Guo, Yuxiao
(2024)
Exploration of central and peripheral neural properties in young and old humans.
PhD thesis, University of Nottingham.
Abstract
Ageing is associated with declines of skeletal muscle mass and function attributable to neural adaptations at both central and peripheral levels. Voluntary movement is achieved via a complex interplay of motor cortex, spinal circuitry and motoneurons, which may be differentially influenced by the ageing process and sex-specific hormonal changes. However, there is limited data exploring age- and sex-related differences in neural inputs and individual motor unit properties in vastus lateralis, which appears to be highly susceptible to age-related functional impairment.
This thesis introduces the structure of the central and peripheral motor nervous system in humans, along with its contributions to motor unit firing properties and its impacts on force generation and physical performance. It then demonstrates the methodological approaches to investigate the neural inputs to skeletal muscles with a focus on age-related adaptation and sex differences.
In Chapter 2 and 3, data indicates young and older females produced muscle force via different neuromuscular recruitment strategies to males which was characterised by smaller motor units discharging at higher rates. However, similar strategies were employed for force gradation in both sexes, manifested by increased motor unit firing rates and greater motor unit size. Notably, greater motor unit firing rate variability observed in older females may contribute to their greater functional deterioration.
In Chapter 4, it was found that muscle function, motor unit firing rates and common synaptic inputs did not differ bilaterally across limbs, whereas there was a significantly increased variability of motor unit firing rate in the dominant limb in both young and older adults. Additionally, an increased bilateral difference in force steadiness was apparent from early to late elderly, indicating a greater likelihood of unilateral functional weakness contributing to frailty in older age. The results also highlight the age-related reduction in muscle strength may be attributable to the decreased motor unit firing rates. In chapter 5, common synaptic inputs and persistent inward currents were found to be lower in VL MUs of older individuals, occurring independently of reductions in motor unit firing rates in this cohort. Furthermore, an age-related decline in force control ability was only observed in the ramp contraction rather submaximal sustained contraction, highlighting the role of task complexity in age- related impairments.
To eliminate the influence of progressive disuse in older age, masters athletes were studied in Chapter 6, and used as a model to explore the effects of inherent ageing and circulating sex hormones on neuromuscular function. The results highlight that lifelong power exercise training had more favourable physical characteristics when compared to inactive and endurance trained masters athletes, and a positive association is reported between androgen derivatives and motor unit firing rates, highlighting the potential of hormone administration as a therapeutic intervention strategy in older age.
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