Preliminary Study for Standardization of Heat Tolerance Test for Young Women

Background:

Heat stroke is defined as a condition where the body temperature raises to dangerous level that may cause harm to body tissues and lead to multi-system clinical and pathological manifestations that might lead to death. Among heat stroke survivors a defect in thermoregulation develops as a consequence. In the IDF, every soldier who had a heat stroke, or is suspected to have had a heat stroke, must pass a heat intolerance test in the Unit of Military Physiology, after a rest period of 4-6 from the incident, which is the approximate period for thermoregulation to recuperate from the injury after a heat stroke. Until the heat intolerance test, the soldier must not train to avoid a repeat incident of heat stroke. The soldiers' ability to return to active duty is based on the results of the exam. The heat tolerance test and its criteria have been developed in the Physiology Unit throughout years of experience, and have been proved and adopted by several labs in Europe and in the United States. The criteria generally include baseline temperature and heart rate, and are based upon the physiological response of healthy men (soldiers) in the physical effort involved in the test.In recent years there is a growing trend in the IDF to integrate female soldiers in combat divisions. As a result, these females are exposed to physical efforts in various climate conditions as a part of their training, or a part of their active duty missions in the division. Therefore, the probability of an incidence of heat injuries increases among these soldiers.Research results indicate differences in the efficiency of thermoregulation mechanisms between men and women:

It has been found that in women the threshold for sweating and vasodilatation is higher than in males. Additionally, although in women the mechanism for sweating is slower, and occurs at a later period, it is found that the relative part of sweating that evaporates is higher, so that women evaporate sweat more efficiently (to make up for the lower sweat quantities compared to men). Reduced efficiency of the thermoregulation mechanisms in women is also attributed to the differences in mass, body composition (fat and muscle mass), and physical fitness between the genders. In addition, the physiological changes that occur during the different phases of the menstrual cycle can affect core temperature at rest and thermoregulation. Different reports in literature indicate a decreased thermoregulation capacity in the luteal phase of the menstrual cycle compared to the follicular phase. Apparently, thermoregulation capabilities in exercise are linked to progesterone blood levels, which are influenced by contraceptive pills and with the phase of the menstrual cycle.

There are two factors that can cause incorrect diagnosis of the level of heat tolerance in females when the specific test is interpreted based on set criteria for men. These criteria are:

1. The physiological differences between men and women
2. The differences in women based on menstrual cycle and using contraceptive pills.

Thus, there is a need to characterize and establish the normal values in a heat tolerance test for young females compared to males, and also to correspond to current menstrual cycle phase and use of contraceptive pills.

Methods:

on the first day of the research, the participants will undergo anthropometric examinations (measuring height, weight, BMI, fat content, maximum oxygen consumption). In addition, the participants will undergo a heat tolerance test protocol. The heat tolerance test will be conducted twice in every participant - once in the follicular phase of the menstrual cycle, and a second time in the luteal phase of the cycle. During the protocol, the following information will be collected from each participant: heart rate, core temperature, skin temperature in three locations (chest, arm, and leg), oxygen consumption and liquid balance.

The results of the examination will be compared to results of male participants who have previously undergone the heat tolerance test in the Unit of Military Physiology.

Contribution to IDF:

the research will allow administrating heat tolerance tests accurately and reliably to female soldiers that have been affected by heat injuries or are suspected to be heat intolerant.