Background Hamstring strain injury (HSI) burdens athletes due to high occurrence and recurrence rates. Previous surface EMG evidence suggests reduced voluntary activation (VA) may contribute to high recurrence rates however no studies have directly assessed VA in previously injured and uninjured hamstrings.

Objective To determine the extent of hamstring VA in limbs with and without a history of HSI. It was hypothesised that previously injured hamstrings would display eccentric-specific VA deficits relative to uninjured contralateral hamstrings and hamstrings of uninjured controls.

Design Retrospective, cross-sectional study.

Setting Recreationally trained males and sub-elite male footballers.

Participants 12 males, with (n=6), and without a unilateral HSI history (n=6) were recruited (average time of 7.7±7.4 months post injury, average age 24.0±2.6 years, stature 180.0±5.7 cm and mass 83.0±7.9 kg).

Assessment of risk factors Twitch interpolation and sEMG were used to measure hamstring VA during 5 sets of 2 maximal voluntary concentric and eccentric contractions, performed on an isokinetic dynamometer. Peak voluntary and evoked torque were recorded in both limbs.

Main outcome measures The percentage value for VA calculated by comparing evoked and voluntary torque and surface EMG recordings.

Results Healthy uninjured limbs displayed VA levels of 96.8%±2.4% during concentric contractions and 93.3%±6.1% during eccentric contractions. Previously injured limbs displayed VA levels of 98.0%±1.1% during concentric contractions and 83.4%±7.1% during eccentric contractions. Eccentric VA was significantly reduced in previously injured limbs compared to the uninjured control group (mean difference=−9.8%, CI=-18.9– −0.7%, P=.033) and uninjured contralateral limb (mean difference=−9.3%, CI=-18.4– −0.3%, P=.043).

Conclusions Previously injured hamstrings are activated less completely than uninjured hamstrings, specifically during eccentric contractions. Such deficits exist despite successful rehabilitation and a return to sport.