Spinal Reflex Excitability does not Associate with Persistent Muscle Inhibition after Anterior Cruciate Ligament Reconstruction

Abstract

Persistent deficits in quadriceps voluntary activation affect muscle function following anterior cruciate ligament reconstruction (ACLR). Previous research has attributed voluntary activation deficits to lower quadriceps spinal reflex excitability following acute knee effusion or injury. There is little evidence to indicate that spinal reflex excitability exists or influences persistent voluntary activation deficits in individuals with an ACLR. PURPOSE: To determine if quadriceps spinal reflex excitability in individuals with a unilateral ACLR differs between limb or compared to a control group. Additionally, we evaluated the ability of quadriceps spinal reflex excitability to predict full voluntary activation following ACLR. METHODS: One hundred and forty-seven individuals (74 ACLR and 73 controls) participated in this cross-sectional, case-control study. Quadriceps spinal reflex excitability was quantified using the Hoffmann reflex normalized to the maximal muscle response (H:M ratio). Quadriceps voluntary activation was evaluated with the burst superimposition technique and calculated via the central activation ratio (CAR). Separate 2x2 ANOVAs were used to compare differences between-limbs and between-groups for H:M ratio and CAR. From a receiver operating characteristic curve analysis, the area under the curve (AUC) was used to determine the accuracy of H:M ratio to predict full voluntary activation (CAR>0.95) in ACLR individuals. We determined the odds ratio (OR) of demonstrating full voluntary activation from spinal reflex excitability cutoff scores that maximized the sensitivity and specificity for predicting full activation. RESULTS: ACLR quadriceps H:M ratios were not different between limbs or compared to the control group (P>0.05). Quadriceps CAR was bilaterally lower in individuals with an ACLR compared to the control group (P<0.01), yet CAR did not differ between limbs. H:M ratio had poor accuracy for predicting ACLR participants exhibiting full voluntary activation (AUC=0.53, 95% CI=0.39,0.67; OR = 2.37, 95% CI=0.91,6.2). CONCLUSIONS: Spinal reflex excitability did not differ between limbs in individuals with ACLR or compared to controls. Quadriceps spinal reflex excitability has poor accuracy for predicting which ACLR individuals will demonstrate a CAR>0.95.