Evaluation of fatigue in young female adults due to repeated exposure to heat in summer and cold in winter.

PURPOSE

The magnitude of fatigue (MF) from psychological and physiological responses during repeated exposure to heat in summer and during repeated exposure to cold in winter was evaluated to test two hypotheses on fatigue models. The first hypothesis is that exposure time (ET, min) would be a factor determining the MF and the second hypothesis is that the same fatigue models as a function of the number of exposure repetitions (NR) could be applied to both repeated exposure to heat in summer and cold in winter.

METHODS

In summer, eight young adult female subjects with clothing insulation (I, clo) of 0.3 clo first stayed in the control room at 26 ℃ for 15 min, moved to the main testing room at 30 ℃ for 25 min, 33 °C for 15 min, or 36 ℃ for 10 min, and then returned to the control room. The product of air temperature difference (ΔT, ℃) and ET was designed to be almost equal among these latter three conditions. The exposure was repeated five times. In winter, the same female subjects with I of 0.84 clo first stayed in the control room at 24 ℃ for 15 min, moved to the main testing room at 18 ℃ for 30 min, 15 ℃ for 20 min, or 12 ℃ for 15 min, and then returned to the control room. Again, the product of ΔT and ET was designed to be equal among these latter three conditions. The exposure was repeated four times. The scores of subjective fatigue feeling (SFF) and salivary amylase value (SAV) were recorded when the subjects returned to the control room. Tympanic temperature, skin temperatures and local sweat rates (S mg/cm/min) at chest, forearm, front thigh, and front shin, and ECG were continuously monitored, except for S in the winter experiment.

RESULTS

In the summer experiment, the SFF showed a threshold value at ΔT = 4 ℃ but continuously increased with NR at ΔT = 7 ℃ and 10 ℃. It was not correlated with ECG variables, but was positively correlated with SAV (R = 0.50) and the mean S (R = 0.76) at ΔT = 7 ℃ and 10 ℃. In the winter experiment, the SFF showed a threshold value at ΔT =  - 6 ℃ but continuously increased with NR at ΔT =  - 9 ℃ and - 12 ℃. It was correlated with SAV at ΔT =  - 9 ℃ (R = 0.77) and score of LF: HF ratio at ΔT =  - 6 ℃ and - 9 ℃ (R = 0.49).

CONCLUSION

It was confirmed that ET may be related to the MF and that different fatigue models may be applied dependent on ΔT during repeated exposure to heat in summer and during repeated exposure to cold in winter. Thus, the two hypotheses were verified.