Relating cortical morphology, corticospinal excitability, corticomotor representation, and quadriceps strength after anterior cruciate ligament injury.

Abstract

The current study investigated the relationship between sensorimotor cortical thickness, corticospinal excitability, corticomotor topography, and quadriceps strength after ACL reconstruction (ACLR). Ten women with a history of unilateral ACLR and 10 controls (CON) received single-pulse transcranial magnetic stimulation during unilateral, submaximal isometric knee extensions. Pulses were delivered to each vastus medialis oblique (VMO) hotspot with concurrent biceps femoris (BF) monitoring. Corticospinal excitability was assessed by delivering 40 pulses at various intensities to each hotspot. Motor-evoked potentials (MEPs) were averaged at each intensity and fitted to a Boltzmann sigmoidal curve using nonlinear regression to derive v50, slope, and MEP_MAX. A motor mapping procedure included 120 pulses delivered in a 6 × 6 cm grid placed around each hotspot. Ultrasonography was used to measure VMO muscle thickness. Structural MRIs were acquired to derive paracentral lobule (PCL) cortical thickness. ACLR group's previously injured leg was weaker than the healthy leg, with no between-leg differences in CON. Regardless of group, v50 was asymmetrical between legs. Slope, MEP_MAX, VMO map measures, and VMO thickness were similar between legs and groups. ACLR tended to have asymmetrical PCL thickness with BF map measures larger in the hemisphere of the previously injured leg than healthy leg, whereas CON had symmetrical PCL thickness and BF map measures. Results indicate that even years after ACLR corticomotor structure plasticity is homotopic with persistent asymmetrical knee extension strength but no differences in corticospinal excitability. Overall, the hemispheric asymmetry in leg-specific brain structure may contribute to the knee extensor strength deficits common after ACLR.