Journal of Human Ergology

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In climatic chamber experiments some physiological responses of a young healthy man were studied. He was able to perform a pursuitmeter task in a fairly wide range of warm climates without decrement of performance. The highly trained and heat-acclimatized subject was exposed nearly undressed to various ambient temperatures (28-55°C) and air humidities (10-92 %) which were combined in such a way that the conditions were tolerable up to 4 h. In 18 experiments the man was sitting quietly all the time, and in a parallel series of 19 experiments (up to 50°C) he performed the pursuitmeter task during the last 2 hours of each exposure. Under equal climatic conditions work caused a slight increase in oxygen uptake corresponding to an increase in metabolic heat generation by about 70 kJ/h (17 kcal/h) on the average. Rectal temperature and temperature next to the eardrum were 0.1-0.2°C higher at work as compared with rest. Weight loss at work exceeded weight loss at rest, the difference cor-responding to an increase in evaporative heat loss by about 110 kJ/h (26 kcal/h) which resulted in a decrease of mean skin temperature of 0.5 to 0.9°C. Superelevation of heart rate due to work reached about 10 beats/ min in a thermally eutral environment, but up to 30 beats/min in hot dry and warm humid surrounding, reflecting the additional effort for maintaining performance under conditions of increasing heat stress.

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Thirteen male subjects lifted boxes of three different weights for 20 min periods each. Ventilation, oxygen uptake and heart rates were measured. Each subject made subjective estimates of the degree of difficulty of the work he was doing. Oxygen uptake and ventilation were found to reach steady state after about four minutes and remain there for the rest of the work session. Heart rate for the two lighter boxes reached steady state after about thirteen minutes while the heart rate continued to rise when lifting was done with the heaviest box. The relationships between oxygen uptake, heart rate and work load tended to be curvilinear, whereas the ventilation was linear when compared to work load. The heart rate and oxygen uptake relationship was also curvilinear, while ventilation and oxygen uptake had a linear relationship. The subjective ratings were expected to be about one-tenth of the heart rate. Only the ratings made while lifting the lightest box reflected this. The heavier boxes were rated as being more difficult to lift than 10 times the heart rate per minute. The mechanical efficiencies were found to range between 4.4 % and 6.2 %.

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