Date of Award

Spring 1-1-2018

Document Type


Degree Name

Doctor of Philosophy (PhD)

First Advisor

Roger M. Enoka

Second Advisor

Alaa A. Ahmed

Third Advisor

Rodger Kram

Fourth Advisor

David E. Sherwood

Fifth Advisor

Brian L. Tracy


This dissertation comprises five experimental studies that investigated the influence of electrical nerve stimulation on motor unit discharge characteristics and mobility in young and older adults. Three studies evaluated the acute effects of electrical nerve stimulation and two were derived from an intervention that aimed to improve motor function in older adults.

In the first study, fine-wire electrodes were used to track single motor unit activity in the biceps brachii of the left arm while electrical nerve stimulation was applied to the biceps brachii of the right arm. Although there was no change in the discharge characteristics of motor units before, during, or after the application of electrical nerve stimulation, there was an increase in the number of motor units detected during and after stimulation. The findings suggest that the activity of motor units in the left elbow flexors during a voluntary contraction was modulated by electrical nerve stimulation applied to the right elbow flexors.

The subsequent two studies examined the influence of age on fluctuations in force during voluntary and evoked contractions with hand and forearm muscles. There were fewer fluctuations in force during a wrist-extension task in older adults during electrical nerve stimulation than during a voluntary contraction, but there was no difference for young adults. Moreover, the greater force fluctuations exhibited by older adults during steady contractions were associated with worse performance on a test of manual dexterity and greater fluctuations in the estimated common synaptic input to motor neurons during steady contractions.

The final two studies examined the influence of an electrical nerve stimulation intervention applied to the calf muscles on motor function in older adults across 6 weeks. The intervention improved mobility, but the time course of the adaptations varied across the different tests of mobility. Additionally, motor unit discharge characteristics during steady contractions partially explained differences in mobility among older adults.

This dissertation demonstrates the unique attributes of electrical nerve stimulation as an approach to modulate motor function in older adults.