The Aging Neuromuscular System and Motor Performance

Age-related changes in the basic functional unit of the neuromuscular system, the motor unit, and its neural inputs have a profound effect on motor function, especially among the expanding number of old (older than ∼60 yr) and very old (older than ∼80 yr) adults. This review presents evidence that age-related changes in motor unit morphology and properties lead to impaired motor performance that includes 1) reduced maximal strength and power, slower contractile velocity, and increased fatigability; and 2) increased variability during and between motor tasks, including decreased force steadiness and increased variability of contraction velocity and torque over repeat contractions. The age-related increase in variability of motor performance with aging appears to involve reduced and more variable synaptic inputs that drive motor neuron activation, fewer and larger motor units, less stable neuromuscular junctions, lower and more variable motor unit action potential discharge rates, and smaller and slower skeletal muscle fibers that coexpress different myosin heavy chain isoforms in the muscle of older adults. Physical activity may modify motor unit properties and function in old men and women, although the effects on variability of motor performance are largely unknown. Many studies are of cross-sectional design, so there is a tremendous opportunity to perform high-impact and longitudinal studies along the continuum of aging that determine 1) the influence and cause of the increased variability with aging on functional performance tasks, and 2) whether lifestyle factors such as physical exercise can minimize this age-related variability in motor performance in the rapidly expanding numbers of very old adults

Exploring mechanisms of disuse atrophy and optimal rehabilitation strategies for the restoration of muscle mass, structure & function

Disuse atrophy (DA) occurs during situations of unloading and is characterised by a loss of muscle mass and function. These reductions may be observed as early as 5 days into a period of unloading. While the reduction of muscle size is well studied, the reduction in muscle function is less well characterised. Furthermore, different muscles of the lower leg have been shown to express diverging profiles of muscle size loss as a result of DA. In particular, the medial gastrocnemius (MG) is relatively susceptible to DA while the tibialis anterior (TA) is resistant to even long-term bed rest of over a month. The average length of stay in hospital in the UK was last reported at 4.5 days which is enough time for DA to occur in the quadriceps. In older individuals, loss of muscle mass and function may reduce quality of life to the point of frailty and are less well suited to performing resistance exercise. Hence, alternative therapies to attenuate DA may be needed. This thesis introduces skeletal muscle and its function as an organ in the human body, along with its composition and how this influences its function. It then discusses the study of DA and the situations in which it occurs, before covering the response of different muscles, the time course and strategies used for rehabilitation. General methods used within this thesis are detailed in Chapter 2. In Chapter 3, results of muscle size, strength, and various aspects of function from the vastus lateralis (VL), the MG and the TA to investigate the difference in response to 15-day unilateral lower limb immobilisation in young adults. In Chapters 4 and 5, this thesis investigates the neuromuscular adaptation to this intervention in the VL compared to the non-immobilised control, and then the immobilised MG and TA, respectively. These results show an impairment of neural input to the VL and the MG following immobilisation which is not seen in the TA. Finally, in Chapter 6, peripheral nerve stimulation is shown to potentially recruit from a broader pool of motor units than traditional neuromuscular electrical stimulation and as such may be more favourable for rehabilitation

A STEP TOWARDS UNDERSTANDING BALANCE CONTROL IN INDIVIDUALS WITH INCOMPLETE SPINAL CORD INJURY

Purpose: Frequent falls are reported by individuals with spinal cord injury (SCI) suggesting impairments in their balance control. This thesis examined balance assessment and balance control in individuals with SCI. Methods and Results: To investigate the effects of light touch on standing balance, center of pressure (COP) sway during standing was measured in 16 participants with incomplete SCI (iSCI) and 13 able-bodied (AB) participants. Participants with iSCI showed reduction in COP sway with light touch similar to AB participants. To study the association between stability during normal walking (NW) and unexpected slip intensity, NW behaviour and intensity of an unexpected slip perturbation were assessed in 20 participants with iSCI, and 16 AB participants. Participants with iSCI demonstrated greater stability by walking slower, taking shorter steps, and more time in double support. Walking slower was associated with lower slip intensity in individuals with iSCI. To study reactive balance control, change in margin of stability with a compensatory step, activation of lower extremity muscles, and change in limb velocity trajectories in response to an unexpected slip perturbation were studied in 16 participants with iSCI and 13 AB participants. Participants with iSCI demonstrated limitations in reactive responses including a smaller increase in lateral margin of stability, slower onset of trail limb tibialis anterior activity, and decreased magnitude of trail limb soleus activity. To identify balance measures specific to individuals with SCI, a systematic review of 127 articles was conducted. Thirty balance measures were identified; 11 evaluated a biomechanical construct and 19 were balance scales designed for use in clinical settings. All balance scales had high clinical utility. The Berg Balance Scale and Functional Reach Test were valid and reliable, while the Mini Balance Evaluation Systems Test was most comprehensive. Conclusions: Individuals with iSCI have impaired balance control, as evidenced by limitations in reactive balance; however, they have the ability to modify their balance, as demonstrated by greater stability during NW and with light touch while standing. No single balance measure met all criteria of a useful measure - high clinical utility, strong psychometric properties, and comprehensiveness in the SCI population. Combined, the findings highlight the need for the comprehensive assessment and rehabilitation of balance control after iSCI

