Leg strength and neural activation deficits in older adults with multiple sclerosis

BACKGROUND: Multiple sclerosis (MS) causes immune-mediated inflammation and neurodegeneration in the central nervous system, often leading to weakness and impaired mobility. In older adults with MS, it can be challenging to distinguish MS vs. age-related contributions to weakness. Studies are needed to examine the neuromuscular mechanisms of age-related declines in muscle strength to better understand their relative contribution to weakness in older adults with MS. OBJECTIVES: To investigate the neuromuscular mechanisms underlying age-related declines in muscle strength and their contribution to weakness in older adults with MS. METHODS: We conducted a study to determine whether older adults with MS exhibit differences in knee extensor voluntary (neural) activation, MRI-derived muscle volume, and skeletal muscle contractile quality compared to age-, sex-, and BMI-matched individuals without MS. Group comparisons were performed using nonparametric statistical analyses. RESULTS: In 10 older adults with MS and 10 age-, sex-, and BMI-matched controls, participants with MS had reduced isokinetic strength relative to body weight (MS 0.95 N·m/kg [0.77, 1.06], controls 1.48 N·m/kg [1.23, 1.66]) and summed torque relative to muscle volume (MS 10.31 × 10³ N·m/cm³ [8.24 × 10³, 11.84 × 10³], controls 13.32 × 10³ N·m/cm³ [12.50 × 10³, 13.59 × 10³]). In MS compared to controls, we also observed reduced peak isometric strength relative to muscle volume (MS 0.47 N·m/cm³ [0.37, 0.51], controls 0.58 N·m/cm³ [0.56, 0.63]) and reduced isokinetic strength relative to muscle volume (MS 0.37 N·m/cm³ [0.33, 0.42], controls 0.56 N·m/cm³ [0.52, 0.56]). Quadriceps muscle volume was similar in participants with MS (168.42 cm³ [158.55, 196.74]) and controls (183.26 cm³ [175.62, 202.25]). There was decreased neural activation parameters in MS including central activation ballistic torque (CAB) ratio (MS 0.31 [0.18, 0.35], controls 0.46 [0.40, 0.57]) and voluntary activation (MS 87.87% [84.09, 90.09], controls 94.82% [92.89, 95.78]). No statistically significant correlations between neuromuscular function and MS clinical outcomes were found. CONCLUSION: Neuromuscular function deficits in older adults with MS are largely attributed to impaired neural activation without significant differences in muscle quality compared with controls.