Neural Excitability Alterations After Anterior Cruciate Ligament Reconstruction

Context: Neuromuscular dysfunction is common after anterior cruciate ligament reconstruction (ACL-R). However, little is known about quadriceps spinal-reflex and descending corticomotor excitability after ACL-R. Understanding the effects of ACL-R on spinal-reflex and corticomotor excitability will help elucidate the origins of neuromuscular dysfunction. Objective: To determine whether spinal-reflex excitability and corticomotor excitability differed between the injured and uninjured limbs of patients with unilateral ACL-R and between these limbs and the matched limbs of healthy participants. Design: Case-control study. Setting: Laboratory. Patients or Other Participants: A total of 28 patients with unilateral ACL-R (9 men, 19 women; age 21.28 +/- 3.79 years, height 170.95 +/- 10.04 cm, mass = 73.18 +/- 18.02 kg, time after surgery = 48.10 +/- 36.17 months) and 29 participants serving as healthy controls (9 men, 20 women; age = 21.55 +/- 2.70 years, height = 170.59 +/- 8.93 cm, mass = 71.89 +/- 12.70 kg) volunteered. Main Outcome Measure(s): Active motor thresholds (AMTs) were collected from the vastus medialis (VM) using transcranial magnetic stimulation. We evaluated VM spinal reflexes using the Hoffmann reflex normalized to maximal muscle responses (H : M ratio). Voluntary quadriceps activation was measured with the superimposed-burst technique and calculated using the central activation ratio (CAR). We also evaluated whether ACL-R patients with high or low voluntary activation had different outcomes. Results: The AMT was higher in the injured than in the uninjured limb in the ACL-R group (t(27) = 3.32, P = .003) and in the matched limb of the control group (t(55) = 2.05, P = .04). The H: M ratio was bilaterally higher in the ACL-R than the control group (F-1,F-55 = 5.17, P = .03). The quadriceps CAR was bilaterally lower in the ACL-R compared with the control group (F-1,F-55 = 10.5, P = .002). The ACL-R group with low voluntary activation (CAR < 0.95) had higher AMT than the control group (P = .02), whereas the ACL-R group with high voluntary activation (CAR >= 0.95) demonstrated higher H: M ratios than the control group (P = .05). Conclusions: The higher VM AMT in the injured limbs of ACL-R patients suggested that corticomotor deficits were present after surgery. Higher bilateral H : M ratios in ACL-R patients may be a strategy to reflexively increase excitability to maintain voluntary activation.