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This thesis describes the physical activity behaviors and factors and settings which influence this behavior in Dutch youth with and without a chronic disease and disability. In addition, the (possible) changes in cardiorespiratory fitness as a result of improved Cystic Fibrosis (CF) care, possible limiting factors and new promising medication were described. The prevalence of severe fatigue and the relation between psychosocial and clinical measured factors were assessed. Higher levels of cardiorespiratory fitness are related to survival and higher quality of life scores in people with CF. Therefore, increasing our knowledge regarding cardiorespiratory fitness in this period subject to change, is clinically relevant. In 2010 up to 2014, only 29% of the Dutch youth met the physical activity guidelines (to be at least moderately active for at least 60 minutes per day) and 43% had at least two hours of screen time. A lot of daily physical activity comes from cycling to school. Unfortunately, physical activity of especially scholars from cluster schools is lower. The Dutch youth with a chronic disease is not able to participate unlimited in sports and exercise. More than half (55%) of our pediatric population (n=60) with CF, with no ventilatory limitation, has still low cardiorespiratory fitness levels, despite improved CF care. Children and adolescents performed a cardiopulmonary exercise test (CPET) to establish cardiorespiratory fitness levels (peak oxygen uptake- VO2peak). Patients were categorized as having low cardiorespiratory fitness when VO2peak relative to body weight < 82% of predicted. Logistic regression analysis showed that glucose intolerance and colonization with P. Aeruginosa were most strongly related to lower levels of fitness. The maximal cardiac output during maximal exercise, determined by stroke volume and heart rate, is lower in both children and adults with CF (n=19) compared to healthy people (n=22). A blunted heart rate response, also named chronotropic incompetence, might be a cause for this lower cardiac output. Training intensities based on predicted heart rates seem inadequate for people with CF. In our adult CF population (n=77), 26% reported to experience severe fatigue. Patients could be categorized as experiencing ‘severe fatigue’ when the score on the Checklist Individual Strength-20 questionnaire was at least 35. Linear regression analysis showed a relation between fatigue and pulmonary function and functional capacity, measured with the Modified Shuttle Test. Stronger relations were however found for fatigue and patient-reported outcome measures such as physical and emotional functioning. In a small group of people (n=7) with CF and the S1251N mutation, we found an increase in pulmonary function and body mass index after a median of 15 [IQR 13-16] months of treatment with ivacaftor, a CFTR modulator drug. Remarkable was a decrease in cardiorespiratory fitness, which might be explained by a lower work of breathing. Maintaining and improving cardiorespiratory fitness remains an important treatment aim for CF physiotherapy care.

Supplemental material, sj-docx-1-tar-10.1177_17534666211070143 for Physiological predictors of cardiorespiratory fitness in children and adolescents with cystic fibrosis without ventilatory limitation by Marcella Burghard, Tim Takken, Merel M. Nap-van der Vlist, Sanne L. Nijhof, C. Kors van der Ent, Harry G.M. Heijerman and H.J. Erik Hulzebos in Therapeutic Advances in Respiratory Disease

The Netherlands is known globally for its widespread use of bicycles and some call it a “cycling nation”. Indeed, many Dutch inhabitants own a bike and cycle frequently. Numbers show that 84% of the Dutch inhabitants from age 4 years and older own a bike. Those owners have an average of 1.3 bikes per person. This results in 18 million bikes in the Netherlands and 13.5 million bike owners.6 The Dutch use their bike as a means of transportation, but also for sports and exercise. Bike-use fits well in an active lifestyle and it is highly plausible that cycling is responsible for a large part of the daily physical activity in Dutch youth. It is estimated that Dutch people have on average a 6 months longer life expectancy attributable to bicycle use.7 It seems that the nation itself is well shaped to cycle: no large mountains, only a few small hills, and an extensive layout of cycle paths and routes in every city and village. In many urban areas separate cycle paths are very common. Our results show that many Dutch children use the bike as their way of transportation. It was demonstrated that active transportation is responsible for a large part of schoolrelated physical activity in Dutch youth.8 80% of 12-17 year-old children cycled three or more days to or from school/work.9 This resulted in an ‘A’ for the indicator active transportation (walking is included in the grade as well). Active transport is associated with increased total physical activity among youth.10,11 Also evidence is reported for an association between active transport and a healthier body composition and healthier level of cardiorespiratory fitness among youth. Although Dutch children accumulate a lot of daily physical activity through cycling, it is not enough to meet the current national physical activity guidelines of 60 minutes of moderate-to-vigorous physical activity per day. Even though cycling is an important component to the amount of daily physical activity, Dutch youth are not cycling to health