Influence of lifelong endurance training on motor unit behaviour in aging

Aging is a physiological process associated with increased fragility, mobility limitations and loss of independence in daily activities. It is influenced by a combination of various factor within the life of a person: diet, sleep and physical activity. One of the main effects of aging is sarcopenia, which is the progressive and generalized decrease of muscle mass and strength, leading to muscle weakness and eventually to movement impairment. In industrialized countries life expectancy is becoming progressively higher and the average age of the population is increasing. In Italy in 2023, 3.86 million older adults experienced difficulties with basic functional activities. However, physical activity, particularly in older adults, helps to slow down the physiological processes described above and mitigate their impact on mobility. To better define strategies for preserving motor function in aging, it is essential to understand how regular physical activity influences neuromuscular activation. Moreover, since these processes may manifest differently in males and females, considering sex differences is also crucial. In this thesis, differences in neuromuscular activation between physically active and inactive older adults have been investigated. Specifically, lifetime endurance trained elders (TLO, 8 males and 8 females, 70.13 ± 3.37 yrs) and control untrained elders (PRO, 8 males and 8 females, 75.44 ± 4.17 yrs) have been studied to investigate the neuromuscular effect of lifelong training. Neuromuscular activity was assessed from vastus lateralis using high density surface EMG (HDsEMG) detected with a 64-electrode grid. Signals were acquired during force-varying isometric contractions with trapezoidal profile at 30% and 50% MVC, a sustained constant-force contraction at 50% MVC until exhaustion, and fatiguing dynamic contractions. Motor unit action potentials were identified through a semi-automatic decomposition algorithm, enabling characterization of motor unit behaviour. During the isometric ramps the mean firing rate was higher for the trained group compared to untrained in both sexes and contraction intensities (PRO F: 9.69 ± 1.32 pps, TLO F: 12.25 ± 2.50 pps, PRO M: 9.47 ± 1.53 pps, TLO M: 10.82 ± 2.16 pps at 30% MVC). No significant differences have been observed in force steadiness or exhaustion time between sexes or training groups (52.06 ± 22.41 s for TLO, 58.09 ± 39.55 s for PRO). No statistical differences have been found in MVC checkpoints comparisons and in power output analysis between TLOs and PROs for both sexes, but males showed higher MVCs than females for both groups in almost all checkpoints. The analysis of fatiguing contractions at 50% MVC to exhaustion showed that TLOs have higher initial firing rate and higher firing decrease than PROs. These findings highlight specific neuromuscular differences between trained and untrained older adults, particularly in motor unit firing behaviour, while other variables such as force steadiness, endurance time, and maximal strength showed no clear distinctions. Some discrepancies with previous literature may be related to the limited sample size and variability within the groups, indicating that further studies with larger cohorts are needed to confirm these results.