Knowledge of the fate and behaviour of the respiratory muscles during ventilatory support is important for the guidance of clinical care. Full support facilitates muscle metabolic repletion, but exposes them to the risk of disuse atrophy. The effect of partial support varies according to the selected mode: assisted mechanical ventilation (AMV) and synchronized intermittent mechanical ventilation (SIMV) result in much less respiratory muscle rest than generally anticipated. On the other hand, inspiratory pressure support (IPS) is able to rest the respiratory muscles and to prevent fatiguing contractions. Opposite interventions have been proposed in case of difficult weaning: either to unload the respiratory muscles by using partial support, or to overload them according to a training programme. The optimal strategy is not known and may combine both approaches.