This study examined the muscle power of Brazilian circumpubertal girls and extended the analysis to a cross-cultural dimension. A total of 462 children, 123 Brazilian girls and 339 French girls, 9–18 years, participated in this investigation. Anthropometric data included body mass (BM), height, skinfold thicknesses, and estimated lean leg volume (LLV). All subjects completed a physical activity questionnaire. Cycling peak power was measured including the flywheel inertia of the device (CPPi). Brazilian girls self-assessed their maturation using pubic hair development. CPPi and optimal velocity (vopt = velocity at CPPi) increased with stages of puberty. A multiple stepwise regression with anthropometric variables as explanatory factors showed only LLV and age explaining the variance of CPPi (R2 = 0.40, P < 0.001). Therefore, 60% of the variance of CPPi in Brazilian girls was related to undetermined qualitative individual factors, which may be related to cycling skill. Even when normalized for anthropometric variables, the anaerobic performance (CPPi and vopt) of Brazilian girls was significantly lower than a cohort of French girls. The latter demonstrated a high participation in sport and training activities, while 50% of the Brazilian girls had only physical education classes in the form of regular physical activity. Moreover, most of the Brazilian girls demonstrated an ineffective sprint cycling skill. The data suggest that motor learning is an important issue in muscle power assessment and might, therefore, partially explain peak power differences in Brazilian compared with French girls. Am. J. Hum. Biol. 14:364–371, 2002. © 2002 Wiley-Liss, Inc.