Muscle strength and fatigue during isokinetic exercise in individuals with multiple sclerosis

Purpose: To compare muscle strength and muscle fatigue of the knee extensors and flexors in individuals with multiple sclerosis (MS) and non-MS control subjects and to evaluate the reliability of muscle strength and muscle fatigue testing in these individuals. Methods: Thirty individuals (13 women and 2 men for both MS and control groups), age (mean +/- SD) 38.8 +/- 10 for MS and 33.1 +/-7.6 yr for controls, participated in this investigation. Peak torque was measured on two occasions separated by approximately 7 d at 30, 60, 90,120,180 degrees .s(-1) with 2 min of recovery between each bout. The nondominant leg was tested followed by the dominant leg after 10 min of recovery. Subjects then performed three bouts of 30 flexions and extensions of the dominant leg at 180.s(-1) with 1 min of recovery between bouts. Results: The reliability of muscle torque was very high for individuals with MS (only 1 of 20 measurements with an ICC below 0.900). Total work was also highly reliable for MS, but the Fatigue Index (work during the last 15 contractions/work during the first 15 contractions) x 100 was not. Peak torque adjusted for age, body mass, and fat free mass (measured by whole body plethysmography; the Bod Pod; Life Measurement Instruments; Concord, CA) was significantly greater for controls than for MS for three of four lower body muscle groups tested. For the muscle fatigue test (3 bouts of 30 knee extensions and flexions at 180 degrees .s(-1)), the Fatigue Index was greater (less fatigue) for the knee extensors for controls than MS for the third bout. For flexion, the Fatigue Index was greater for controls than MS over the three bouts (group effect). Total work was significantly greater for controls than MS for the flexors (group effect) and approached significance for the extensors. Conclusions: Individuals with MS were weaker than controls when data were adjusted for age, body mass, and fat free mass. This latter finding (force relative to age and fat free mass) suggests that there is a reduced ability to activate muscle mass in MS and/or the muscle mass of individuals with MS is of lower quality (i.e., reduced force/unit muscle mass) than controls.