Enhanced corticomotor excitability with dynamic fatiguing exercise of the lower limb in multiple sclerosis.

OBJECTIVE :

To determine if task performance and fatigability during rapid repetitive exercise of the lower limb differ in a group of MS subjects compared to a control group, and what central changes accompany the development of fatigue and the period of recovery.

METHODS :

Transcranial magnetic stimulation (TMS) was used to compare motor evoked potential (MEP) responses between 10 clinically definite MS subjects (7F, 33- 64 years of age; EDSS </= 4; MRC grade >/= 4/5) and 13 control subjects during and after 5 bouts of a 15- second maximum rate foot-tapping task performed at 1 minute intervals.

RESULTS :

Maximum voluntary contraction (MVC) force of ankle dorsiflexion was lower (15 %) in the MS group compared to controls; however there were no differences in the rate of foot tapping. The rate of foot tapping decreased during each bout of exercise to a comparable degree in both groups, but there was no overall deterioration in performance across the 5 repeat bouts in either group.

MS subjects showed a greater decline in strength than controls after exercise (20.7 +/- 7.7 % vs. 6 +/- 3.6 %; p < 0.05). MEP amplitude increased significantly for the exercised limb in both groups, but the increase was greater in MS subjects (65.9 +/- 27 % vs. 31 +/- 19.6 %; MS vs. control; p < 0.05).

MEP amplitude also increased for the non-exercised limb in controls (40.6 +/- 15.6 %, p < 0.01) but not in MS subjects.

CONCLUSIONS :

Mild to moderately affected MS subjects can perform a fatiguing exercise requiring a high level of central motor control but this is associated with a greater strength loss and increase in cortico-motor (cortico = cortex) excitability compared to unaffected individuals.

SIGNIFICANCE :

Central adaptive processes are likely to have a significant role in maintaining task performance in MS.

Centre for Neuromuscular and Neurological Disorders,

University of Western Australia,

Queen Elizabeth II Medical Centre,

Perth, Western Australia WA 6009,