Individuals with Duchenne muscular dystrophy (DMD) often exhibit delayed motor 
and cognitive development, including delayed onset of ambulation. Data on age 
when loss of independent ambulation occurs are well established for DMD; 
however, age at onset of walking has not been well described. We hypothesize 
that an effective medication given in early infancy would advance the age when 
walking is achieved so that it is closer to age-matched norms, and that this 
discrete event could serve as the primary outcome measure in a clinical trial. 
This study examined three data sets, Muscular Dystrophy Surveillance, Tracking, 
and Research Network (MD STARnet); Dutch Natural History Survey (DNHS); and 
Parent Project Muscular Dystrophy (PPMD). The distribution of onset of 
ambulation in DMD (mean ± SD) and median age, in months, at the onset of 
ambulation was 17.3 (±5.5) and 16.0 in MD STARnet, 21.8 (±7.1) and 20.0 in DNHS, 
and 16.1 (±4.4) and 15 in PPMD. Age of ambulation in these data sets were all 
significantly later (P <0.001) than the corresponding age for typically 
developing boys, 12.1 (±1.8). A hypothetical clinical trial study design and 
power analyses are presented based on these data.