Heart rate variability (HRV) changes during the fatigue progression of manual material handling in acute hypobaric hypoxia environments

To support National Aeronautics and Space Administration's (NASA) lunar habitat plans utilizing hypobaric hypoxia (HH) environments, this study investigated the combined effects of acute hypoxia severity and physical fatigue on astronauts' autonomic nervous system and cardiac responses during manual tasks. Ten subjects conducted repeated 25-kg box manual material handling (MMH) tasks to fatigue across normobaric normoxia (NN, 0 m), moderate (MH, 3500 m), and severe hypobaric hypoxia (SH, 4500 m), while electrocardiogram-derived heart rate variability (HRV) indices were analyzed. Increasing severity of hypoxia evoked a significant decline in parasympathetic markers (root mean square of successive interbeat intervals differences (lnRMSSD, p = 0.032), Poincaré plot standard deviation perpendicular the line of identity (SD1, p = 0.032), natural logarithm of high frequency power (lnHF, p = 0.018)), coupled with a significant increase in heart rate and sympathovagal balance indices (SD2 (Poincaré plot standard deviation along the line of identity)/SD1, p = 0.008; lnLF (natural logarithm of low frequency power)/lnHF, p = 0.001). The number of MMH repetitions at the onset of moderate fatigue under SH was significantly lower than that under NN (p = 0.039). In the SH condition, both the time for each MMH set before (p = 0.036) and following (p = 0.018) the onset of moderate fatigue were significantly longer than those observed under the NN condition. At the moderate fatigue phase, the SD2/SD1 was significantly higher in SH than that in NN(p = 0.01) and SH(p = 0.01). The remaining HRV indices only demonstrated a main effect of time. The findings offer valuable insights into the design of hypoxic environments in extraterrestrial habitats and into the monitoring and safeguarding of astronaut fatigue and cardiovascular safety.