Ambulant young people with cerebral palsy (CP) have reduced muscle volumes in their lower limbs (as low as 50% of their weight-matched typically-developing peers). Yet, they may complete a 'timed up-and-go' test at similar speeds to unaffected persons. Perhaps, these individuals are able to maintain high levels of function because their muscle deficits have not fallen below the threshold values required to perform certain motor tasks. This is consistent with data from studies of progressive strengthening in children with mild CP. These programmes improve muscular output but have limited immediate effect on functional capacity. Sarcopenia is responsible for much of the loss of muscle mass in the typically developing adult. The decline in muscle mass begins in the mid-twenties and occurs rapidly after the 7th decade. It is possible that the muscle deficits characteristic of young people with CP, coupled with the decline of muscle properties in adulthood, contribute to an early loss of mobility in this group. In the typically developing elderly, progressive strengthening is thought to extend mobility. Perhaps, the real value of strengthening programmes in CP is to improve muscular reserve in the short-term and to maintain muscle mass above critical thresholds in the long-term.