Date of Award

Spring 2014

Document Type


Degree Name

Doctor of Philosophy (PhD)



First Advisor

Hunter, Sandra K.

Second Advisor

Pan, Lawrence G.

Third Advisor

Nielson, Kristy A.


Despite the critical role of the lower limb during functional tasks such as walking, most studies examining the role of the cortex during muscle contractions have been conducted in upper limb muscles. Modulation of force by the cortex in the lower extremity and the influence of cortical inputs are poorly understood. The purpose of this dissertation was to investigate the role the cortex plays in modulating force control during static contractions with the lower limb and to determine the influence of manipulating cortical inputs.

Aim 1 determined the cortical regions involved in force-related changes between low and high forces and those areas that modulate steadiness (force fluctuations) during sustained isometric ankle dorsiflexion contractions in young men and women. This was achieved using functional magnetic imaging (fMRI). Both motor and some typically associated non-motor brain areas were active during lower limb force production and scaled linearly as force increased. Steadiness was associated with both motor and non-motor brain areas with minimal differences in areas activated between men and women. Aim 2 examined the influence of cognitive demand (null, low-cognitive demand, high-cognitive demand) on fatigability and steadiness of low- to moderate-force isometric contractions in young and older men and women. Women demonstrated greater force fluctuations than men during both the low- and moderate-force contractions and their motor output was influenced by changes in cognitive demand. Older adults were less steady than young during low- and moderate-force contractions, had greater age-related reductions in steadiness, and greater variability in fatigability when cognitive demand was increased.

This dissertation shows that cortical inputs are very important to lower limb motor control of static voluntary contractions. Cortical motor and non-motor regions that are important for control of force intensity and steadiness of lower limb contractions were identified and are key areas for potential cortical plasticity with impaired or enhanced leg function. Steadiness was altered by increasing cortical inputs (cognitive demand) especially in older adults whose motor performance was impaired and more variable than young. These results have important performance implications for cognitively demanding and low- to moderate-force tasks that are common to daily function in older adults.