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Thermal Imaging of Cutaneous Temperature Modifications in Runners During Graded Exercise
The thermo-regulatory system of the human body maintains a constant temperature against a wide range of environmental and/or physical work conditions. The control of heat transfer with the environment plays a fundamental role for body temperature regulation. At the human/environment interface (skin), heat is carried away from the body by conduction, convection, radiation and evaporation. A general energy-balance equation determines the thermal state of the body.
Intense exercise activates body compensatory vasoregulation through reduction of blood flow in the splancnic region and tegumentary apparatus. During intense physical activity, core structures and muscle mass produce heat. Peripheral vasodilatation enables massive transfer of ‘‘warmer’’ blood from the body core to the surface, where the above-cited processes regulate heat exchange. Muscular blood flow increases, while the total volume of muscular blood remains almost unvaried because of the reduced vascular compliance in this district. The blood volume within the core region and cutaneous layers decreases, thus increasing the venous return and the cardiac output.
Johnson and Kenney and Johnson found that the modification of cutaneous blood flow during exercise depends on the individual grade of vasodilatation and vasoconstriction. The cutaneous blood flow also influences the cutaneous temperature (Tc). Therefore, Tc variations depending on exercise-induced modification of cutaneous blood flow may occur as well.
Therefore, monitoring Tc provides useful information concerning thermolysis modalities and peripheral vasoregulation during exercise. Usually, Tc is estimated by averaging values of the cutaneous temperature recorded in predetermined regions of interest (ROI) through contact temperature probes. A more accurate method for estimating the overall Tc distribution and its variation during exercise would provide important information for improving athletic performance. This can be achieved by using modern touchless high-resolution thermal imaging devices, which quantitatively and precisely map the cutaneous temperature distribution and its time evolution. These digital devices are capable of precisely measuring and storing real-time thermal images. The major characteristics of these Focal Plane Array based thermal cameras are high temperature sensitivity (i.e., the capability of discriminating in ideal conditions temperature differences as low as 0.01 C), high sampling frequency (up to 100 non-interlaced images per second), and high spatial resolution (up to 1 mrad per sensor).
Figure 1. Anterior view of the upper and lower body of a representative subject taken during a duplicate run without markers.
Although extensively used for diagnostic purposes, these devices have been rarely used to investigate the variation of Tc during exercise. We investigated Tc distribution and variations associated with graded exercise in well-trained athletes using highresolution thermal imaging. Fifteen male volunteers underwent a graded treadmill test until reaching their individual maximal heart rate. Total body Tc decreased as the subjects started the exercise. Thighs and forearms exhibited the earliest response. A further Tc diminution occurred with the progress of the exercise. At the exercise interruption, Tc values were in average 3–5 C lower than at baseline. Tc increased during recovery from exercise. Forearms and thighs exhibited the earliest increase, followed by total body Tc increase. Thermal imaging documented the presence of hyperthermal spots (occasionally tree-shaped) due to the presence of muscle perforator vessels during baseline and recovery, but not during exercise.
The results we report indicate that thermal infrared imaging permits the quantitative evaluation of specific cutaneous whole body thermal adaptations which occur during and after graded physical activity. Thus providing the basis for evaluating local and systemic cutaneous blood flow adaptation as a function of specific type, intensity and duration of exercise, and helping to determine the ideal conditions (in terms of environment and apparel) in which physical activities should be conducted in order to favor thermal regulatory processes.
In conclusion, the temperature of the entire anterior cutaneous surface of the body was monitored using thermal infrared imaging during graded exercise yielding a specific time evolution of cutaneous temperature modifications. The results of this study indicate that when performing graded exercise as a warm-up, the subject should not wear heavy clothing, thus favoring vasoconstriction of the cutaneous vessels increasing the blood flow to muscles.
Arcangelo Merla, Peter A. Mattei, Luigi Di Donato, et al. Thermal Imaging of Cutaneous Temperature Modifications in Runners During Graded Exercise. Annals of Biomedical Engineering. 38(1):158-163, 2010.
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