Forty percent of maximum capacity has been suggested to approximate the upper loading limit before fatigue of the diaphragm occurs (20). POWERbreath® is a pressure-threshold device that requires continuous application of inspiratory pressure throughout inspiration in order for the inspiratory regulating valve to remain open. As with the maximal
inspiratory pressures, subjects were instructed to initiate every breath from RV. They continued the inspiratory
effort up to the lung volume where the inspiratory capacity for the given resistance limited further
excursion of the thorax. Powerful execution of the maneuvers was encouraged to ensure maximal voluntary
output for the given loading conditions. Because of the increased tidal volume, a decreased but spontaneous
breathing frequency was adopted by the subjects in order to avoid hyperventilation. This breathing pattern resulted
in a very low duty cycle (inspiratory time/total breath duration) and further ensured that fatigue was avoided.
The respiratory warm up was performed before the RWU.
This protocol has been shown to enhance the strength of the inspiratory muscles (26).