Longitudinal changes in muscle and mobility in septuagenarian men and women

Cameron, James Andrew (2019) Longitudinal changes in muscle and mobility in septuagenarian men and women. Doctoral thesis (PhD), Manchester Metropolitan University.

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Abstract

The effects of old age on musculoskeletal structure and function have been characterised in the literature through cross-sectional studies, comparing data from young and older adults in an attempt to uncover the mechanisms behind decreases in muscle mass and function. However, such work is prone to potential bias associated with differences in changes in lifestyle and genotype during human ageing. Longitudinal studies can overcome such bias, though these are of in the minority and generally outline relatively simple measures in larger epidemiological studies. The overall aim of the work described in this thesis was to assess longitudinal changes in muscle mass and function, and how these influenced the ability to perform daily life activities over a 5-year period in older individuals. Characterising a more robust model of ageing than its cross-sectional counterparts in more detail than previously reported. This aim was addressed through several objectives that are described across the Chapters. In Chapter 2, we highlight that the use of dual-energy X-ray absorptiometry (DXA) to track changes in muscle mass is a viable method when compared to magnetic resonance imaging (MRI), though DXA exhibited a positive intercept with MRI and therefore consistently overestimated muscle volume. It was reported in Chapter 2 that over the 5-year period there might be an accelerated decline in muscle mass, with no difference in relative rate of muscle loss between genders or baseline muscle mass. Loss of muscle mass in ageing is related to deficits in functional capacity, in Chapter 3 the contribution of reduced voluntary activation, fibre atrophy and fibre loss to muscle weakness are investigated. Muscle quality, measured by patella tendon specific force was found to contribute significantly to the loss of age-related muscle weakness in early ageing, though loss of muscle mass was found to be the main cause. The objective of Chapter 4 was to investigate the influence of muscle weakness on mobility performance. Significant decreases in 6-min walk and timed up and go were noted, with changes in muscle power being the key contributor to this change. Rather than the intrinsic slowing of the muscle seen in early ageing, power loss was primarily due to reductions in maximal voluntary contraction. The main conclusion of this work was that free-living septuagenarian’s show an accelerated decline in muscle mass and functional performance over a 5-year period compared to cross sectional data previously reported, suggesting upon reaching the eighth decade humans neuromuscular system ages at an faster rate than preceding decades. The preferential atrophy of type II and loss of fibres are the key contributors to this loss of mass. These findings highlight the need for septuagenarians in relatively good health to have appropriate interventions designed to mitigate these age-related changes.