Darcy Ummels

10 | Chapter 1 physical activity behaviours (e.g., increase one’s physical activity level). Physical activity can be measured by numerous variables, such as step count, active minutes, heart rate, walked distance, type of activity (e.g., sitting), and burned calories. Most variables of physical activity can be measured in either an objective or subjective way. Subjective measures include self ‐ report questionnaires and diaries 23 ; the former is currently the most used method to measure physical activity. 24 Frequently used questionnaires are the Global Physical Activity Questionnaire (GPAQ), 25 International Physical Activity Questionnaire (IPAQ), 26 Physical Activity Scale for the Elderly (PASE), 27 and the Short Questionnaire to Assess Health ‐ enhancing physical activity (SQUASH). 28 In a review, 85 self ‐ reported questionnaires for adults were presented. 29 The validity and reliability properties of the questionnaires were assessed in 76 different questionnaires, from which only a few had sufficient validity and reliability. 29 This review stated that there is a lack of standardisation among questionnaires and concluded that no questionnaires could be (strongly) recommended above others. Overall, several studies showed that using questionnaires and diaries to self ‐ report physical activity tend to result in over ‐ or underestimations of actual physical activity. 30 In addition to their lack of good clinometric properties, questionnaires, and diaries also lack feasibility; they are time ‐ consuming and contingent on the participant’s memory. 31 Objective measurements of physical activity can be performed with doubly labelled water or indirect calorimetry. 23 Contrary to questionnaires and diaries, these measurement tools have good validity and reliability and do not rely on the participant’s memory. 32,33 However, these methods also lack feasibility for daily practice; they are not accessible to the average individual, cost more and require more expertise than questionnaires and diaries, and are equally time ‐ consuming. 34 Another method of objectively measuring physical activity involves the use of activity trackers, which are potentially objective, easy to use, and commercially available. Activity trackers Activity trackers are primarily used in research and sports. Recently, however, activity trackers have become available for commercial use and are gaining popularity. A distinction can be made between consumer ‐ grade activity trackers (e.g., Fitbit) and non ‐ consumer grade activity trackers (e.g., activPAL). Non ‐ consumer ‐ grade activity trackers are predominantly used in research settings and often require special software or additional licenses. Meanwhile, there is a plethora of consumer ‐ grade activity trackers from which to choose. In 2016, the top five brands combined sold 102.4 million activity trackers worldwide. 35 In 2016, one out of three Dutch citizens was using either an app, smartwatch, or activity tracker. 36 Depending on the type of activity tracker, it can