Motor unit characteristics in young and old skeletal muscle

The neuromuscular changes associated with increasing age include a reduction in the number of motor units (MU) and an increase in the size of those that remain. Electromyography (EMG) has been used to investigate these changes, however the available literature is largely concerned with small peripheral muscles and little is known of the large leg muscles, in particular the vastus lateralis (VL) which is particularly susceptible to age-related atrophy. In Chapter 2 we highlight the variations in MU structure across a single large muscle, the VL. Motor unit properties differed according to location, recording depth and contraction intensity. These findings were applied in Chapter 3 to study differences in MU properties of the VL between Young and healthy Old participants. It was noted that the Old males had fewer, but larger MUs than the young. The Old MUs also showed signs of greater instability in neuromuscular transmission and lower firing rates. It has been suggested that lifelong exercise may have beneficial effects on neuromuscular changes, in Chapter 4 this was investigated in the VL and tibialis anterior (TA) of competitive Masters Athletes (MA). It was found the MA were no different to the Old, and showed significant signs of neuromuscular decline and MU remodelling. The MU remodelling in VL and TA was further assessed in Chapter 5, where it was found the effects of age on MU characteristics were greater in the TA when compared to the VL. The age-related differences also differed from low to moderate contraction intensities, and indicated a more homogenous MU pool in Old participants. Older people show clear differences in MU properties than Young, and lifelong exercise does not appear to have any beneficial effects on these changes. The MU remodelling process likely has consequences beyond the loss of muscle fibres, and the lack of heterogeneity in the MU pool may well cause functional limitations