Sex Difference in Oxygen Intake and Heart Rate during Assuming Ten Basic Postures under Different Air Temperature Conditions

Abstract

Twenty four young female and male subjects, twelve in each, all in good physical conditions, volunteered for this study. Examinations were made during the following ten basic postures: lying awake, sitting with crossed legs on the floor, sitting on a chair, squatting with hands on knees, bending with trunk forward from standing posture, standing at ease, standing with attention, half rising posture with knees nearly extended, half rising posture with knees bending at 120° and 90°. The measurements were performed at the air temperature conditions of 20°C, 30°C and 40°C in a climatic chamber. The relative humidity and air velocity were maintained at 50% and 18 cm/sec, respectively, throughout all the air temperature conditions.In all the temperature conditions the lying posture gave the smallest oxygen intake and the lowest heart rate and the half rising posture with knees deeply bent the largest values for both parameters. The smaller intraclass correlation coefficients for heart rate in both sexes and three temperature conditions and the smaller coeficients in females for both items indecate obviously the relatively larger variations within each posture for heart rate than for oxygen intake and the relatively larger variations in females than in males.The pooled variance for 60 groups classified by the posture, air temperature, and sex was devided into the variances for the factor of posture, that of air temperature in the same posture, that of sex in the same temperature in the same posture, and that of subject in the same sex in the same temperature in the same posture. The result shows that the differences in the means of both parameters between postures were statistically significant and the sex difference in both parameters was highly significant in the same temperature in the same posture. The temperature effect was insignificant for oxygen intake, though it was significant for heart rate.The analysis of covariance indicates that sex difference in the regression coefficients was statistically significant only in the case of 20°C. The rank correlation between the adjusted heart rate by the differences in oxygen intake and projected height of each posture was confirmed to be highly significant in all the temperature conditions. A certain increment of skin blood flow, the hemodynamics effect associated with redistribution of blood, and the static property of keeping a posture are suggested to be responsible for the above tendencies. Although the sex difference in the regression coefficients disappeared in 30°C and 40°C, physiological strain for keeping a standing posture including half rising one was estimated to be remarkably severe in females.