The primary findings of this study were as follows. First, most of the skin temperatures were reduced after exercise in both treadmill and crunch exercise. Second, the skin temperature of the pectoral, umbilical, and infrascapular regions were more significantly reduced than the other ROIs whether it was a whole-body exercise or a selective abdominal muscle exercise. These findings indicate that these regions may play an important role in thermally controlling of the body during exercise.
The pattern of skin temperature following exercise in this study can be caused by redistribution of peripheral blood flow to the active muscles during exercise, as suggested in previous studies [2–4]. From this point of view, the skin temperature in the abdominal region after crunch exercise should not be reduced more than other regions because the main activated muscles of crunch exercise located in that region. However, the pattern of skin temperature in crunch exercise were similar to those of the treadmill exercise which was whole-body exercise; the pectoral, umbilical, and infrascapular regions reduced more significantly than in the other ROIs. This contradictory finding is consistent with hypothesis of this study that there would be a specific regions that act as a thermal controller of the body. Therefore, the authors deduced that the unique pattern of skin temperature in the pectoral, umbilical, and infrascapular regions are sufficient to consider these regions as a thermal controller of the body.
Two mechanisms are possible to explain the unique patterns of skin temperature following exercise in the pectoral, umbilical, and infrascapular regions. As a thermoregulation mechanism of the body, the evaporation effect of sweat can more greatly affect these regions [21, 22]. However, a morphometric study on the distribution of hair and sweat glands on the body showed that there was no evidence of an abundance of sweat glands in the pectoral, umbilical, and infrascapular regions [23]. In addition, a study that mapped the sweating patterns of the body revealed that there was no correlation between local skin temperature and regional sweat rate [24]. Furthermore, in our study, sweat was wiped off throughout the exercises to minimize the effect of evaporation. Therefore, the authors believe that the evaporation effect of sweat did not affect the skin temperature change in these regions.
The other possible mechanism is the cutaneous vasoconstriction and vasodilation of the body. In the beginning of exercise, the increasing sympathetic adrenergic activity cause a cutaneous vasoconstriction which reduce blood flow on the skin [25, 26]. As exercise continues, the initial cutaneous vasoconstriction is attenuated and vasodilation begins; the threshold for the shift to the vasodilation is depend on the degree of internal temperature [27]. Because these cutaneous vascular responses can happen independently according to the body regions [28], it can lead a significant reduction of the skin temperature in the pectoral, umbilical, and infrascapular regions than other regions. Although the reason for the unique patterns of skin temperature in these regions is not clear yet, the different skin temperature changes might be activated to protect internal organs from thermal damage or to increase blood flow to the active skeletal muscle for exercise as Seto et al. [29] deduced. However, to confirm this hypothesis, a further study that quantifying the cutaneous vascular flow in these regions through exercise is warranted.
Several studies with IRT were found that skin temperature decreased after various types of exercise [7]. Most of these studies were performed using exercises involving some parts of the body, and only a few studies measured skin temperature changes through whole-body exercise. Fernandes et al. [30] and Merla et al. [12] measured skin temperature during treadmill exercise and found that the skin temperatures in ROIs of the body decreased as the subjects started the exercise and increased during recovery from the exercise. They also found a significant reduction of skin temperature in most ROIs except the lower limbs and the skin temperatures in the forearms and thighs exhibited the earliest response to exercise. However, the findings of their study were insufficient to understand response of skin temperature to exercise because the comparison between whole-body exercise and selective muscle exercise was not performed. Therefore, the results of this study will play an important role in supplementing this point.
This study has a limitation that all participants in study were majoring in professional physical education which tend to have higher physical status than the ordinary people. Therefore, it is questionable whether the results of the study can be generalized. However, the previous studies that evaluated the thermal behavior of the subjects with varying physical levels indicated that there was no difference depending on the physical level of the subjects [31, 32]. In addition, Formenti et al. [11] found that lesser-trained subjects release more heat than highly-trained subjects during exercise and maintain the heat for a longer period of time during the recovery process, but the pattern of skin temperature were not differ according to levels of training. Therefore, the authors thought that the training level of the participants in this study would not biased to the results of this study.