Quadriceps motor unit properties following ACL reconstruction are associated with corticospinal excitability and motor cortex activations.

Abstract

Quadriceps weakness is a primary concern following anterior cruciate ligament reconstruction (ACLR), and neuroimaging studies have revealed higher motor inhibition and structural atrophy of the corticospinal tract. To investigate the contributions of supraspinal mechanisms underlying spinal motoneuron impairments and quadriceps weakness, this study explored the firing patterns of motor units (MUs) in the vastus medialis muscle following ACLR. Twenty individuals with primary ACLR and 20 matched controls performed a unilateral knee extension torque-control task at 50% of maximal voluntary effort. High-density electromyographic activity of the vastus medialis muscle was decomposed into constituent MU action potentials. Electroencephalography was used to localize cortical activity to sensory and motor brain regions. Active motor thresholds were acquired using transcranial magnetic stimulation. We compared motor unit properties, cortical activity, and corticospinal excitability between groups and limbs using mixed-effects models and Cohen's d effect sizes. Participants with ACLR had weaker quadriceps compared with contralateral and control limbs. Strength deficits were accompanied by recruitment of larger MUs with lower firing rates in the involved limb. Those with ACLR also had lower corticospinal excitability and lower contralateral hemisphere motor cortex activations during quadriceps contractions. Lower corticospinal excitability and lower activations in the sensory and motor cortices were weakly associated with smaller MU action potential amplitudes, whereas group was not. Larger, slower-firing quadriceps MUs are recruited at lower absolute and mass-normalized recruitment thresholds, but not relative recruitment thresholds after ACLR. Lower corticospinal excitability and motor cortex activity were associated with the recruitment of smaller MUs irrespective of ACLR.NEW & NOTEWORTHY Individuals with anterior cruciate ligament reconstruction (ACLR) exhibited earlier recruitment of larger MUs with lower firing rates at both absolute and mass-normalized recruitment thresholds in their involved limb when compared with contralateral and control limbs. Individuals with ACLR had lower corticospinal excitability and lower contralateral hemisphere motor cortex activations, which were weakly associated with lower firing rates as larger motor units were recruited. Reduced excitatory cortical drive may contribute to quadriceps motor unit impairments and persistent quadriceps weakness after ACLR.