Effect of age and gender on sEMG signals and force steadiness

Challenges encountered during daily activities are easily overcome by young adults but may be potential risk for falls and injuries among the elderly due to age-associated sensorimotor deficits. To mitigate these risks, early detection of neuromuscular changes is essential and it is important for these to be cost-efficient, non-invasive, high throughput and non-hazardous. Electromyogram (EMG) is a non-invasive recording of the muscle activity that uses inexpensive equipment and hence may be considered for this purpose. However, it is a gross non-specific signal and thus there is need for careful investigation to identify its suitability for studying age-associated changes to the muscles. This research has investigated non-invasive, superficially recorded EMG signals to identify the differences between young healthy adults (20-35 years) and older (60-80 years) subjects of both genders while they were performing isometric ankle plantar flexion and dorsiflexion. The study also studied age and gender differences in the maximal voluntary force, its steadiness, the time to reach steadiness and modulus of the force output prior to steadiness as measured at the foot plate during dorsi- and plantar-flexion. This study has experimentally demonstrated the significant increase in co-activation index around the ankle joint, decrease in the agonistic activity and increase in antagonistic activity in the major lower leg muscles due to ageing. Female participants were noted to have a higher co-activation index in comparison to the males of corresponding age group. From the analysis, it was observed that ageing causes an overall decline in muscle signal complexity affecting the whole muscle strength in both genders. Furthermore, it was also established that within the triceps surae muscle group, Soleus and the gastrocnemii showed varied effects of aging. Another key finding is the significant age and gender difference in the maximal force and its steadiness around the ankle joint during dorsiflexion. However, these differences are less significant during plantarflexion. Results of this study revealed that with age, there was an increase in the total modulus of the force used by the participant to stabilize the foot at a desired level of contraction, difference being more significant during dorsiflexion. This study highlights the age associated neuromuscular adaptations in plantarflexor and dorsiflexor muscles. This is reflected in the altered activity of agonistic and antagonistic muscles during isometric contractions, the reduction in the overall muscle signal complexity, and decreased strength and steadiness of the force exerted by the calf muscles. It has established gender differences in the reduction of the co-activation index and decreased force strength during ankle flexion movements

Exploring mechanisms of disuse atrophy and optimal rehabilitation strategies for the restoration of muscle mass, structure & function

Disuse atrophy (DA) occurs during situations of unloading and is characterised by a loss of muscle mass and function. These reductions may be observed as early as 5 days into a period of unloading. While the reduction of muscle size is well studied, the reduction in muscle function is less well characterised. Furthermore, different muscles of the lower leg have been shown to express diverging profiles of muscle size loss as a result of DA. In particular, the medial gastrocnemius (MG) is relatively susceptible to DA while the tibialis anterior (TA) is resistant to even long-term bed rest of over a month. The average length of stay in hospital in the UK was last reported at 4.5 days which is enough time for DA to occur in the quadriceps. In older individuals, loss of muscle mass and function may reduce quality of life to the point of frailty and are less well suited to performing resistance exercise. Hence, alternative therapies to attenuate DA may be needed. This thesis introduces skeletal muscle and its function as an organ in the human body, along with its composition and how this influences its function. It then discusses the study of DA and the situations in which it occurs, before covering the response of different muscles, the time course and strategies used for rehabilitation. General methods used within this thesis are detailed in Chapter 2. In Chapter 3, results of muscle size, strength, and various aspects of function from the vastus lateralis (VL), the MG and the TA to investigate the difference in response to 15-day unilateral lower limb immobilisation in young adults. In Chapters 4 and 5, this thesis investigates the neuromuscular adaptation to this intervention in the VL compared to the non-immobilised control, and then the immobilised MG and TA, respectively. These results show an impairment of neural input to the VL and the MG following immobilisation which is not seen in the TA. Finally, in Chapter 6, peripheral nerve stimulation is shown to potentially recruit from a broader pool of motor units than traditional neuromuscular electrical stimulation and as such may be more favourable for rehabilitation