K.A. Pribanic1, M.C. Fogarty2, D.P. Heil, FACSM3

1Montana State University/Montana Overland Research Foundation, Bozeman, MT, 2University of Hull, Kingston-upon-Hull, United Kingdom, 3Montana State University, Bozeman, MT

Numerous indices relate heat stress to changes in physical performance, but many are too complex to be practical for field use, or in cases where clothing creates a microclimate distinct from ambient conditions. The Physiological Strain Index (PSI), however, requires only rectal temperature (Tre) and heart rate (HR) data, and has been shown to be accurate in predicting composite heat stress in a variety of conditions. This case study - in which a 37-year old male professional firefighter in the UK ran the 2015 Humber Bridge Half Marathon while wearing his full firefighting kit - allowed us to collect data needed to examine the relationship between PSI (as a measure of overall physiological stress) and changes in half-marathon running characteristics. PURPOSE: To describe the relationship between PSI and changes in half-marathon running characteristics in full firefighting kit. METHODS: A GPS-enabled heart rate monitor (HRM) and a rectal temperature probe were fitted to the subject, who wore his full kit of non-breathable turnout coat and pants, leather boots, helmet, and breathing apparatus (full-face mask, hoses, air tank). The subject planned to walk (not run) up hills and change air cylinders at 2.4 km intervals, or as needed. Heart rate (HR), speed, and distance were recorded every two seconds. Tre was recorded when air cylinders were changed. Post hoc calculation of Tre progression and PSI were then descriptively compared to changes in running characteristics: Mean Speed (MSp), Run Time/Walk Time Ratio (RWR), and Run/Walk Oscillation Rate (OR). RESULTS: Despite the generally cool ambient conditions (17.1 °C, 80% humidity), the subject’s Mean PSI climbed steadily from 3.6 to 9.5 (on a scale of 0-10) over the course of the event. Peak MSp of 7.4 kph was reached near the 9 km mark, at which time the calculated PSI was 8.7, while RWR was 5.9, and OR remained at 2 cycles per segment (cps, with a segment defined as a section of the course covered between cylinder changes or rest breaks). From this point on, MSp and RWR declined steadily while OR increased from 2 cps to 10. These waves of rising speed and PSI - followed by dropping speed and PSI - became more prominent, especially as PSI neared 10.0, indicating that a physiological limit had been reached that required alteration of running characteristics in order to maintain forward progress. CONCLUSION: PSI was shown be closely related to observed changes in running characteristics. Given its ease of use, and its applicability in varied environments, further investigation into the use of PSI as a predictor of impending performance change is warranted, especially where the maintenance of set performance characteristics is crucial.

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