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RESPIRATORY MUSCLE FATIGUE AND REDUCED OPERATING LUNG VOLUMES WITH PROLONGED LOAD CARRIAGE IN HYPOXIA

Abstract

BACKGROUND: Prior research indicates that thoracic load carriage induces shallower breathing patterns and respiratory muscle fatigue in normoxic conditions. The purpose of this study was to determine whether load carriage compromises the compensatory ventilatory responses to hypoxic exposure. METHODS: Pulmonary function (i.e., forced vital capacity [FVC], forced expiratory volume in 1 sec [FEV1], and peak flow [PEF]), respiratory muscle fatigue (i.e., maximal inspiratory [MIP] and expiratory [MEP] pressure), dyspnea, and ventilatory responses (i.e., operating lung volumes and ventilation [VE]) to prolonged exercise were assessed in healthy male subjects (n=12) under 3 conditions: unloaded normoxic (FiO2=20.93%; UN), unloaded hypoxic (equivalent to ~3,650m; FiO2=~13.0%; UH), and loaded hypoxic (~30 kg pack; FiO2=~13.0%; LH). Exercise consisted of 45 min uphill (8%) walking at speeds individually customized to elicit an equivalent relative intensity in hypoxia (64.0±2.6%VO2max) and absolute VO2 across conditions (2.0±0.2 L/min). One-way ANOVAs and t-tests were employed to analyze pulmonary function and respiratory muscle fatigue (pre- vs. post-exercise), respectively. Exercise data were analyzed via two-way repeated measures ANOVA with post hoc one-way ANOVA tests to identify differences in the case of significant interactions (α=0.05). RESULTS: FVC (-7.8-8.5%), FEV1 (-9.0-10.2%), and PEF (-8.3-11.0%) were reduced with LH relative to the unloaded conditions (p<0.05). Additionally, MIP (-12.2±9.7%) and MEP (-10.2±11.2%) were reduced following exercise with LH (p<0.05) but were unchanged with UN and UH. In both hypoxic conditions relative to normoxia, VE and dyspnea were increased throughout exercise (p < 0.05). However, with LH vs. UH, VE and dyspnea were further increased starting at 30 min (p < 0.05), which coincided with increases in deadspace VE (i.e., starting at 20 min; p < 0.05). In addition, breathing frequency and tidal volume were increased and decreased (p < 0.05), respectively, throughout exercise due to reductions in end inspiratory lung volumes (p < 0.05). CONCLUSIONS: Load carriage reduces respiratory efficiency and increases breathing discomfort during exercise in hypoxia. This may compromise health and performance during occupational tasks in high altitude environments. GRANT OR FUNDING INFORMATION: This study was funded by the Jackson-Hope New Directions in Research grant.

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