Voluntary Activation and Reflex Responses after Hamstring Strain Injury

Abstract

Introduction There is a lack of definitive evidence supporting deficits in voluntary activation in participants with prior hamstring injury; moreover, it remains unknown if spinal mechanisms contribute to suspected deficits. Purpose This study aimed to determine the effect of prior hamstring strain injury on knee flexor concentric and eccentric strength, voluntary activation, surface electromyographic (sEMG) activity, and stretch and tendon reflex amplitudes. Methods Twenty-five participants were recruited, 12 with a history of unilateral hamstring strain injury of at least moderate severity. Voluntary activation, strength, and sEMG activity were recorded during maximal eccentric and concentric knee flexor contractions at 60°·s⁻¹. Stretch and tendon reflexes were also recorded at rest. Results Previously injured limbs exhibited lower levels of voluntary activation (mean difference = −24.1%, 95% confidence interval [CI] = −34.1% to −14.0%, P < 0.001), strength (mean difference vs control group = −0.37 Nm·kg⁻¹, 95% CI = −0.71% to −0.03 N·m·kg⁻¹, P = 0.03), and normalized sEMG (mean difference = −17%, 95% CI = −32% to −2%, P = 0.02) during maximal eccentric knee flexor contractions compared with control group. No such differences were seen in concentric contractions. Stretch reflexes (mean difference = −3.8%, 95% CI = −6.8 to −0.8, P = 0.02) and tendon reflexes (mean difference = −13%, 95% CI = −26% to 0%, P = 0.04) were also lower in previously injured compared with control biceps femoris muscles. Conclusion Moderate to severe hamstring strain injury is associated with long-term deficits in voluntary activation during maximal eccentric contraction. Hamstring injury history is also associated with deficits in stretch reflex and tendon reflex amplitude.