A Body Sensor Network for Fluid Management during Sport

This thesis presents a concept for a bioimpedance-based measurement system to monitor the fluid balance during sports and its validation. Two subject areas are combined, on one hand a so-called “Body Sensor Network” to measure different physical parameters and to quantify the exercise intensity, and on the other hand, a model-based compensation of exercise-related fluid shifts. Both parts were validated by a clinical trial.

A balanced hydration status is required for an optimum sports performance. An imbalance can reduce the sports performance and can even have a negative effect on health. However, exercise by itself influences the hydration status. It has been reported that athletes can loose up to 4 l of fluids per hour in the form of sweat during high intensity exercise and unfavorable conditions. Literature also states that even professional athletes are not adequately hydrated. Therefore, monitoring the hydration status can contribute to maximize the sports performance.

Bioimpedance spectroscopy (BIS) is a clinically established measurement technology to assess the hydration status of a person. State of the art devices can even distinguish between extracellulare water and intracellulare water. However, a strict measurement regime has to be followed in order to obtain reproducible results. Any physical exercise is prohibited 24 - 48 h before a measurement because it may falsify the measurement. The reason for that are metabolic changes during exercise which lead to an additional fluid shift within the body. This shift is not regarded in the state of the art BIS calculation models, yet. Therefore, the applicability of BIS measurements for athletes is very limited.

A wearable system, called “Integrated Posture and Activity Network by MedIT Aachen” (IPANEMA BSN) was developed to improve the applicability of BIS measurements for athletes. The Body Sensor Network (BSN) includes e.g., accelerometers, temperature sensors and a textile integrated ECG T-Shirt. The processed data is used to quantify the exercise intensity while the BIS measurements have been performed with a commercial device.

The theoretical focus of this thesis is the extension of a BIS model to incorporate for the first time the dynamic changes of the metabolism and its related fluid shifts. The essential physiological processes e.g., sweat production in relation to the exercise intensity, have been modeled. Furthermore, the pressure conditions in the active muscles have been modeled to include the increased perfusion due to self-heating and shifting of blood constituents. Also, endogenous compensation of fluid shifts are integrated.

Finally, the IPANEMA BSN and the enhanced fluid shift model were validated by a clinical trial with 12 athletes. Measurements were performed in a laboratory, in a climate chamber and on an outdoor track. It could be shown that the simulation results closely resembled the measured characteristic fluid shifts related to running. Thus, the influence on the BIS-measurement results could be compensated and the BIS-measurement errors of the extracellulare resistance (Re) were reduced by 80 % on average.

In conclusion, the results of this thesis show that the hydration status monitoring of athletes during sports is possible with BIS measurement technology provided there is a proper compensation of exercise related perturbations.