Increasing protein and energy intake above the anabolic capacity of a given individual may result in increased lipogenesis and excessive fat deposition. Adults with reduced mobility or with neuromuscular conditions will develop excessive fat deposition if they receive normal to high protein and energy intakes. Excessive fat deposition have been demonstrated in preterm infants receiving high energy intakes. Given that physical activity in preterms is often reduced for the prematurity itself, the associated sickness and the numerous medications, the investigators speculate that physical stimulation may have a beneficial effect on protein accretion and on lean mass accretion. The investigators further hypothesize that today's recommended daily intakes of proteins and energy cannot be fully incorporated into lean body mass without a concomitant physical activity. In spite of the fact that this notion is well accepted in adult physiology and in the elderly, it has never been tested in preterm infants.