Injury History, Not Corticospinal Excitability, Explains Slower Quadriceps Motor Unit Firing After Acl Reconstruction

Corticospinal inhibition and slower motor unit firing rates following anterior cruciate ligament reconstruction (ACLR) suggest supraspinal mechanisms underly persistent quadriceps weakness, yet the relationship between each has not been explored. PURPOSE: To assess whether corticospinal excitability mediates quadriceps motor unit (MU) firing rates after ACLR. METHODS: 20 individuals with ACLR (12 female, 21.2 ± 2.2 years, 15.1 ± 9.6 months from surgery) and 20 matched controls performed a series of isometric knee extension force-matching contractions at 50% effort during one laboratory visit. Surface electromyographic activity of the vastus lateralis (VL) was recorded concurrently and decomposed into constituent MU action potentials. The relationship between MU firing rate (FR) and recruitment threshold (RT) was quantified with regression. Active motor thresholds (AMTs) of the VL were measured in each limb using transcranial magnetic stimulation. We assessed differences in AMT and FR-RT slope between groups and limbs using a linear mixed effect model and Cohen’s d effect sizes as appropriate. To explore whether corticospinal excitability mediated this relationship, we considered the direct and indirect effects of group and limb on FR-RT slope, with AMT as a mediator. We controlled for the y-intercept of the FR-RT relationship in all models. RESULTS: Figure 1 depicts data and results. The ACLR group exhibited higher AMTs than controls, without difference between limbs. FR-RT slopes were steeper in ACLR limbs compared to uninvolved and control limbs. Higher AMT independently associated with steeper FR-RT slope, but did not mediate the involved limb effect in the full model. CONCLUSIONS: The ACLR limb recruited slower firing MUs than did uninvolved and control limbs. Although lesser corticospinal excitability associated with slower firing rates, it did not mediate the effect of ACLR, suggesting a direct effect of ACLR history on the FR-RT relationship.