Unilateral grip fatigue reduces short interval intracortical inhibition in ipsilateral primary motor cortex.
OBJECTIVE
This study was designed to examine whether exhaustive grip exercise of the left hand affected intracortical excitability in ipsilateral motor cortex.


METHODS
Ten healthy male subjects (aged 21-24 years) participated in experiment 1 in which paired-pulse transcranial magnetic stimulation (TMS) was used to test corticospinal and corticocortical excitability in right (relaxed) first dorsal interosseous (FDI) muscle during the recovery period after exhaustive forceful grip exercise of the left hand. Seven of the same subjects participated in experiment 2, in which the intensity of the test stimulus was adjusted so that the amplitude of motor evoked potential (MEP(TEST)) was kept constant throughout the measurement.


RESULTS
In experiment 1, MEP(TEST) was slightly reduced from 5 to 15min after exercise whilst short interval intracortical inhibition (SICI) at interstimulus interval (ISI) of 2 and 3ms became less effective. Intracortical facilitation (ICF) was unchanged. In experiment 2 when the MEP(TEST) was maintained at a constant size there was again no change in ICF, and the reduction in SICI was still present at the same intervals.


CONCLUSIONS
We conclude that unilateral exhaustive grip exercise reduced the excitability of the corticospinal output of the ipsilateral motor cortex whilst simultaneously reducing the excitability of SICI. These results would be compatible with the idea that fatigue increases the tonic level of interhemispheric inhibition from the fatigued to the non-fatigued cortex.


CONCLUSIONS
Muscle fatigue to the point of exhaustion has lasting effects on the excitability of intracortical circuits in the non-exercised hemisphere, perhaps via changes in the tonic levels of activity in transcallosal pathways.
Takahashi
Kyohei
K
Biological Functions and Engineering, Kyushu Institute of Technology, Kitakyushu, Japan.
Maruyama
Atsuo
A
Maeda
Masato
M
Etoh
Seiji
S
Hirakoba
Kohji
K
Kawahira
Kazumi
K
Rothwell
John C
JC
D000704:Analysis of Variance; D001703:Biophysics; D004558:Electric Stimulation / Q000379:methods; D004576:Electromyography / Q000379:methods; D019054:Evoked Potentials, Motor / Q000502:physiology; D005221:Fatigue / Q000503:physiopathology*; D007839:Functional Laterality / Q000502:physiology*; D018737:Hand Strength / Q000502:physiology*; D006801:Humans; D008297:Male; D009044:Motor Cortex / Q000503:physiopathology*; D018482:Muscle, Skeletal / Q000294:innervation; D009433:Neural Inhibition / Q000502:physiology*; D011930:Reaction Time / Q000502:physiology; D013997:Time Factors; D050781:Transcranial Magnetic Stimulation; D055815:Young Adult