Ventilatory Responses to Prolonged Exercise with Heavy Load Carriage

Abstract

The purpose of this experiment was to study breathing pattern, operational lung volume and respiratory muscle strength during 45 minutes of exercise with a heavy backpack (25 kg). Fifteen males completed randomly ordered graded exercise tests on a treadmill with (L) and without (U) a correctly sized and fitted 80 L pack weighing 25 kg. Subsequently, each subject completed two exercise challenges (L and U conditions, in random order) that consisted of 45 minutes of treadmill walking at 67 ± 4% VO2peak. Maximal inspiratory and expiratory pressures (MIP and MEP) were measured before and immediately following exercise. During exercise, ventilatory and gas-exchange data were recorded every five minutes. Perceptual responses were recorded in the first five-minute measurement cycle and were repeated every ten minutes during exercise. Between-condition comparisons were made during exercise every five minutes, while within- condition comparisons were made between the 15 and 45 min time points only. During loaded exercise, breathing frequency (BF) and ventilation (VE) increased by 21.7 and 15.1% (P<0.05), respectively, while tidal volume (VT) and end-inspiratory lung volume (EILV) were reduced by 6.3 and 6.4% (P<0.05), respectively. Following exercise in the loaded condition, maximal inspiratory pressure decreased by 6.7% (P<0.05) with no change in maximal expiratory pressure. No changes in maximal inspiratory or expiratory pressures were observed following exercise in the unloaded condition. Although aerobic demand was matched between conditions, exercise stress, leg fatigue and breathing stress were always perceived to be higher (P<0.05) in the loaded condition. In summary, the mechanical disadvantage placed on the respiratory system during prolonged exercise with a heavy pack suggests that work of breathing (WOB) was increased and this resulted in a progressive alteration in ventilatory mechanics. The decrease in maximal inspiratory pressure and compensatory changes in breathing pattern and EILV is suggestive of respiratory muscle fatigue. We suggest that in an attempt to minimize the WOB, subjects adopted a shallow and frequent breathing pattern; however, this breathing pattern increased dead space and minute ventilation, increasing perceived exercise stress and breathing discomfort.