh contractile properties. These HS modifications should not be ignored from a clinical perspective.