Physical fitness is of importance in both our daily lives and medical procedures. It not only determines our capacity to perform daily activities but also has the potential to prevent diseases and limitations. This thesis has established reference values for exercise capacity by subjecting healthy individuals to submaximal or maximal exercise tests, aiming to assess the level of physical fitness within the Dutch population.The second part of the thesis focused on the level of physical activity in patients undergoing major abdominal surgery. The research indicated that a higher level of physical activity before an operation resulted in a shorter time till functional recovery. Interestingly, self-reported measurements of physical activity, such as questionnaires, showed a higher level of activity than objective measurements using accelerometers. This difference underscores the need to measure physical activity with objective instruments.This thesis emphasizes the importance of physical fitness in daily life and during medical procedures. Reference values and models provide a foundation for assessing physical fitness, while promoting physical activity can expedite the time till functional recovery after surgeries. The thesis encourages regular physical activity efforts to improve health and well-being.