2 Physical activity as a risk or protective factor | 21 Participants were asked to perform the grip strength exercise twice with each hand. Grip strength was determined as the average of the highest score of the left and the right hand. The position of the participants was seated with the back straight and elbow bended in a 90° angle. The dynamometer was adjusted for hand size. Falls and fractures A fall was defined as ‘an unintentional change in position resulting in coming to rest at a lower level or on the ground’ (97). From T1 up to and including T5, participants kept a fall and fracture diary and were interviewed over telephone every nine months about falls and fractures. If a fall was reported over the past nine months, participants were asked whether a fracture resulted from their fall. Physical activity To measure physical activity objectively, participants were sent an Actigraph tri-axial inertial sensor (Model GT3X; Actigraph, Pensacola, FL, USA) by mail together with the instructions for wearing the inertial sensor at T1. The inertial sensor was attached to a three centimetre wide, tight elastic belt and was worn superior to the left iliac crest. Two days after sending the inertial sensor, a phone call to the participant was made to ensure that the package was received and the inertial sensor was properly worn. Participants were instructed to wear the inertial sensor for seven consecutive days during waking hours and to remove the inertial sensor only during bathing, showering and swimming. Participants kept a diary to log the time the inertial sensor was put on after waking and removed before sleeping. When the inertial sensor was not worn for some period during the day, participants recorded the start and end time of the period not wearing the inertial sensor. Physical activity was defined as the time spent on at least light-high intensity in minutes per day, defined as the inertial sensor registering at least 760 counts per minute (a count is a relative measure of change in momentum as measured in 3D by the inertial sensor) (98-100). Frailty At T1, frailty was determined using the frailty index, which is a valid and reliable instrument to determine frailty in older adults (86, 101, 102). The frailty index includes 32 health deficits from physical, mental and cognitive domains. These health deficits include self-reported chronic conditions (11 items), functional limitations (six items), self-rated health (two items), mental health (six items from the Centre for Epidemiologic Studies Depression Scale), physical activity (one item), cognitive health (five items, based on self-reported memory complaints and domains of the Mini-Mental State Examination), and physical performance measured by gait speed (one item) (86, 103, 104). All deficits were scored as 0 or 1, where 0 indicates the absence of the deficit and 1 the presence of the deficit. The frailty score was not calculated if participants had more than 20% missing items. This criterion is commonly used and allows
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