Objective:​ To examine quadriceps spinal-reflexive, corticospinal, and intracortical excitability before, at 2-weeks post and at 6-months post-anterior cruciate ligament reconstruction (ACLr) compared to healthy controls. A secondary aim of this study was to investigate the association between quadriceps neural excitability and neuromuscular, biomechanical and self-reported function at times of pre-surgery and 6-months post-surgery.

Patient and Other Participants:​ Seventeen ACL injured patients scheduled to undergo surgical reconstruction (9 Female, 8 Male;​ age:​21.0 ± 4.8years;​ height:​173.1±7.3cm;​ weight:​77.7±11.2kg;​ 35.8±14.8days-post-injury) volunteered and were compared to seventeen healthy controls (9 Female, 8 Male;​ age:​22.1 ± 3.8years;​ height:​173.3±10.7cm;​ weight:​75.3±21.3kg).

Methods:​ This investigation utilized a case-control study design. For aim 1, quadriceps spinal-reflexive, corticospinal, and intracortical excitability were tested at pre-surgery, 2-weeks post-surgery (2-wks;​ average:​ 15.8 ± 2.5days post-surgery) and 6-months post-surgery or when returned to participation (6-mo;​ average:​ 28.1 ± 2.8wks post-surgery). For aim 2, quadriceps strength, voluntary activation, knee joint biomechanics during stair walking and self-reported function were collected at the pre-surgery and 6-mo post time points. For aim 1, all measures were collected bilaterally in the ACLr group and in a dominance matched limb of the control group. For aim 2, outcome measures were collected in the injured limb of ACLr patients and a matched limb from the healthy control group for aim 2.

Main Outcome Measures:​ Quadriceps spinal-reflexive excitability was assessed using Hoffmann reflexes normalized to maximal muscle responses. Corticospinal excitability was evaluated with active motor thresholds (AMT) and motor evoked potentials at 120% of AMT. Intracortical excitability was assessed using short interval intracotical inhibition and intracortical facilitation. Quadriceps strength and voluntary activation were quantified using maximal voluntary isometric contractions and the central activation ratio, respectively. Peak knee joint angles and internal extension moments were calculated during the first 50% of stance phase during stair ascent and descent gait trials. The International Knee Documentation Committee questionnaire was used to evaluate self-reported function.

Statistical Analyses:​ Aim 1:​ 3x3 (limb x time) repeated measures ANOVAs were performed with Tukey post-hoc tests where appropriate. Aim 2:​ Spearmen Rho correlation matrices were performed at the pre-surgery and 6-mo post-surgery time points on the injured limb of the ACLr group and matched limb of the control group independently.

Results:​ Aim 1:​ ACLr patients demonstrated lower spinal-reflexive excitability than controls at pre-surgery and 2-wks. At 6-mo post-surgery, spinal-reflexive excitability was not different between groups. Over-time, spinal-reflexive excitability in the ACLr group decreased from pre-surgery to 2-wks, and increased higher than pre-surgery at 6-mo, while controls did not change. Corticospinal excitability was not different between groups at pre-surgery or 2-wks post-surgery, however ACLr patients had lower corticospinal excitability at 6-mo compared to controls. Corticospinal excitability in the ACLr group increased from pre-surgery to 2-wks, and at 6-mo was decreased compared to pre-surgery, while controls did not change. Intracortical excitability was not different between groups, nor did the values change over time in either group. Aim 2:​ At pre-surgery, corticospinal excitability was related to knee joint moments during stair ascent in ACLr patients, with higher corticospinal excitability relating to higher joint moments during gait. At 6-mo post-surgery, intracortical facilitation was inversely related to spinal-reflexive excitability. In addition, quadriceps strength had strong correlations with self-reported function and knee joint angle during stair ascent, indicating that ACLr patients with weaker quadriceps muscles reported higher levels of dysfunction and demonstrated straighter knee joint angles during stair ascent.

Conclusions:​ Spinal-reflexive deficits are present before surgery and at 2-wks post-reconstruction, but not at a time when these individuals are cleared for full activity. In contrast, corticospinal deficits existed at 6-mo post-surgery, however not prior to or at 2-wks post-surgery. Deficits in neural excitability are present following ACL injury, potentially due to loss of ligament mechanoreceptors or joint effusion. Separate neuromuscular alterations occur at different stages of injury, with decreases in spinal-reflexive excitability early (pre-surgery, 2-weeks post) and deficits in corticospinal excitability observed at a time when individuals are returned to participation. Early rehabilitation strategies targeting spinal-reflexive excitability may help to improve post-operative outcomes, while later-stage rehabilitation may benefit from modalities aimed at improving corticospinal excitability. In addition, these neural alterations may have negative effects on neuromuscular, biomechanical and self-reported function. Clinically, targeting these neural changes may benefit patients who are recovering from ACL injury and surgical reconstruction, and future research should examine the efficacy of the treatments proposed to target neural alterations.