The Physiological Strain Index Modified for Trained Heat Acclimatized Individuals in Outdoor Heat
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2019-07Author
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<jats:p><jats:italic><jats:bold>Purpose</jats:bold>:</jats:italic>To determine if the Physiological Strain Index (PSI), in original or modified form, can evaluate heat strain on a 0–10 scale, in trained and heat-acclimatized men undertaking a competitive half-marathon run in outdoor heat.<jats:italic><jats:bold>Methods</jats:bold>:</jats:italic>Core (intestinal) temperature (TC) and heart rate (HR) were recorded continuously in 24 men (mean [SD] age = 26 [3] y, VO<jats:sub>2</jats:sub>peak = 59 [5] mL·kg·min<jats:sup>−1</jats:sup>). A total of 4 versions of the PSI were computed: original PSI with upper constraints of TC 39.5°C and HR 180 beats·min<jats:sup>−1</jats:sup>(PSI<jats:sub>39.5/180</jats:sub>) and 3 modified versions of PSI with each having an age-predicted maximal HR constraint and graded TC constraints of 40.0°C (PSI<jats:sub>40.0/PHRmax</jats:sub>), 40.5°C (PSI<jats:sub>40.5/PHRmax</jats:sub>), and 41.0°C (PSI<jats:sub>41.0/PHRmax</jats:sub>).<jats:italic><jats:bold>Results</jats:bold>:</jats:italic>In a warm (26.1–27.3°C) and humid (79–82%) environment, all runners finished the race asymptomatic in 107 (10) (91–137) min. Peak TC and HR were 39.7°C (0.5°C) (38.5–40.7°C) and 186 (6) (175–196) beats·min<jats:sup>−1</jats:sup>, respectively. In total, 63% exceeded TC 39.5°C, 71% exceeded HR 180 beats·min<jats:sup>−1</jats:sup>, and 50% exceeded both of the original PSI upper TC and HR constraints. The computed heat strain was significantly greater with PSI<jats:sub>39.5/180</jats:sub>than all other methods (<jats:italic>P</jats:italic> < .003). PSI >10 was observed in 63% of runners with PSI<jats:sub>39.5/180</jats:sub>, 25% for PSI<jats:sub>40.0/PHRmax</jats:sub>, 8% for PSI<jats:sub>40.5/PHRmax</jats:sub>, and 0% for PSI<jats:sub>41.0/PHRmax</jats:sub>.<jats:italic><jats:bold>Conclusions</jats:bold>:</jats:italic>The PSI was able to quantify heat strain on a 0–10 scale in trained and heat-acclimatized men undertaking a half-marathon race in outdoor heat, but only when the upper TC and HR constraints were modified to 41.0°C and age-predicted maximal HR, respectively.</jats:p>
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