THE EFFECTS OF LEARNING ON THE VENTILATORY RESPONSES TO INSPIRATORY THRESHOLD LOADING

Progressive threshold loading (PTL) is frequently used to assess inspi ratory muscle endurance in health and disease. We and others have note d a systematic increase in endurance with the first few exposures to t he task in subjects previously naive to PTL, which may not be related to conditioning of the muscles themselves. The purpose of this study w as to investigate the mechanisms responsible for this increased endura nce by examining the ventilatory responses to 3 PTL tests, each > 24 h apart, in 18 healthy subjects. During PTL, threshold pressure (Pth) w as increased by approximately 10% every 2 min until task failure. Subj ects were allowed to adopt any breathing pattern. Respiratory muscle s trength (maximal inspiratory pressure [PImax]) was unchanged over succ essive tests while maximal Pth (Pth,,) during PTL increased (69 +/- 17 , 77 +/- 16, and 86 +/- 11% of PImax respectively, p < 0.05) (mean +/- SD), indicating that the increased Pth,,, could not be attributed to improved respiratory muscle strength. Breathing pattern changed with s uccessive tests, so that for comparative loads inspiratory time (TI), respiratory frequency (f), and duty cycle (TI/Ttot) decreased. This ch ange in breathing pattern did not alter respiratory muscle efficiency (respiratory muscle VO2/work), which was similar in each test (2.4 +/- 2.2%), but perceived effort (Borg Score), which was maximal at task f ailure in each test, decreased at comparative loads with successive te sts. Thus, Pth,,, during initial tests appeared to be limited by disco mfort rather than respiratory muscle function. These findings suggest that the increased Pth,,, with successive tests is a consequence of di fferences in the breathing pattern adopted, reflecting neuropsychologi cal rather than respiratory muscle conditioning. Measurements from PTL should only be used to assess respiratory muscle performance after al lowing time for learning.