Moran et al developed the physiological strain index (PSI) to evaluate a person's response to heat strain during exertion. If accurate temperature data is not available, then heart rate (HR) data alone can be used to calculate the PSI-HR. The authors are from the US Army Research Institute of Environmental Medicine in Natick, Massachusetts, and universities in Israel.
Parameters:
(1) heart rate at baseline in beats per minute
(2) heart rate during heat stress in beats per minute
physiological strain index based on heart rate alone (PSI-HR) =
= (5 * ((heart rate under heat stress) - (heart rate at baseline)) / (180 - (heart rate at baseline)))
PSI-HR
Physiological Strain
0
none
1
minimal
2
low
3
moderate
4
high
5
very high
If the person's physical fitness is taken into account:
PSI-HR Zone
Heart Rate for Low Fitness
Heart Rate for Moderately Fit
Heart Rate for High Fitness
0 to 1.0
70 - 96
60 - 88
50 - 80
1.01 to 2.0
97 - 122
89 - 116
81 - 110
2.01 to 3.0
123 - 148
117 - 144
111 - 140
3.01 to 4.0
149 - 174
145 - 172
141 - 170
4.01 to 5.0
175 - 200
173 - 200
171 - 200
The maximum of 200 beats per minute assumes a 20 year old male.
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