Intracortical Inhibition Associates with Quadriceps Voluntary Activation in Individuals with an Anterior Cruciate Ligament Reconstruction

Abstract

Quadriceps strength is important for maintaining high physical function following anterior cruciate ligament reconstruction (ACLR), and deficits in voluntary activation (VA) lead to quadriceps weakness. ACLR individuals with deficits in quadriceps VA also demonstrate diminished corticomotor excitability, which reflects the balance of intracortical inhibitory and facilitatory activity. Therefore, specific alterations in intracortical inhibition and facilitation may be responsible for persistent deficits in quadriceps VA following ACLR. PURPOSE: To determine if short interval intracortical inhibition (SICI), intracortical facilitation (ICF), and the SICI: IFC ratio associate with quadriceps VA in individuals with ACLR. METHODS: Twenty-seven individuals (74% Female; 169±11cm; 73±19kg; 44±37 months post ACLR) with a history of a primary unilateral ACLR were enrolled into this cross-sectional study. All outcome measures were collected with the participant seated in an isokinetic dynamometer with the knee flexed at 90°. Paired pulse transcranial magnetic stimulation was used to assess SICI and ICF. SICI and ICF paradigms utilized two stimuli delivered at 80% and 120% of active motor threshold (AMT) at 3 and 15 msec apart, respectively. Motor evoked potentials (MEPs) elicited via SICI and ICF paradigms were then normalized to MEPs elicited at 120% of AMT. A reduction in SICI indicates greater inhibition, whereas an increase in ICF indicates greater facilitation. Quadriceps VA was evaluated with the superimposed burst technique and calculated via the central activation ratio (CAR). We used Spearman correlations (ρ) to evaluate associations between the non-normally distributed outcome measures (α = 0.05). RESULTS: Greater inhibition (i.e. lower SICI) was associated with lower VA (ρ = .502, P = 0.008). A greater SICI: ICF ratio, indicating more inhibition relative to facilitation, was associated with lower VA (ρ = -.530, P = 0.004). ICF was not associated with VA (ρ = -.089, P = 0.661). CONCLUSIONS: Reduced intracortical inhibition is associated with higher quadriceps VA in individuals with ACLR. Interventions that seek to reduce intracortical inhibition may improve VA in ACLR patients demonstrating persistent deficits in VA.