Selective Strength Loss and Decreased Muscle Activity in Hamstring Injury

STUDY DESIGN: Cross-sectional, controlled laboratory study. OBJECTIVES: To determine whether thigh muscle isokinetic torque patterns and activity, measured by electromyography (EMG), of individuals with hamstring injury differ from control individuals. BACKGROUND: Neuromuscular control during thigh muscle strength assessment following hamstring injuries has not been reported. METHODS: Fifteen athletes with prior hamstring injury (hamstring-injured group [HG]) were compared to 15 uninjured athletes (control group [CG]). The injuries were incurred 6 weeks to 12 months prior to participation, and all injured athletes had returned to at least partial training. Participants performed 5 isokinetic concentric extensor, concentric flexor, and eccentric flexor torque tests at 60 degrees/s in the seated position. Peak torque was determined for each contraction type, as well as average torque for each of 4 time-based movement quartiles. EMG root-mean-squares were calculated in these movement quartiles for the biceps femoris and medial hamstrings. RESULTS: No significant differences were found for peak torque for all contractions, when comparing HG injured and uninjured sides to CG bilateral averages. The HG injured limb eccentric flexor torque was significantly lower in the fourth quartile (approximately 25 degrees to 5 degrees knee flexion, hamstring lengthened range) compared to the CG bilateral average (P = .025). Eccentric flexor biceps femoris and hamstrings EMG root-mean-squares of the HG injured and the uninjured sides were significantly lower in the second to fourth quartiles (towards the lengthened range), compared to the CO bilateral averages (P<.05). CONCLUSION: Decreased strength and EMG activation in a lengthened hamstrings range for the athletes with prior hamstring injury suggested a change in neuromuscular control. Lengthened range assessment of isokinetic eccentric flexor torque may be useful for the assessment of athletes with a prior injury; however, results should be confirmed with prospective studies. J Orthop Sports Phys Ther 2011;41(5):354-363, Epub 2 February 2011. doi:10.2519/jospt.2011.3268