Complexities of ageing: An investigation of age-group differences in neuromuscular complexity.

Neuromuscular complexity quantifies the temporal structure of muscle torque or force signals, providing a non-invasive measure of the neuromuscular system's functionality. Ageing has been identified as a contributing factor to the loss of neuromuscular complexity, proposed to reflect an age-related reduction in neuromuscular function, specifically a reduced capacity to effectively control voluntary muscle contractions. Notable gaps remain in the current understanding of how age influences neuromuscular complexity, particularly in knee extensor (KE) muscles and small hand muscles involved in precision pinch grip (PG) tasks. Additionally, the neurophysiological mechanisms underpinning age-related changes in

neuromuscular complexity are not yet fully understood, and the functional importance of changes in neuromuscular complexity remains unclear. Therefore, the current thesis investigated age-group differences in the complexity of isometric KE torque and precision PG force signals, as well as the neural mechanism(s) underpinning the complexity of isometric muscle torque signals.

Chapter Four assessed the intra- and inter-day reliability of the complexity of isometric KE muscle torque signals in younger and middle-aged adults. The results indicated that measures of complexity exhibit moderate to excellent intra- and inter-day reliability across both age groups. These findings support neuromuscular complexity as a reliable, non-invasive technique for assessing and monitoring age-group related differences in neuromuscular function.

Chapter Five investigated age-group differences in the complexity of isometric KE muscle torque signals at 40% MVT and MVT. No differences in neuromuscular complexity were observed between middle-aged adults (58.6 ± 5.1 years) and younger adults (21.9 ± 3.7 years). However, age-related changes were evident in the temporal organisation of neuromuscular output, suggesting a possible restructuring of neuromuscular control strategies, that maintains motor performance with age.

Chapter Six investigated the effect of contraction intensity on age-group related differences in the complexity of isometric KE muscle torque signals. The study also examined the relationship between neuromuscular complexity and postural sway. Older adults (64.9 ± 11.3 years) demonstrated lower neuromuscular complexity across a range of contraction intensities (10% to 80% MVT) compared to younger adults (22.5 ± 3.4 years), with the most pronounced age-group related differences occurring above 20% MVT. These findings suggest that impairments in muscle torque regulation in older age are exacerbated at higher contraction intensities. Additionally, lower neuromuscular complexity was associated with greater postural sway, highlighting the potential functional relevance of complexity.

Chapter Seven investigated age-group differences in the neuromuscular complexity of isometric precision PG force signals. Adults over the age of 70 years exhibited lower neuromuscular complexity than younger adults, aged 18 to 30 years. However, no differences in neuromuscular complexity were observed between younger adults and those aged 50 to 70 years. These findings suggest that measurable changes in hand muscle force regulation begin to emerge in the eighth decade of life, providing insight into the potential time course of age related changes in hand muscle neuromuscular function.

Chapter Eight investigated the neural mechanism(s) that may underpin the complexity of isometric KE muscle torque signals. Chapter 8A investigated the association between the strength of common synaptic input to the motor neuron pool of the vastus lateralis muscle and the complexity of isometric KE torque in adults aged between 18 and 90 years. Stronger alpha

frequency band oscillations in common synaptic input were found to be associated with higher complexity isometric KE torque signals. Chapter 8B investigated whether fatigue-related changes in the strength of common synaptic input to the motor neuron pool could explain fatigue-related changes in isometric KE torque complexity. Fatigue-related changes in the strength of alpha band oscillations in common input accounted for a significant percentage of the fatigue-related changes in isometric KE torque complexity. These findings suggest that the complexity of isometric muscle torque is attributable, in part, to the strength of oscillations in common synaptic input (an adaptive neural response) and is therefore indicative of the neuromuscular system's strategy to adapt muscle torque to meet task demands.

In conclusion, the thesis demonstrates that older adults exhibit lower neuromuscular complexity in both lower and upper extremity muscle groups compared to younger adults. The observed age-related differences in neuromuscular complexity were likely attributable, in part, to alterations in the strength of oscillations in common synaptic input. These findings highlight the potential of neuromuscular complexity as a reliable, functionally relevant, non-invasive biomarker for assessing and monitoring age-related changes in aspects of neuromuscular system function.