Corticospinal Excitability Is Associated With Quadriceps Motor Unit Action Potential Amplitudes Irrespective Of ACL Reconstruction

Quadriceps weakness after anterior cruciate ligament reconstruction (ACLR) may be linked to supraspinal mechanisms, as indicated by corticospinal inhibition and alterations in motor unit properties (e.g., firing rates and size). Yet, the relationship between corticospinal inhibition and motor unit firings rates and size has not been explored. PURPOSE: To investigate whether corticospinal excitability is associated with motor unit (MU) action potential amplitude in the quadriceps 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 8 trials of a knee extension force-control task at 50% of maximal voluntary effort. We recorded vastus lateralis (VL) surface electromyographic activity and decomposed the activity into constituent MU action potentials. A regression analysis quantified the exponential relationship between firing rate (FR) and action potential amplitude (APAMP), with decay rate defined as the rate at which FR slowed relative to increase in APAMP. We measured active motor threshold (AMT) of the VL in each limb using transcranial magnetic stimulation. We assessed the relationships between group*limb and AMT with FR-APAMP decay rate using separate linear mixed effect models and Cohen’s d effect sizes. We controlled for FR-APAMP scaling factor in all models. RESULTS: Figure 1 shows all results. The ACLR group exhibited higher AMTs than controls, with no limb differences. FR-APAMP decay rate did not differ between groups or limbs. Higher active motor threshold associated with faster decay rate. CONCLUSION: We found no differences in FR-APAMP decay rate between groups or between limbs. However, each unit increase in active motor threshold associated with a 0.22 pps/μV faster decay in MU firing rate relative to increasing MU size. These results suggest reduced corticospinal excitability is associated with slower and smaller MU recruitment, regardless of ACLR status.