Assessment of muscle strength and motor fatigue with a knee dynamometer in subjects with multiple sclerosis: a new fatigue index

Objective: To measure muscle strength and motor fatigue with a knee dynamometer and to assess the intra-rater reliability of measurements for maximal isometric extensor and flexor torques and the reliability of a new fatigue index (FI) in patients with mild to moderate multiple sclerosis (MS). Design: Repeated assessments with one-week intervals. Setting: The Masku Neurological Rehabilitation Centre, Masku, and the Social Insurance Institution, Research Department, Turku, Finland. Subjects: Twenty-eight MS patients. Outcome measures: Maximal isometric torque during 5 s and fatigue of knee flexors and extensors during isometric contractions of 30 s were assessed. A new FI was established and compared with the two previously used indices (FI1 and FI2). All three indices are based on the calculated area under the force versus time curve (AUFC), with FI1 using the 30-s recording time in its entirety and F2 omitting the initial 5 s in the calculation. In the new fatigue index (FI3), the time point of maximum (TPM) torque achieved by the subject is used as the starting point in the calculation. The patient's subjective fatigue was measured by Fatigue Severity Scale (FSS). Results: The intraclass correlation coefficient (ICC) was 0.97 in maximal isometric torque measurements. FI3 showed good intra-rater reliability (ICC 0.68-0.86). None of the fatigue indices correlated with FSS. Conclusions: Maximal isometric torque and motor fatigue of knee flexor and extensor muscles can be reliably measured using a knee dynamometer in MS patients. The new FI proved to be a reliable model for MS patients.