Effects of respiratory equipment on endurance in hard work.

Men ran to a voluntary end point under the following conditions listed in increasing order to resistance, expressed as inspiratory/expiratory in mmH2O at a flow of 4 l-s-1: 1) bare-headed 0/0; 2) face mask 0/0; 3) face mask -6/14; 4) mouthpiece and valves -25/29; 5) face mask -96/62; and 6) face mask -148/60. Average endurance was 545, 490, 434, 392, 343, and 338 s, respectively. Despite the diverse features of the equipment in conditions 2-6, there seemed to be a rather smooth relation between resistance and endurance. There was no indication of a threshold below which endurance would be indifferent to resistance. The duration of the phases of the respiratory cycle was measured by means of a fine thermocouple in the airway. The variability in the duration of the expiratory phase at the end of the run seemed to weaken the concept that the attainment of a critically short expiratory time might be related to the subject's decision to halt running.