INTRODUCTION:
The anterior thigh muscles are particularly susceptible to muscle loss and weakness during ageing, but it remains unclear how this is associated with changes to neuromuscular structure and function in terms of motor unit (MU) number, size and MU potential (MUP) stability.

METHODS:
Intramuscular and surface electromyographic (EMG) signals were recorded from the vastus lateralis (VL) during voluntary contractions held at 25% maximal knee extensor strength in 22 young (25.3 ± 4.8yrs) and 20 physically active older men (71.4 ± 6.2yrs). MUP size, firing rates, phases, turns and near fibre (NF) jiggle were determined and motor unit number estimates (MUNEs) were made by comparing average surface MUP with maximal electrically-evoked compound muscle action potentials. Quadriceps cross-sectional area was measured by magnetic resonance imaging.

RESULTS:
379 individual MUs were sampled in young and 346 in older men. Compared with young, the older MUs had 8% lower firing rates and larger MUP size (∼+25%) as well as increased complexity evidenced as, phases (+13%), turns (+20%) and NF jiggle (+11%) (all P < 0.0005). The MUNE values (derived from the area of muscle in range of the surface-electrode) in older subjects were around 70% of the young (P < 0.05). Taking into consideration the 30% smaller cross-sectional area of the VL, the total number of MUs in the older muscles was between 50-60% lower than in the young (P < 0.0005).

CONCLUSION:
A large portion of the VL MU pool is lost in older men and those recruited during moderate intensity contractions were enlarged and less stable. These MU changes were evident before clinically relevant changes to muscle function were apparent; nevertheless, the changes in MU number and size are likely to be a prelude to future movement problems.