Progression of spontaneous breathing trials administered during inspiratory muscle strength training study interventions. Progression of spontaneous breathing trials administered during inspiratory muscle strength training study interventions. Subjects who could not initially tolerate at least 1 hour of spontaneous, unassisted breathing (right) underwent trials of reduced pressure support or CPAP (left). Barbara Kellerman Smith et al. Respir Care 2014;59:22-31 (c) 2012 by Daedalus Enterprises, Inc.