Lower-limb functional capacity is impaired in most people with multiple sclerosis (PwMS). Reductions in lower-extremity muscle mechanical function (e.g., muscle strength) appear to have critical implications for lower-limb functional capacity. However, no review has summarized the current knowledge about the importance of muscle strength for functional tasks in PwMS. Expanding the current knowledge would advance the design of both clinical and research interventions aiming to improve functional capacity in PwMS.

(1) To identify studies that measured lower-extremity muscle mechanical function and lower-limb functional capacity outcomes in PwMS, and (2) to map associations between muscle strength and functional capacity.

This review was based on a literature search (databases: PubMed, Embase). Included studies had to report data on lower-extremity muscle mechanical function and lower-limb functional capacity outcomes in PwMS. The associations between muscle strength and functional capacity were analyzed by using the reported correlation coefficients (R) recalculated to the determination coefficient R². Randomized trials and observational studies were included.

A total of 59 articles were reviewed; 17 (773 participants) reported associations between muscle strength and functional capacity. Lower-extremity muscle mechanical function explained a significant part of the variance in most lower-limb functional capacity tests (approximately 20–30%). This was particularly evident in muscle strength from the weakest leg. Muscle strength was predominantly tested on knee extensors and knee flexors by using isokinetic dynamometry during maximal isometric (0°/s) and dynamic (30–60°/s) contractions. Walking tests such as the timed 25-Foot Walk Test and 10-Min, 2-Min and 6-Min Walk Test were the most frequently performed functional capacity tests.

In PwMS, muscle strength of particularly the weakest limb explains 20% to 30% of the variance across a number of lower-limb functional capacity tests. Thus, exercise programs should focus on increasing lower-extremity muscle mechanical function in PwMS and minimizing strength asymmetry between limbs.