This factorial randomised controlled trial will evaluate the effect of increasing amino acid intake (by 1 g/kg/d) and energy intake (by 20 kcal/kg/d) during parenteral nutrition and also of increasing protein intake and energy intake by an extra 1 g/kg/d during enteral nutrition, with and without the stimulation of the infant's physical activity, on growth of extremely low birth weight infants. The investigators aim at demonstrating that increasing energy and protein intake above the standard of care intakes will result in better growth only in association with adequate physical activity, in particular in relation to body composition and lean mass accretion.