Electronic Thesis and Dissertation Repository

Thesis Format

Integrated Article

Degree

Doctor of Philosophy

Program

Health and Rehabilitation Sciences

Supervisor

Doherty, Timothy J.

2nd Supervisor

Hunter, Susan W.

Joint Supervisor

Abstract

Objectives: The objective of this thesis was to evaluate and compare neuromuscular system function in a group of young, healthy old and mobility-impaired older females (gait speed less than 1m/s) in an attempt to identify which neuromuscular factors contribute most to the loss of mobility and functional performance with age.

Methods: The ankle dorsiflexor and knee extensor muscles were used as a model in these studies. First, in Chapter 2 quantitative electromyography (EMG) was used to evaluate motor unit (MU) numbers, properties and the fidelity of the neuromuscular junction. Next, Chapter 3 conducted magnetic resonance imaging (MRI) to examine contractile and non-contractile muscle components, as well as a novel technique known as magnetization transfers (MT) to assess muscle protein quality. Finally, to determine how changes in the neuromuscular system result in impaired mobility and a decline in functional performance, Chapter 4 explored isometric muscle strength, power and velocity of muscular contraction in the lower extremity muscle groups.

Results: Mobility impaired older adults had significantly lower motor unit number estimates (MUNE) in the tibialis anterior (TA) than young adults (Chapter 2). In the ankle dorsiflexor and knee extensor muscles the proportion of non-contractile muscle tissue in both healthy and mobility-impaired older adults was approximately twice that of young adults (Chapter 3). Additionally, muscle protein quality, as determined via the magnetization transfer ratio (MTR) was significantly reduced in both groups of older adults compared to young, and further reduced in healthy old compared to mobility impaired adults (Chapter 3). While isometric strength was maintained in the ankle dorsiflexor muscles of older adults, muscular power was significantly reduced in mobility-impaired, but not healthy older adults (Chapter 4). Muscle contraction velocity was determined to be the critical component in power production, and was significantly reduced in the knee extensor muscles of mobility impaired older adults compared to young and healthy older adults (Chapter 4).

Significance: This thesis provides novel information regarding the neuromuscular factors that contribute to the loss of mobility with age. It highlights the importance of power and muscle contraction velocity for sustaining lower extremity functional performance with advanced age. These studies provide insight into important outcomes that can be used for further research to help maintain mobility into old age.

Summary for Lay Audience

The purpose of this research was to compare nerve and muscle health in young adults, healthy older adults and older adults with impaired mobility. We identified mobility impairment as older adults who walked slower than 1 m/s. The aim of this thesis was to identify factors that may contribute to the loss of mobility with age. To accomplish this we conducted three experiments to: 1) record the electrical activity that comes from muscle during normal muscle contraction, 2) use magnetic resonance imaging (MRI) to image muscles and compare their composition and 3) record measures of strength, power and performance in the lower extremity. We found that older adults with mobility impairment had reduced nerves supplying a muscle in the lower leg compared to young adults. Using MRI we also discovered that both groups of older adults had a greater proportion of fat within their muscles and poorer quality of muscle protein compared to young adults. Finally, we found that muscle power, which is calculated by multiplying force by angular velocity of contraction, declined more than strength with age. Specifically, we revealed that the velocity of muscle contraction seems to be the critical factor leading to the loss of power in older adult’s muscles, and that those with mobility impairment had the greatest loss of muscle contraction velocity. These findings are important because they highlight some of the adaptations to nerve and muscle that occur with aging and lead to impairment in older adult’s mobility. The results of this thesis provide important outcome measures that can be used to design programs aimed to maintain mobility in older adults.

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