Quantitative assessment of motor fatigue and strength in MS

Objective: To determine the test-retest reliability of strength and fatigue measurements in patients with MS and in healthy control subjects, and to e xamine associations among motor fatigue, strength, and ambulatory impairmen t in MS patients. Background: Motor fatigue, defined as the loss of the max imal capacity to generate force during exercise, and weakness are common in patients with MS. Method: Twenty ambulatory MS patients and 20 age- and se x-matched healthy control subjects participated in the study. Test-retest r eliability was assessed in two identical testing sessions, separated by 3 t o 5 days. Maximal voluntary isometric strength was determined by fixed myom etry of seven muscle groups on each side. Motor fatigue was assessed using three exercise protocols: sustained maximal contractions (static fatigue), repetitive maximal contractions, and walking as far as 500 m. Four analysis models for static fatigue were examined for their test-retest reliability and their ability to discriminate between normal fatigue and pathologic fat igue from MS. Results: Test-retest reliability in MS patients was excellent for isometric strength and very good for static fatigue. Test-retest relia bility was lower for exercise protocols that involved repetitive contractio ns or ambulation. Compared with healthy control subjects, MS patients were weak in lower extremity muscles, but upper extremity strength was relativel y preserved, Fatigue was greater in MS patients, even in muscles that were not clearly weak. There were no significant associations between strength a nd fatigue in any of the muscles tested. A fatigue analysis model based on the area under the force-versus-time curve gave the best combination of rel iability and sensitivity to detect differences between MS patients and heal thy control subjects. Conclusions: Strength and motor fatigue can be measur ed reliably in patients with MS. MS patients experience more fatigue than h ealthy control subjects during sustained contractions, repetitive contracti ons, and ambulation. Motor fatigue appears to be distinct from weakness bec ause the degree of fatigue was not associated with the degree of weakness i n individual muscles. Quantitative assessment of strength and fatigue may b e useful to monitor changes in motor function over time in MS patients.