Changes in corticospinal excitability during the preparation phase of ballistic and ramp contractions.

Abstract

Changes in corticospinal excitability prior to a contraction may depend on its characteristics, including the rate of torque development. This study compared the specific modulation of cortical and spinal excitability during the preparation phase (last 500ms before contraction) of fast (ballistic) and ramp contractions of ankle dorsiflexors, using transcranial magnetic stimulation and peripheral nerve stimulation. The results indicate earlier changes at the cortical than at the spinal level during the preparation phase of both contraction types. However, these adjustments are delayed prior to ballistic relative to ramp contractions. This study suggests that the time course of change in cortical and spinal excitability during the preparation phase of a voluntary action is specific to the intended rate of torque development of the upcoming contraction. The present study investigated cortical and spinal excitability during the preparation phase of ballistic (BAL) and ramp (RAMP) isometric contractions. To this end, young adults performed BAL and RAMP (1500ms torque rise time) contractions, reaching a similar torque level, with the ankle dorsiflexor muscles. Transcranial magnetic stimulation of the motor cortex was randomly applied to record motor evoked potentials (MEP) in the tibialis anterior during the last 500ms preceding the contraction (n=16). Short-interval intracortical inhibition (SICI; n=10) and spinal motor neurone excitability (F-wave occurrence; n=8) were also assessed during this period. Data were averaged over 100ms time windows beginning 500ms prior to the onset of contractions. An increase in MEP amplitude and a decrease in SICI were observed from the 200-100ms and 300-200ms time windows prior to BAL and RAMP contractions (P<0.05), respectively, with greater changes prior to RAMP than to BAL within the 300-200ms time window (P<0.05). F-wave occurrence, used to assess spinal motor neurone excitability, increased prior to RAMP (200-100ms, P<0.05) but not BAL contractions. Data obtained in a few participants during the last 100ms confirmed a delayed and steeper rise in corticospinal excitability prior to BAL contractions. These results indicate earlier changes at the cortical than at the spinal level, with delayed changes prior to BAL contractions. This study suggests that the time course of change in cortical and spinal excitability during the preparation phase of a voluntary action is specific to the intended rate of torque development of the upcoming contraction.