The purpose of this pilot study was to determine the critical resistance necessary to add to inspiration to produce an increase in respiratory muscle (RM) strength and/or endurance in COPD patients. Prior to training we measured (1) pulmonary function, (2) RM strength as maximum inspiratory (PImax) and maximum expiratory (PEmax) pressures, and (3) RM endurance with a 2-minute incremental threshold loading test. Patients trained on the inspiratory muscle trainer for 6 weeks. The critical orifice was determined as the maximum resistance each patient could inspire against for 5 minutes. Patients were randomly assigned into 3 groups. Group 1 trained on the critical orifice. Group 2 progressed up to the critical orifice. Group 3, the control group, trained on the largest orifice. Patients were re-tested every 2 weeks during training. We found no difference in any of the above measurements between tests or between groups pre and post training (ANOVA p < 0.10). After calculating the tension time index, we discovered the majority of patients were not training on a fatiguing load. The pressure-flow characteristics of the trainer are such that as inspiratory flow rates decrease, relatively low pressures are required to breathe through even the smallest orifice. We concluded that we did not determine the critical resistance necessary to train the inspiratory muscles, probably because we did not control pressure generation and breathing pattern during training sessions.

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