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Corticospinal-evoked responses in lower limb muscles during voluntary contractions at varying strengths

Schools of ¹Human Movement Studies and ²Health and Rehabilitation Sciences, University of Queensland, Brisbane, Australia

Submitted 29 April 2008 ; accepted in final form 9 September 2008

This study investigated corticospinal-evoked responses in lower limb muscles during voluntary contractions at varying strengths. Similar investigations have been made on upper limb muscles, where evoked responses have been shown to increase up to 50% of maximal force and then decline. We elicited motor-evoked potentials (MEPs) and cervicomedullary motor-evoked potentials (CMEPs) in the soleus (Sol) and medial gastrocnemius (MG) muscles using magnetic stimulation over the motor cortex and cervicomedullary junction during voluntary plantar flexions with the torque ranging from 0 to 100% of a maximal voluntary contraction. Differences between the MEP and CMEP were also investigated to assess whether any changes were occurring at the cortical or spinal levels. In both Sol and MG, MEP and CMEP amplitudes [normalized to maximal M wave (M_max)] showed an increase, followed by a plateau, over the greater part of the contraction range with responses increasing from 0.2 to 6% of M_max for Sol and from 0.3 to 10% of M_max for MG. Because both MEPs and CMEPs changed in a similar manner, the observed increase and lack of decrease at high force levels are likely related to underlying changes occurring at the spinal level. The evoked responses in the Sol and MG increase over a greater range of contraction strengths than for upper limb muscles, probably due to differences in the pattern of motor unit recruitment and rate coding for these muscles and the strength of the corticospinal input.

transcranial magnetic stimulation; electromyography; motor system

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				B. W. Hoffman, T. Oya, T. J. Carroll, and A. G. Cresswell Increases in corticospinal responsiveness during a sustained submaximal plantar flexion J Appl Physiol, July 1, 2009; 107(1): 112 - 120. [Abstract] [Full Text] [PDF]