1 12 CHAPTER 1 BACKGROUND Running a marathon at the Olympic level represents the ultimate challenge for an elite athlete, whereas walking independently to the supermarket may be an outstanding achievement for a frail older person. While both engage in physical activity, ‘any bodily movement produced by skeletal muscle that requires energy expenditure’ [1], the difference in the level of physical performance between an Olympic athlete and a frail older person illustrates that valuing physical performance depends on an individual’s physical capacities and context [2], [3]. Zehr categorises the physical performance of humans as a continuum from the lowest category, humans being hindered from performing basic activities due to illness or injury, through a middle category of humans being able to perform in an everyday day setting to the highest category of humans that can perform at an athletic level [4]. In this thesis, we concentrate our research on humans functioning in daily life and elite sports. In everyday life, being physically active helps to maintain general functioning as long as it is done regularly and with sufficient duration and intensity [5]. Physical activity in daily life can be undertaken in many ways, such as walking, cycling, gardening, household activities, sports, and work. Regular physical activity generally prevents chronic or noncommunicable diseases such as heart disease, stroke, and diabetes and supports mental health, quality of life, and well-being [5]. Inversely, insufficient physical activity may lead to deterioration of physical performance in daily life and higher risks of health problems, illness, and lower life expectancy [6]–[8]. Consequently, it is essential to monitor physical activity and to intervene to enhance physical activity when the physical activity is insufficient to maintain general functioning. In elite sports, for example, high-level running or professional soccer, humans require many years of targeted physical activity (i.e., training). During these years of training, a delicate balance between training (referred to as ‘load’), recovery, and individual capacity must be maintained to increase physical performance and prevent injuries and illnesses [9]. A short-term increase in training frequency, duration, or intensity provokes a short-term decrement in performance, termed functional overreaching (FOR) [10]. FOR is considered a necessary component of a training programme to improve physical performance [11]–[13]. On the other hand, high training loads without sufficient recovery over a longer period may lead to overtraining, causing a decrease in physical performance and a higher risk of sustaining an injury [14]–[19]. Therefore, balancing load, recovery, and capacity is crucial for elite athletes to maintain or increase physical performance and prevent injuries. Hence, monitoring the balance and adjust the balance is essential for elite athletes to maintain and increase physical performance.
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