Project Outline

Aim

To elucidate the neurophysiological and morphological processes affecting the generation of muscle force in older individuals and study the optimal loading strategies ameliorating declines in strength.

Objectives

1. Identify main neural and contractile mechanisms affecting the generation of muscle force in older individuals.
2. Investigate mechanisms regulating the amount of motor neuron/motor unit activation required to produce force during low-load contractions at the point of failure and high-load contractions.
3. Determine the effects of low-load resistance training (LLRT) vs high-load resistance training (HLRT) on mechanisms of force generation in older individuals

Background

As we age, both muscle mass and the number of motor units decrease. Muscle fibre atrophy is linked to issues in protein synthesis, and reductions in motor units can lead to muscle atrophy due to denervation. Although some fibres can be reinnervated, they cannot generate as much force as in youth. The mechanisms behind muscle mass reduction with age have been extensively studied, but there's limited knowledge about how aging affects force generation mechanisms. Muscle properties like volume, thickness, and pennation angles change with age, potentially reducing muscle force. Nevertheless, older adults also experience reductions in motor unit discharge rate and recruitment, affecting strength further. Resistance training can increase motor unit firing properties, potentially compensating for muscle changes, but the interaction between motor unit firing properties and muscle morphology/architecture in older individuals remains unknown.

In ageing research, a key question is the optimal loading for maximizing muscle hypertrophy and strength. Current guidelines recommend high loads for older adults to boost muscle mass and strength. However, recent studies suggest that similar benefits can be achieved with lower loads until exhaustion (task failure). When low-load contractions are done to failure, there is a gradual increase in motor unit recruitment and fibre activation. Recent research also indicates that both low and high-load resistance training activate type II muscle fibres with higher hypertrophic potential. However, it is important to note that neural drive (motor unit firing) during low-load contractions until failure is lower than during high-force contractions. This may be due to inhibitory mechanisms that could limit strength gains in older individuals. Some studies suggest that high-load resistance training is more effective for increasing strength, while others show similar gains in muscle mass and strength for both low and high-load training in older adults. This debate highlights the need to understand the mechanisms underlying resistance training's effects on muscle mass and strength in older individuals. The proposed techniques will allow to compare motoneuron excitability and motor unit behaviour at both low and high loads and examine the interaction between muscle changes and motor unit activity. These factors are crucial since changes in motoneuron excitability and motor unit activity can impact an older adult's ability to gain strength, regardless of muscle mass gains. Determining the optimal training approach for increasing both strength and muscle mass in older adults is essential.

Methodology

Cross-sectional comparison of mechanisms of force generation in young and old individuals at different loads, followed by a 12-week resistance intervention comparing LLRT vs HLRT to understand the effects of these training paradigms on both neural and contractile muscle properties. By using state-of-the-art electrophysiological and imaging techniques, this project will address the following key outstanding questions:

1. To what extent do age-related reductions in muscle mass affect mechanisms of motor unit activation and muscle contraction?
2. What is the effect of repeated low-loads until failure versus high-loads on motor unit activation and motor neurone excitability in the elderly?
3. What are the effects of a 12-wk LLRT vs HLRT intervention on motor unit activation, strength, and changes in whole-muscle and muscle-fibre cross-sectional area in older adults?

References

Schoenfeld, B. J., et al. (2017). "Strength and Hypertrophy Adaptations between Low- Vs. High-Load Resistance Training: A Systematic Review and Meta-Analysis." Journal of Strength and Conditioning Research 31(12): 3508-3523.

Del Vecchio, A., et al. (2019). "The increase in muscle force after 4 weeks of strength training is mediated by adaptations in motor unit recruitment and rate coding." J Physiol 597(7): 1873-1887

Techniques

• High density surface electromyography
• Transcranial magnetic stimulation
• Ultrasound imaging
• Muscle biopsies
• Isokinetic Dynamometry