Task-specific physical fitness tests were previously designed for the Queensland Mines Rescue Brigades. Although physical fitness has been shown to be an important factor in heat tolerance, these tests were not designed to evaluate ability to tolerate work in the heat. Other factors that may affect the response to work in the heat are age, body surface area to mass ratio, cardiovascular adjustments, ability to sweat and acclimatisation.
The purpose of this project was to assess the means of determining heat tolerance in Brigade members and to develop, if possible, a short, easily administered test of heat tolerance which could be administered at rescue stations.
Three tests were devised for the purposes of this study: a 2 hour criterion test at 36°C and 100% humidity for the assessment of heat tolerance during simulated rescue tasks, a 15 min predictive test at 40°C and 100% humidity with similar tasks and a physical work capacity test involving graded treadmill walking to voluntary fatigue.
The main study was conducted on 37 volunteers from the Central Queensland Mines Rescue Brigade at Blackwater during a six-week period in the months of September and October. All testing was conducted in the hot room and a temporary laboratory set up at the Blackwater Mines Rescue Station.
Each subject reported on three occasions, generally on alternate weekdays so as to ensure complete recovery after each test. On the first day, the participants were familiarised with the procedures, a 15 min predictive test was carried out and, after about one hour’s rest, a physical work capacity test was performed. On Day 2, the 15 min predictive test was repeated, a set of anthropometric measures was taken, and then a physical work capacity test was again perfomed on the treadmill. On Day 3, the subjects attempted a 2 hour test of heat tolerance wearing standard breathing apparatus and rescue clothing. Physiological responses were monitored every half hour.
Of the 36 subjects who attempted the heat tolerance test, 27 completed the full 120 min and 9 stopped before the end of the full period. The non-finishers were marginally older, fatter and lower in physical work capacity, but these differences were not statistically significant. Core temperatures, heart rates, sweat rates and subjective reports indicated that those who completed the test were severely stressed. Percentile norms were developed for the indices of physiological strain. The non-finishers were less physiologically strained and appeared to stop for in motivational or other reasons. It was, therefore, difficult to predict the non-finishers from the 15 min predictive test, although their rectal temperatures were significantly higher than those of the finishers after that test.
Physiological strain in response to heat stress is most commonly determined from the core (usually rectal) temperature or some combination of core temperature, heart rate and sweat rate. Its mmeasurement was complicated in this study by the fact that these three measures were almost totally uncorrelated with each other (r • -.06 to .15). A Physiological Strain Index which combines all three was not able to improve the prediction of heat tolerance compared with rectal temperature alone. Oral temperature was not closely related to rectal temperature throughout the 120 min test in many subjects and their terminal values were not well correlated (r - .37), indicating that oral temperature cannot satisfactorily be used to estimate rectal temperature in these conditions. Oral temperature, on the other hand, may be more sensitive to the breathing load as the subject breathes through his oxygen supply unit.
The 15 min predictive test provided a substantial heat stress, producing a mean loss of body mass (sweat loss) of 0.48 kg, a rectal temperature rise of 0.74°C and a terminal heart rate of 145 b/min. There were no significant differences between the mean values of the measures on the two tests, but test-retestcorrelations were only moderate (r • .43 - .69), indicating that some familiarity with the procedures is desirable for reliable results to be obtained. Measures from the two 15 min predictive tests were used to predict both rectal temperature and Physiological Strain Index during the 120 min heat tolerance test. No one predictor variable was closely correlated with either measure of heat tolerance (r • -.32 to .51). From a correlation matrix of 58 variables on 27 subjects, multiple regression equations were developed to allow prediction from several measures in combination. Those selected were two of the most accurate and practical equations which predicted rectal temperature from six anthropometric and blood pressure measures. Approximately 802 of the variance in rectal temperature could be explained by these variables, far above the predictive accuracy reported for tests currently used in other countries.
A detailed discussion is presented of these results and current knowledge regarding the management of heat stress so that appropriate education, testing and management practices can be instituted in the Queensland Mines Rescue Brigades.