Coen Bongers

In Chapter 1 we provided a historical overview of the measurement of core body temperature (Tc) and we introduced the general concepts of the human thermoregulation and fluid balance control. Exercise is associated with the development of an increased core body temperature (hyperthermia) and dehydration. Both hyperthermia and dehydration may negatively impact on exercise performance and the development of heat-related illnesses, which emphasizes the importance of interventions to attenuate the exercise-induced increase in Tc. Therefore, the general aim of this thesis was to evaluate the thermoregulatory and fluid balance responses to exercise in young and elderly individuals, using State-of-the-Art equipment. Second, we aimed to get more insight into cooling strategies to determine the most beneficial cooling strategy to improve exercise performance in the heat.

An accurate measurement of Tc is of great importance to quantify the thermal strain in rest and during exercise. Ingestible telemetric temperature capsules have been described as a valid surrogate marker for Tc. In Chapter 2 we provided a detailed prescription of a measurement protocol using the CorTemp ingestible temperature capsule. Furthermore, we identified important factors that should be addressed, while using an ingestible temperature capsule to measure Tc in lab- and field based settings. Current available capsule systems are expensive and have restrictions related to the battery life time and expiry. Therefore, we assessed the validity and test-retest reliability of a novel ingestible temperature capsule (myTemp) in well-controlled ex-vivo circumstances in Chapter 3. A total of n=15 capsules were tested twice in a highly temperature controlled water bath, in which the water temperature was gradually increased from 34°C to 44°C. Based on the low systemic bias, narrow 95% limits of agreement, high intraclass correlation coefficient and a low standard error of the mean, we concluded that the myTemp ingestible temperature capsule is a valid and reliable technique to measure (water) temperature. Subsequently, we compared the validity, reliability and inertia characteristics of four different temperature capsule systems (n=10 capsules per system) in Chapter 4, using an ex-vivo water bath. We demonstrated that all capsule systems are valid and reliable to measure (water) temperature. The best test–retest reliability was found for the myTemp and VitalSense system, whereas CorTemp and e-Celsius demonstrated a small, but negligible, systematic bias. Furthermore, the VitalSense system showed the slowest response to increases in water bath temperature, whereas the other systems had a comparable time delay. These findings may implicate that ingestible temperature capsules are eligible to measure Tc.

Strategies to reduce the thermal strain prior to (pre-cooling), during (per-cooling) and directly after exercise (post-cooling) are able to improve exercise performance and enhance recovery from exercise. In Chapter 5 we performed a meta-analysis, in which we demonstrated that pre- and per-cooling are equally effective in improving exercise performance in the heat (ambient temperature ≥30°C). Moreover, we revealed that a combination of cooling techniques (for pre-cooling) or ice vests (for per-cooling) is most effective in improving exercise performance. Pre-cooling was also effective in reducing the maximum Tc, whereas no difference in Tc was found after per-cooling. Additionally, no correlation was found between maximal Tc and improvement in exercise performance, which suggests that a lower Tc at the end of exercise does not necessarily improve exercise performance in the heat.

In Chapter 6 we provided a comprehensive overview of current scientific knowledge in the field of pre-, per- and post-cooling, in which we discussed the effectiveness of cooling interventions, the underlying physiological mechanisms and the practical regulations regarding the use of cooling techniques. Our findings suggest that pre-cooling, per-cooling and a combination of both are equally effective in improving exercise performance. The beneficial effects of pre- and per-cooling can be explained by thermoregulatory as well as cardiovascular and metabolic mechanisms, while the beneficial effects of post-cooling can be explained by faster recovery of thermoregulatory and cardiovascular strain and a reduced inflammatory response to exercise. In short, any opportunity to reduce thermal strain prior to, during or after exercise is effective to improve exercise performance and recovery from a stressful bout of exercise.

The effects of wearing an evaporative cooling vest during a 5-km running time trial on exercise performance and thermoregulatory responses is examined in Chapter 7. A total of 10 well-trained subjects were included and they completed a 5-km time trial during three study visits (familiarization, cooling and control session). We showed that wearing an evaporative cooling vest during exercise did not improve 5-km time trial performance or attenuate the increase in Tc in moderate ambient conditions (25°C and 55% relative humidity), but did improve thermal comfort during exercise. These findings suggest that although the cooling vest did not enhance exercise performance, it might be comfortable during practice.

Individuals with spinal cord injury are known to have a reduced thermoregulatory function below the level of the lesion. Therefore, the effects of wearing an evaporative cooling vest on the Tc response of individuals with thoracic spinal cord injury during submaximal exercise was examined in Chapter 8. We included n=10 subjects with a thoracic lesion and they performed two 45-minute exercise bouts at 50% of maximal workload in moderate ambient conditions (25°C). Wearing a cooling vest effectively lowered skin temperature, perception of thermal sensation and increased the core-to-skin temperature gradient, but cooling was not effective in limiting or delaying the increase in Tc. Therefore, an evaporative cooling vest might be comfortable for individuals with spinal cord injury during exercise in moderate ambient conditions despite no impact on Tc was observed.

The impaired thermoregulatory and fluid balance responses to exercise in older individuals are well established, however it was not yet known whether these responses further deteriorate with advanced aging. Therefore, in Chapter 9 we compared the thermoregulatory and fluid balance responses between sexagenarians (60±1 year, n=40) versus octogenarians (81±2 year, n=39). All subjects participated in the first day of the Nijmegen Four Days Marches and walked 30 km at a self-selected pace. We found a larger exercise-induced increase in Tc in octogenarians compared to sexagenarians. Furthermore, octogenarians reported a lower fluid intake and body mass loss compared to sexagenarians, whereas no differences traditional fluid balance parameters were found. Our findings suggest that the thermoregulatory control declines with advanced aging, while the fluid balance control did not deteriorate with aging.