Comparing Neuromuscular System Function in Healthy and Mobility-Impaired Older Females
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.