Acute Prolonged Hamstrings Vibration Increases Quadriceps Spinal And Corticospinal Excitability Following Anterior Cruciate Ligament Reconstruction

Prolonged hamstrings vibration increases voluntary activation of the quadriceps following anterior cruciate ligament reconstruction (ACLr). This increase may be due to an increase in spinal excitability, a persistent increase in excitability of corticospinal projections to the quadriceps, or both. PURPOSE: To investigate the immediate and persistent effects of prolonged hamstrings vibration on quadriceps spinal and corticospinal excitability in individuals with ACLr and matched non-injured controls (NI). METHODS: Voluntary activation of the quadriceps was evaluated with the central activation ratio, spinal (H/M) corticospinal (MEP/M), and cortical (MEP/H) excitability of the vastus medialis muscle were assessed in 42 participants (14 ACLr/14 NI/14 Sham) immediately before and 0, 30 and 60 minutes after 20 min of sustained vibration (30 Hz) applied directly to the hamstrings. The Sham group received no vibration. RESULTS: Vibration increased quadriceps voluntary activation (p < 0.001) in the ACLr group and this increase was positively correlated with increases in spinal (r = 0.82; r2 = 0.67; p = 0.003), corticospinal (r = 0.74; r2 = 0.54; p = 0.002), and cortical (r = 0.56; r2 = 0.32; p = 0.15) excitability. These effects were not observed in the NI or Sham group. In the ACLr group only, hamstrings vibration increased spinal excitability (p < 0.001), and this effect persisted until 60 min after stimulation (p < 0.001). For the ACLr and NI groups, vibration increased corticospinal excitability (p < 0.001), and this effect persisted until 60 min post-vibration. Vibration increased cortical excitability beginning 30 min post-vibration in the ACLr group (p = 0.01) and NI group (p = 0.03); these increases persisted until 60 minutes post-vibration. CONCLUSIONS: Voluntary activation of the quadriceps is increased predominantly by spinal mechanisms in response to prolonged hamstrings vibration in patients with ACLr. Increases in corticospinal excitability are likely cortical in nature in the NI group and a combination of spinal and cortical in the ACLr group. The persistence of these effects suggests that vibration applied at the beginning of a rehabilitation session may benefit voluntary activation of the quadriceps and enhance the efficacy of exercises performed throughout the rehabilitation session.