Hamstring strength and morphology progression after return to sport from injury.

PURPOSE Hamstring strain reinjury rates can reach 30% within the initial 2 wk after return to sport (RTS). Incomplete recovery of strength may be a contributing factor. However, relative strength of the injured and unaffected limbs at RTS is currently unknown.The purpose was to characterize hamstring strength and morphology at the time of RTS and 6 months later. METHODS Twenty-five athletes who experienced an acute hamstring strain injury participated after completion of a controlled rehabilitation program. Bilateral isokinetic strength testing and magnetic resonance imaging (MRI) were performed at RTS and 6 months later. Strength (knee flexion peak torque, work, and angle of peak torque) and MRI (muscle and tendon volumes) measures were compared between limbs and over time using repeated-measures ANOVA. RESULTS The injured limb showed a peak torque deficit of 9.6% compared to the uninjured limb at RTS (60°·s, P < 0.001) but not 6 months after. The knee flexion angle of peak torque decreased over time for both limbs (60°·s, P < 0.001). MRI revealed that 20.4% of the muscle cross-sectional area showed signs of edema at RTS with full resolution by the 6-month follow-up. Tendon volume of the injured limb tended to increase over time (P = 0.108), whereas muscle volume decreased between 4% and 5% in both limbs (P < 0.001). CONCLUSIONS Residual edema and deficits in isokinetic knee flexion strength were present at RTS but resolved during the subsequent 6 months. This occurred despite MRI evidence of scar tissue formation (increased tendon volume) and muscle atrophy, suggesting that neuromuscular factors may contribute to the return of strength.