An evaluation of the physiological strain index during a prolonged submaximal exercise with sleep deprivation

INTRODUCTION: The physiological strain index (PSI) is a prevention tool to detect incidences of heat-related illnesses at an individual level. The PSI is based on core temperature (Tc) and heart rate (HR) to assess the strain on the cardiovascular and thermoregulatory systems on a scale of 0 – 10 (Moran et al., 1998). Fatigue is known to induce physiological and psychological perturbations, hence, impairs (un)conscious behaviors for thermoregulation (Westwood et al., 2021). During prolonged exercise and sleep deprivation, fatigue can weaken the ability to maintain thermal homeostasis, and therefore, acts as risk factor for heat-related illnesses. This study aimed to evaluate whether the PSI reflects increasing and cumulative fatigue during prolonged submaximal exercise with sleep deprivation. METHODS: Twenty one soldiers (1 female; 21.0 ± 1.1 years; 180.6 ± 8.5cm; 78.9 ± 11.9kg) of the Swiss Armed Forces performed a submaximal 100km march (total duration 24h11 – 25h38). HR and Tc were measured using the chest belt Open Body Area Network (USARIEM & MIT, USA) in 1-min epochs. At five time points (at km 0, 30, 55, 72, and 93) four fatigue indicators were assessed: rate of perceived physical and mental exertion (RPEp; RPEm) using two CR100 scales, physical fatigue (PF) measuring the absolute acceleration during a counter movement jump and mental fatigue (MF) using a questionnaire. The PSI was calculated as PSI = 5•[(Tc(t)–Tc(0))•(39.5–Tc(0))-1]+5•[(HR(t)–HR(0))•(180–HR(0))-1] and compared to the fatigue indicators at the five time points; Tc(t1-5) and HR(t1-5) were calculated as average Tc and HR values of one hour prior every time point, and Tc(0) and HR(0) as average Tc and HR values of the first hour after the start. RESULTS: PSI ranged from 2.56 – 3.09 with a peak of 3.58 at km 55 and the repeated measures ANOVA revealed no significant differences compared to km 0 (p > 0.05). However, all fatigue indicators differed significantly from almost each time point to the other (p < 0.05). The PSI reported a weak to medium correlation with MF and RPEp (r = 0.24 / 0.34, both p < 0.05), however no significant correlation with PF and RPEm (r = 0.10 / 0.18, both p > 0.05). CONCLUSION: The four fatigue indicators showed that physical and mental fatigue had increased significantly during the march. In contrast, the PSI remained steady at a low level of physiological strain and showed no association with fatigue. When shorter or longer breaks are taken, PSI may lack to identify cumulative fatigue. Whether the PSI would miss a heat- or exertion-related occurrence cannot be conclusively assessed, as no subject experienced such conditions. REFERENCES: 1. Moran, D. S., Shitzer, A., & Pandolf, K. B. (1998). A physiological strain index to evaluate heat stress. Am J Physiol, 275(1), R129-134. 2. Westwood, C. S., Fallowfield, J. L., Delves, S. K., Nunns, M., Ogden, H. B., & Layden, J. D. (2021). Individual risk factors associated with exertional heat illness: A systematic review. Exp Physiol, 106(1), 191-199.