MATTHEW S. HARKEY, BRITTNEY A. LUC-HARKEY, ADAM S. LEPLEY, TERRY L. GRINDSTAFF, PHILLIP GRIBBLE, J. TROY BLACKBURN, JEFFREY T. SPANG, BRIAN PIETROSIMONE

Persistent voluntary quadriceps activation deficits are common following anterior cruciate ligament reconstruction (ACLR), but the direct causes are unclear. The primary purpose of this study was to determine whether spinal reflex excitability deficits are present in individuals with a history of ACLR, and secondarily to determine if spinal reflex excitability predicts which individuals possess full voluntary quadriceps activation. One hundred and forty-seven individuals (74 Healthy and 73 ACLR) 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). Voluntary quadriceps activation was evaluated with the burst superimposition technique and calculated via the central activation ratio (CAR). Separate 2x2 ANCOVAs were used to compare differences between-limbs and between-groups for H:M ratio and CAR. A receiver operating characteristic (ROC) curve was used to determine the accuracy of H:M ratio to predict if ACLR participants present with full voluntary activation (CAR≥0.95). ACLR H:M ratio was not different between limbs or compared to the healthy group (P>0.05). While ACLR CAR was lower bilaterally compared to the healthy group (p<0.001), it did not differ between limbs. H:M ratio has poor accuracy for predicting which individuals exhibit full voluntary activation (ROC area under the curve=0.52, 95% CI=0.37,0.66; odds ratio = 2.2, 95% CI=0.8, 5.9). Spinal reflex excitability did not differ between limbs in individuals with ACLR or compared to healthy participants. The level of quadriceps spinal reflex excitability has poor accuracy at predicting which ACLR individuals would demonstrate full voluntary quadriceps activation.