Objectives: To investigate whether five-weeks of concentric (CON) or eccentric (ECC) hamstring strength training have different effects on recovery from sprint running, eccentric strength and architecture of the biceps femoris long head (BFLH). Design: Cohort study. Methods: Thirty males (age, 22.8 ± 4.1y; height, 180.1 ± 6.4cm; weight, 85.2 ± 14.6kg) were allocated into either a CON or ECC group, both performing nine sessions of resistance training. Prior to and immediately after the five-week intervention, each participant's BFLH fascicle length (FL), pennation angle (PA), muscle thickness (MT), peak isometric KF torque and Nordic eccentric strength were assessed. Postintervention, participants performed two timed sprint sessions (10x80m) 48 hours apart. Blood samples 2 and passive KF torques were collected before, immediately after, 24 hours and 48 hours after the first sprint session. Results: After five-weeks of strength-training, fascicles lengthened in the ECC (p<0.001; d = 2.0) and shortened in the CON group (p<0.001; d =0.92), while PA decreased for the ECC (p=0.001; d = 0.52) and increased in the CON group (p<0.001; d = 1.69). Nordic eccentric strength improved in both ECC (p<0.001; d =1.49) and CON (p<0.001; d = 0.95) groups. No between-group differences were observed in peak isometric strength (p=0.480), passive KF torques (p=0.807), sprint performance decrements between sprint sessions (p = 0.317) and creatine kinase (p=0.818). Conclusion:Despite inducing significant differences in BFLH muscle architecture, there were no significant between group differences in sprint performance decrements across two sprint sessions.

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.

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