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

KEY POINTS: 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 500 ms 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. ABSTRACT: 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 (1500 ms 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 500 ms 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 100 ms time windows beginning 500 ms prior to the onset of contractions. An increase in MEP amplitude and a decrease in SICI were observed from the 200-100 ms and 300-200 ms time windows prior to BAL and RAMP contractions (P < 0.05), respectively, with greater changes prior to RAMP than to BAL within the 300-200 ms time window (P < 0.05). F-wave occurrence, used to assess spinal motor neurone excitability, increased prior to RAMP (200-100 ms, P < 0.05) but not BAL contractions. Data obtained in a few participants during the last 100 ms 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.