Cortical and segmental excitability during fatiguing contractions of the soleus muscle in humans

Objective: The aim of this study was to examine the cortical and segmental excitability changes during fatigue of the soleus muscle. Methods: Ten healthy young subjects performed 45 plantar flexion maximal voluntary contractions (MVCs) (7-s on/3-s off) in 9 epochs of five contractions. Motor evoked potentials (MEPs) using transcranial magnetic stimulation and H-reflexes were assessed during the task. Results: The torque and the soleus EMG activity both showed the greatest decline during the 1st epoch, followed by a gradual, but significant decrease by the end of the task (similar to 70% pre-fatigue). The H-reflex sampled at rest after each epoch decreased to 66.6 +/- 18.3% pre-fatigue after the first epoch, and then showed no further change. The MEP on 10% pre-fatigue MVC after each epoch increased progressively (252.9 +/- 124.2% pre-fatigue). There was no change in the MEPs on the 3rd MVC in each epoch. The silent period on the MVC increased (109.0 +/- 9.2% pre-fatigue) early with no further changes during the task. Conclusions: These findings support that the motor cortex increases excitability during fatigue, but with a concomitant inhibition. Significance: These findings are in contrast to upper extremity muscles and may reflect a distinct response specific to postural, fatigue-resistant muscle. (C) 2011 International Federation of Clinical Neurophysiology. Published by Elsevier Ireland Ltd. All rights reserved.