Motor unit (MU) numbers decline in older age and contribute to reductions of muscle mass, strength and function [1]. The compensatory process of MU remodelling enables the rescue of denervated muscle fibres via axonal sprouting and reinnervation, helping to reduce fibre loss and atrophy. A number of findings suggests older master athletes are more successful at this remodelling process, displaying  larger MU potentials (MUP) assessed via muscle electrophysiology [2], and fewer histochemical markers of denervation [3] when compared to less active controls. Furthermore, data from healthy young has demonstrated that MUs are not randomly located within a muscle, with those located superficially larger than those located deeper within the muscle [4]. The aims of this study were to determine the effects of ageing and exercise on the heterogeneity of MU properties across deep and superficial aspects of the vastus lateralis (VL). Intramuscular electromyography (iEMG) was used to sample individual MUPs from the VL of 85 males consisting of 15 young controls (Y, 26±5yrs), 19 young athletes (YA, 27±4yrs), 22 old controls (O, 70±4yrs), and 29 competitive master athletes (MA, 70±5yrs, sprint and endurance). All MUPs were recorded during a sustained isometric contraction held at 25% of the participants maximum voluntary contraction. A total of 1414 MUs were isolated, with a mean of 17 ± 7 per person from which multiple MU properties were assessed. MUP amplitude was measured from the maximal positive and negative peaks and is indicative of MU size. The number of turns from the template MUPs indicates the level of firing synchronicity of individual muscle fibres within a single MU. The near fibre MUP (NF MUP) shows the contributions from fibres closest to the recording electrode as a measure of fibre density. Multilevel mixed effects linear regression models were performed on each group to account for within-subject variability and to determine effects of depth on these parameters. Significance was assumed when p<0.05. Motor unit potential amplitude was larger in deep MUPs from Y, YA and O (all p<0.05), when compared to superficial, but did not differ across depth in MA (p=0.182). Near fibre (NF) MUP area was greater in deep MUPs from Y, YA and O (all p<0.01), but similar to MUP amplitude, it did not differ in MA across depth (p=0.067). MU complexity, represented by the number of turns in the MUP template, was also greater in deep MUPs compared to superficial MUPs from Y, YA and O (all p<0.01), with no difference across depth in the MA (p=0.716). These data suggest MUs of the VL are not randomly located within the VL and exhibit a level of plasticity that is influenced by both age and activity levels. Importantly, the finding of a more homogenous distribution of MU characteristics across muscle depth adds further evidence to a greater level of age-related MU remodelling in life-long exercisers.