Local Vibration of the Hamstrings Induces Changes in Cortical and Spinal Excitability to the Antagonist Quadriceps following ACL Reconstruction

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INTRODUCTION: Prolonged vibration of the hamstrings increases voluntary activation of the quadriceps in patients with anterior cruciate ligament reconstruction (ACLR). To gain insight into the mechanisms responsible for this increase, we investigated the effects of prolonged vibration of the hamstrings on spinal and cortical excitability of motor pathways to the quadriceps and on the time course of these effects in patients post-ACLR and in noninjured (NI) age-matched participants. METHODS: Spinal and cortical excitability of neural pathways to the vastus medialis were assessed in 42 participants (14 ACLR, 14 NI, and 14 NI Sham) before and at 0, 10, 20, 30, 45, and 60 min after prolonged (20 min) vibration of the hamstrings. The NI Sham group received no vibration. Voluntary activation of the quadriceps was evaluated with the central activation ratio technique. RESULTS: In both vibration groups (ACLR and NI), vibration induced a persistent increase in corticospinal ( P < 0.001) excitability for 60 min after vibration. In the ACLR group only, vibration induced a sustained increase in spinal (H-reflex amplitude) excitability for up to 60 min following vibration ( P < 0.001). In the ACLR group, an increase in voluntary activation was positively correlated with increases in spinal and corticospinal (motor evoked potential amplitude) excitability. CONCLUSIONS: The increase in voluntary activation of the quadriceps following prolonged vibration of the hamstrings is due to an increase in quadriceps spinal excitability. Furthermore, prolonged vibration of the hamstrings increases cortical and spinal excitability of neural pathways to the quadriceps for at least an hour following cessation of the vibration. Understanding these mechanisms will enable clinicians to optimize this technique and enhance rehabilitation outcomes.

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