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. 2025 Jan;13(1):e70197.
doi: 10.14814/phy2.70197.

Thoracic load carriage impairs the acute physiological response to hypoxia in healthy males

Daniel A Baur  1 , Caroline M Lassalle  1 , Stephanie P Kurti  2
Affiliations

Thoracic load carriage impairs the acute physiological response to hypoxia in healthy males

Daniel A Baur et al. Physiol Rep. 2025 Jan.

Abstract

To assess the impact of thoracic load carriage on the physiological response to exercise in hypoxia. Healthy males (n = 12) completed 3 trials consisting of 45 min walking in the following conditions: (1) unloaded normoxia (UN; FIO2:20.93%); (2) unloaded hypoxia (UH; FIO2:~13.0%); and (3) loaded hypoxia (LH; 29.5 kg; FIO2:~13.0%). Intensity was matched for absolute VO2 (2.0 ± 0.2 L·min-1) across conditions and relative VO2 (64.0 ± 2.6 %VO2max) across hypoxic conditions. With LH versus UH, there were increases in breathing frequency (5-11 breaths·min-1; p < 0.05) and decreases in tidal volume (10%-18%; p < 0.05) throughout exercise due to reductions in end inspiratory lung volumes (p < 0.05). Consequently, deadspace (11%-23%; p < 0.05) and minute ventilation (7%-11%; p < 0.05) were increased starting at 20 and 30 min, respectively. In addition, LH increased perceived exertion/dyspnea and induced inspiratory (~12%; p < 0.05 vs. UN) and expiratory (~10%; p < 0.05 vs. pre-exercise) respiratory muscle fatigue. Expiratory flow limitation was present in 50% of subjects during LH. Cardiac output and muscle oxygenation were maintained during LH despite reduced stroke volume (6%-8%; p < 0.05). Finally, cerebral oxygenated/total hemoglobin were elevated in the LH condition versus UH starting at 15 min (p < 0.05). Thoracic load carriage increases physiological strain and interferes with the compensatory response to hypoxic exposure.

Keywords: altitude; expiratory flow limitation; hemodynamics; near‐infrared spectroscopy; respiratory muscle fatigue; ventilation.

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Conflict of interest statement

The authors report no conflicts of interest.

Figures

FIGURE 1
FIGURE 1
Experimental design and protocol. MEP, maximal expiratory pressure; MIP, maximal inspiratory pressure; NIRs, near‐infrared spectroscopy.
FIGURE 2
FIGURE 2
(a) Minute ventilation; (b) tidal volume; (c) breathing frequency; and (d) deadspace ventilation during exercise. Data are presented as mean ± SE. LH, loaded hypoxic; UH, unloaded hypoxic; UN, unloaded normoxic. a,b,cDenotes different from prior time point within UN, UH, and LH, respectively (p < 0.05).
FIGURE 3
FIGURE 3
Operating lung volumes during exercise. Data are presented as mean ± SE. EELV, end expiratory lung volume; EILV, end inspiratory lung volume. a,b,cDenotes different from prior time point within UN, UH, and LH, respectively (p < 0.05).
FIGURE 4
FIGURE 4
(a) Cardiac output; (b) heart rate; (c) stroke volume during exercise. Data are presented as mean ± SE. LH. Loaded hypoxic; UH, unloaded hypoxic; UN, unloaded normoxic. a,b,cDenotes different from prior time point within UN, UH, and LH, respectively (p < 0.05).
FIGURE 5
FIGURE 5
(a) Muscle deoxygenated hemoglobin; (b) muscle oxygenated hemoglobin; (c) muscle total hemoglobin. Data are presented as mean ± SE. LH, loaded hypoxic; UH, unloaded hypoxic; UN, unloaded normoxic. a,b,cDenotes different from prior time point within UN, UH, and LH, respectively (p < 0.05).
FIGURE 6
FIGURE 6
(a) Cerebral deoxygenated hemoglobin; (b) cerebral oxygenated hemoglobin; (c) cerebral total hemoglobin. Data are presented as mean ± SE. LH. loaded hypoxic; UH, unloaded hypoxic; UN, unloaded normoxic. a,b,cDenotes different from prior time point within UN, UH, and LH, respectively (p < 0.05).
FIGURE 7
FIGURE 7
The relationship between dyspnea and minute ventilation. Data are presented as mean ± SE. LH, loaded hypoxic; UH, unloaded hypoxic; UN, unloaded normoxic; VE, minute ventilation.
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