Anterior cruciate ligament reconstruction is associated with increased corticospinal excitability and rate of force development

Background: After anterior cruciate ligament reconstruction, asymmetries in central activation are suspected to prevent complete functional recovery. This cross-sectional study investigated the motor function of both lower limbs in ACLR patients using morphological, mechanical, and neurophysiological measures after surgical repair with a semitendinosus-gracilis graft. Methods: Ten male patients (age 20–31 years; 6/4 right/left knee surgery; 6–12 months after ACLR) were recruited. Muscle trophism was quantified through ultrasound estimates of quadriceps volume and mid-thigh circumference; knee extensors’ rate of force development (RFD), maximum torque and voluntary activation (interpolated twitch technique, ITT) were assessed through dynamometry during maximal isometric effort; spinal excitability was measured with the Soleus H-reflex; transcranial magnetic stimulation was used to assess corticospinal excitability (resting motor threshold (rMT) and recruitment curve of motor evoked potentials (MEP) during submaximal contraction) and intracortical excitability (short-interval intracortical inhibition (SICI)) of the Vastus medialis (VM) and Tibialis anterior (TA) muscles. Results: The quadriceps muscle on the operated side showed significant volume loss (mean [SD] of 2264.6 [345.1] cm³ and 2082.9 [386.2] cm³ for the non-operated and operated sides, respectively; p = 0.035) and mid-thigh circumference (52.2 [2.7] cm and 50.2 [3.3] cm; p = 0.035). In the VM recruitment curves, the increase in MEP amplitude with increasing stimulation intensity was steeper on the operated side (p = 0.001). The operated limb also showed a higher RFD (p = 0.026). No inter-limb differences were found for the remaining outcomes. Conclusions: The steeper rise of knee extensor torque, paralleled by an increased corticospinal excitability of the operated side VM muscle, suggests that an increased drive from the motor cortex is needed to engage the quadriceps in fast contractions following ACLR. This may represent a compensatory phenomenon aimed at counteracting the decline in muscle power associated with reduced muscle mass and altered quadriceps morphology. Trial registration: ClinicalTrials.gov NCT04837417 (submitted On 2021-03-31).