Introduction
Following anterior cruciate ligament (ACL) injury, ACL reconstruction (ACLR) and rehabilitation, only 33%–61% of athletes return to their same level of play.1–4 Despite rehabilitation, quadriceps strength deficits persist after ACL injury and ACLR.3 5 6 Knee joint trauma induces a presynaptic reflex inhibition of musculature surrounding the joint, termed arthrogenic muscle inhibition (AMI).6 7 This inhibition is initially protective of the joint as deactivation of the muscle can protect the joint from large forces; however, AMI ultimately prevents complete muscle activation and can impede recovery and return to sport (RTS).6 The mechanism for persistent muscular strength deficits remains unclear and could be due to sensorimotor or psychological corticomotor inhibition. A plethora of data demonstrates that many athletes report fear of RTS after ACL injury.1 8–18 Furthermore, fear of re-injury is highly associated with secondary injury.1 9 13 17 Fear of re-injury could be attributed to decreased activation of the musculature or contrarily, fear could induce cortical inhibition of voluntary strength and motor unit (MU) activity. Regardless of the causation of lowered muscle activation, it is evident that psychological readiness is an important factor for RTS.13 18 19
Psychological responses to injury include fear of re-injury,8 9 16 17 20 21 lack of self-confidence,22 kinesiophobia,9 12 17 20 depression23 24 and anxiety.22 25 26 Psychological readiness for RTS after ACLR is commonly assessed with the ACL Return to Sport after Injury (ACL-RSI) survey,27 28 a validated questionnaire to measure psychological responses, particularly confidence in the injured body part and risk appraisal.28 As psychological readiness may affect motor control (CTRL), potentially with the induction of neuroplasticity,29 30 it is vital to understand the motor CTRL deficits related to these psychological responses. In addition, combining objective measures that correlate with subjective questionnaires is vital for appropriate clinical care decisions.31 Future patient-centric health approaches will require holistic integration of physical, psychological, cultural and metaphysical dimensions.32 Thus, it is important to characterise the function of MUs with psychological readiness.
This work aimed to assess the effect of psychological readiness for RTS with measures of motor CTRL. Understanding motor CTRL characteristics is vital to avoid future injury occurrences (and potentially primary injury). Approximately 1 in 3 ACL-injured individuals will suffer a secondary ACL injury within 2 years, wherein both ipsilateral and contralateral limbs are at risk.33 34 Furthermore, individuals with a smaller increase in ACL-RSI scores were more likely to experience a second ACL injury.13 Contrarily, another study with a smaller sample size has also demonstrated that females with higher ACL-RSI scores were more likely to have a second ACL injury.35 Thus, with the discrepancy of ACL-RSI scores with re-injury, this study was uniquely designed to differentiate objective MU characteristics of thigh musculature between those with disparate ACL-RSI scores (RSI-high and RSI-low) at 6 months and 12 months post-injury and healthy CTRLs.31 36
This study aimed to determine the relationship of a subjective psychological readiness metric to RTS to objective measures of bilateral motor CTRL. Specifically, we measured MU common drive, MU recruitment, MU rate coding, MU action potential (MUAP) peak-to-peak amplitude and delta frequency (ΔF).37 38 The last two metrics are surrogate measures of MU size39 40 and synaptic activity,41–43 respectively. We hypothesised that ACL-injured athletes with lower ACL-RSI scores would recruit smaller MUs and have increased synaptic neuromodulation (ΔF) values across the recruitment of MUs compared with athletes with higher ACL-RSI scores and CTRLs. Demonstrating objective neural deficits that directly associate with psychological responses may help improve future physical, psychological, and emotional health outcomes.31