No association of leg strength asymmetry with walking performance, fatigability or fatigue in multiple sclerosis

2018 Spring.Includes bibliographical references.Background: One of the first signs of Multiple Sclerosis (MS) is weakness on one side of the body, which is associated with an increased reliance on the stronger leg during walking as indicated by asymmetric muscle activity. The role of leg strength asymmetry on walking performance, fatigue and fatigability is unknown in people with MS (PwMS). Objective: The purpose of this cross-sectional study was to determine whether leg strength asymmetry is associated with walking performance, objective measures of fatigability, or subjective perceptions of fatigue during a 6-minute walk test (6MWT). Methods: Maximal knee extensor strength was assessed in 19 PwMS, and a symmetry index was calculated based on the objectively defined more- and less-affected leg. Walking ability was determined by measuring the total distance covered during a 6MWT and fatigability by calculating the change in distance covered between minutes six and one. Perceptions of fatigue were assessed by obtaining ratings of perceived exertion (RPE) using the modified Borg 10-point scale during the first and the final minute of the 6MWT. Results: PwMS covered less distance (P=0.01) and perceived greater exertion (P<0.01) during minute six compared to minute one. Maximal knee extensor strength was different between the more- and less-affected side (P<0.01). The magnitude of asymmetry did not correlate with walking performance, fatigability, or perceptions of fatigue. Conclusions: Maximal knee extensor strength asymmetry may not play an important role in walking performance, fatigability or fatigue in PwMS. Future asymmetry studies should include the flexor muscles and measures of sensory function

Commentaries on viewpoint : physiology and fast marathons

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Time course of disuse-induced corticomotor plasticity in individual human brains: a precision TMS study

The organization of the human brain can be modified by behavior. Disuse reduces skeletomotor and corticomotor function, but the exclusive use of endpoint measures in the disused limb without consideration of spinal or peripheral influences has left the time course, origin and extent of disuse-induced corticomotor adaptations unclear. PURPOSE: 1) to determine the time course, origin and extent of corticomotor plasticity in response to skeletal muscle disuse, 2) to assess the relationship between disuse-induced changes in corticomotor and skeletomotor function and 3) to examine whether mental imagery (MI) can counteract the disuse-induced skeletomotor and corticomotor loss of function. METHODS: Six (3W, age: 22.7yrs, BMI: 24.4kg/m2) healthy young adults performed daily assessments of upper- (casted and un-casted first dorsal interosseus) and lower-extremity (non-dominant tibialis anterior) skeletomotor function, corticospinal, spinal and peripheral excitability over the course of twenty-one days. To induce disuse, three participants completed a 7-day immobilization intervention (Cast) after seven days of baseline testing (Pre), which was followed by another seven days of recovery testing (Post). The remaining participants performed a 5-day MI counter-intervention that started 48h after the onset of immobilization. Changes in corticomotor white matter microstructure were assessed with differential tractography between diffusion scans obtained before the first day of testing, three times throughout the intervention and at the end of the study. Changes in skeletomotor and corticomotor function were determined within-subject using ANOVAs (Pre, Cast, Post) with Benjamini-Hochberg corrections for multiple comparisons. RESULTS: Immobilization markedly reduced casted hand use, strength and fine motor skill. Skeletomotor deficits coincided with reduced white matter microstructure in smaller corticomotor regions and rapid homotopic reductions in corticospinal excitability (CSE) that occurred independent of changes at the spinal or peripheral level and reversed with the recovery of function after cast removal. MI preserved skeletomotor function when CSE was maintained, but had no beneficial effects when CSE decreased. DISCUSSION: Our results indicate that disuse-induced corticomotor plasticity is homotopic, that the skeletomotor consequences of such adaptations depend on the interplay between supraspinal and peripheral excitability, and that MI may attenuate the loss of skeletomotor function by preserving CSE

Effects of Muscle Function and Limb Loading Asymmetries on Gait and Balance in People With Multiple Sclerosis

People with MS (PwMS) often have a more- and less-affected side of the body which results in a variety of asymmetries, including measures of power, strength, muscle activity, and limb loading. Though many studies have identified asymmetries, their impact on gait and balance in PwMS is currently unclear. In this mini-review we first summarize previous findings of asymmetries in muscle function and limb loading and their impact on gait and balance in PwMS. We then provide potential explanations for this lack of consistency in the current literature, and propose study guidelines to improve future lower limb asymmetry studies. Making use of a unified approach to study lower limb asymmetry may then provide more clarity regarding their impact on mobility, specifically gait and balance, in PwMS