Introduction

Rupture of the anterior cruciate ligament (ACL) is a common problem in sports medicine. ACL reconstruction (ACL-R) is often the preferred treatment. Hamstring (HS) autograft is the most common technique (53%) for primary ACL-R. The isokinetic test has been widely identified as the gold standard for assessing muscle strength at return to sport (RTS) after ACL-R. The test measures the overall thigh muscles’ strength not distinguishing between lateral and medial HS contribution to total force output. Since HS injuries can lead to a non-physiologic lateral Vs medial HS intramuscular activation pattern and grafting patients' HS in ACL-R generates a medial HS iatrogenic muscle injury, the isokinetic test could prevent clinicians from detecting an HS non-physiologic activation at RTS.
This study aimed to evaluate ACL-Rs at RTS with concurrent isokinetic test and surface electromyography (sEMG) of the lateral Vs medial HS compared to a group of healthy controls to identify abnormal HS patterns otherwise missed by isokinetic testing.

Methods

Mean lateral Vs medial HS sEMG amplitude and timing were measured during isokinetic tests performed at 60-180-300°/s in 92 subjects divided between 46 primary HS grafted ACL-Rs and 46 healthy controls matched per age (18-45 yrs), sex, and level of physical activity (Tegner 4-9). After the 8th post-operative month, ACL-Rs were evaluated with an isokinetic test combined with sEMG of the lateral and medial HS. The controls were tested using the same procedure. The subjects’ sEMG data were included in the analysis only if they passed the minimum criteria to be cleared for RTS according to the published literature (HS/quadriceps ratio >=60%, quadriceps & HS interlimb peak torque difference at 60-180-300°/s <=10%).

Results

The isokinetic peak torque normalized to body weight resulted significantly lower for the ACL-Rs at all tested angular velocities (60-180°/s p=0.01, 300°/s p=0.02). Mean sEMG amplitude for the lateral HS of ACL-R Vs control group resulted respectively n.s. at 60°/s, whereas significantly lower at 180°/s (p=0.005), and 300°/s (p=0.01). The same sEMG data for the medial HS of ACL-Rs Vs controls was n.s. at all angular velocities. The medial/lateral HS ratio in ACL-R Vs control group emerged n.s. at 60-300°/s, whereas significantly higher at 180°/s (p=0.01). Moreover, the sEMG mean timing of the lateral HS of ACL-Rs Vs controls resulted n.s. at all angular velocities but significantly faster for the medial HS at all angular velocities (60°/s p=0.006, 180°/s p=0.002, 300°/s p=0.02).

Discussion

ACL-Rs presented lower strength values than controls. The mean sEMG amplitude of the medial HS showed no intergroup difference at any angular velocity. However, the sEMG mean timing of the medial HS highlighted a consistently faster time-to-peak across all the isokinetic tests in ACL-Rs. The simultaneous significantly lower mean sEMG amplitude of the lateral HS at 180-300°/s in the ACL-Rs proves an altered latero-medial HS muscular activation pattern at angular velocities closer to sports-specific tasks. These findings highlight a higher neuromuscular demand on the grafted medial HS in ACL-Rs possibly exposing them to a previously undetected higher risk of injury at RTS.