Date of Award

Spring 2020

Document Type


Degree Name

Doctor of Philosophy (PhD)


Physical Therapy

First Advisor

Ng, Alexander V

Second Advisor

Bobholz, Julie A.

Third Advisor

Hunter, Sandra K.


Activities of daily living require steady, non-fatiguing, isometric muscular contractions to maintain postural control and stabilize body segments to facilitate interaction with the environment. Furthermore, typical activities often require simultaneous performance of cognitive and motor tasks. This may challenge people with multiple sclerosis, a chronic neurodegenerative disease of the central nervous system associated with motor and cognitive impairments. Despite functional relevance, isometric force steadiness in both the upper and lower extremities has not been explored in this population. Additionally, dual task experiments in multiple sclerosis have primarily used gait, a dynamic activity, as the motor task. Thus, the purpose of this dissertation was to examine isometric force steadiness performed under single and dual task conditions in people with multiple sclerosis. It was hypothesized that people with multiple sclerosis would be less steady and have greater dual task costs of cognitive-motor tasks.Study one measured steadiness of the ankle dorsiflexors and elbow flexors across a range of low to moderate force targets during a single task condition. Absolute force fluctuation at each target was measured and relative fluctuation was calculated using the coefficient of variation. In the elbow flexors, people with multiple sclerosis were less steady than controls only at very low forces and were less steady at nearly all force targets in the ankle. However, magnitudes of upper and lower extremity force fluctuation did not correlate within either sample.Study two determined dual task effects of simultaneous performance of a steady ankle dorsiflexion contraction and a cognitive task involving working memory and processing speed. Both controls and people with multiple sclerosis experienced negative dual task effects on motor and cognitive performances. Although those with multiple sclerosis did not perform as well as controls for all tasks, there was no difference in motor effects.This dissertation shows that 1.) isometric steadiness is impaired in the upper and lower extremities of people with multiple sclerosis at very low forces under single task conditions, 2.) people with multiple sclerosis experience cognitive-motor interference when dual tasking, and 3.) the relative dual task motor effects are nonetheless comparable to what is experienced by healthy controls.