The age-related loss of muscle mass and function may be explained by the combination of individual muscle fibre atrophy and fibre loss. Although the relative contribution of each remains largely unknown, the loss of fibres is associated with the loss of the innervating motor neurons, and is in part compensated for by an expansion of surviving motor units. This talk will address in vivo methods recently applied in the exploration of age-related loss and consequential remodelling of human motor units, and present data supporting the corresponding adaptation of neural input as a defining factor in neuromuscular decrements.