[How I investigate...respiratory muscle disorders].

As for other skeletal muscles, ventilatory muscle performance can be described in terms of strength and endurance. Ventilatory muscle strength is measured, for example, as the maximum inspiratory and expiratory mouth pressures. It is now a routine procedure in many pulmonary function laboratories. Measurements of ventilatory muscle endurance are more difficult but two general types of tests are used: maximum voluntary ventilation and inspiratory threshold loading.