Differences in psychological readiness for return to sport after anterior cruciate ligament injury is evident in thigh musculature motor unit characteristics

BACKGROUND: Following anterior cruciate ligament (ACL) injury, many athletes that undergo surgery and 6–9 months of rehabilitation struggle to return to sport. Evidence suggests that psychological factors contribute to this failure to return-to-sport. OBJECTIVE: Determine the motor control relationship between thigh musculature motor unit characteristics and psychological readiness to return to sport between ACL-injured and healthy controls. STUDY DESIGN: A longitudinal cohort study. METHODS: Athletes longitudinally completed the ACL Return to Sport after Injury (ACL-RSI) survey and isometric strength measures with a measurement of electromyography (EMG) of the vastus lateralis, vastus medialis, biceps femoris, and semitendinosus. A score cut-off of 61 on the ACL-RSI was used to divide ACL-injured groups. EMG was decomposed to provide each identified motor unit’s characteristics (amplitude, average firing rate, etc). RESULTS: Data demonstrated increased average firing rate for hamstrings (p<0.001), decreased average firing rate for vastus lateralis (p<0.001) and decreased motor unit size for both the quadriceps and hamstrings at return-to-sport post-ACL reconstruction compared with sex-matched and age-matched healthy controls (p<0.001). Furthermore, there were marked differences in disparate ACL-RSI scores between ACL-injured athletes. CONCLUSIONS: At return to sport, ACL-injured athletes have major alterations of thigh musculature motor control, with smaller motor units used by those with low ACL-RSI scores. This study uniquely demonstrates objective thigh muscle motor unit characteristics that coincide with subjective reports of psychological readiness. This information will be important to address psychomotor complexes of injury for future rehabilitation protocols.