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 × 10 3 N·m/cm 3 [8.24 × 10 3 , 11.84 × 10 3 ], controls 13.32 × 10 3 N·m/cm 3 [12.50 × 10 3 , 13.59 × 10 3 ]). In MS compared to controls, we also observed reduced peak isometric strength relative to muscle volume (MS 0.47 N·m/cm 3 [0.37, 0.51], controls 0.58 N·m/cm 3 [0.56, 0.63]) and reduced isokinetic strength relative to muscle volume (MS 0.37 N·m/cm 3 [0.33, 0.42], controls 0.56 N·m/cm 3 [0.52, 0.56]). Quadriceps muscle volume was similar in participants with MS (168.42 cm 3 [158.55, 196.74]) and controls (183.26 cm 3 [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.