Regular physical activity (PA) and high cardiorespiratory fitness (CRF) have numerous scientifically documented health benefits.1,2 The aerobic part of the recently updated PA guidelines for adults recommends at least 150 to 300 min of moderate (MPA) or 75 to 150 min of vigorous PA (VPA) weekly, or some combination of them.3,4 A striking change from the previous PA guidelines is that the at least 10-minute bouts are no more required for the accumulation of relevant physical activity. Recent studies employing device-measured PA have indicated that the total volume of moderate-to-vigorous physical activity (MVPA) is related to many health benefits whereas time-specific bouts are not essential.2,5
PA-related health benefits and self-reported MVPA have shown an inverse, curvilinear dose-response relationship.2 Even small amounts of PA confer health benefits while they are most evident for the least active individuals.6 The health benefits continue across the full range of commonly achievable volumes, although they have diminishing returns for MVPA levels over 150 to 300 min per week.2 However, it seems that daily 30-40 min of accelerometer-measured MVPA may attenuate the association between sedentary time and risk of death,7 being substantially lower than previously estimated 60-75 min based on self-reported data.8
High cardiorespiratory fitness (CRF) is associated with a significant reduction in all-cause mortality at any level of habitual PA, without evidence of a plateau effect or U-shaped association.1 CRF is more strongly associated with all-cause mortality than self-reported PA in men and women.9 The minimum CRF conferring substantial risk reduction is estimated to be 7.9 MET (metabolic equivalents, 1 MET = 3.5 mL/kg/min of oxygen consumption).10
To assess trends regarding the adherence to PA guidelines, it is important to regularly measure population-level PA with valid methods. Such methods should be able to measure the frequency, duration, and intensity of PA, and desirably also the type of activity and its context. These methods can be divided into self-reports and device-based.11 The self-reports are known to overestimate the exercise time while underestimate the activity time accumulated during daily routines. The device-based methods can assess PA in a more standardized manner regardless of the current fitness level and body weight which both may influence the subjectively perceived and reported intensity of PA.12–43 Self-reported data can supplement device-based data, for example, by providing information on the specific type or context of PA.15,16
The intensity of aerobic PA can be expressed in either absolute or relative terms. Absolute intensity is the amount of energy expended during the given activity without considering a person’s CRF or aerobic capacity. Relative intensity denotes the level of effort relative to a person’s individual maximum aerobic capacity.4,7 The use of relative intensity has been recommended when it is feasible in device-based PA studies, but in large-scale population studies, it can be too laborious and costly to conduct individual exercise testing in laboratory conditions.17 However, the 6-min walk test (6MWT), a cost-effective and well-documented field test of CRF,18,19 has recently been validated for predicting VO2max also among healthy adults.20
It is known that the choice of parameters employed in the analysis of device-measured PA data can substantially affect the results. The use of relative intensity thresholds may lead to paradoxical results regarding the total amount of MVPA time.21 Likewise, the use of different epoch lengths and cut-points to define the intensity (MPA or VPA) can essentially change the estimates of the accumulated PA time.21 Shorter epoch lengths will capture instantaneous and sporadic instances of movement, which are most likely missed with longer epochs due to the inherent smoothing effect. On the other hand, the smoothing effect of the long epoch allows the intensity temporarily to drop below the cut-point.22,23 The cut-points together with the epoch length determine the time spent in MPA and VPA levels. The selected intensity cut-points should be validated in a sample population closely matching the study group of interest and the selected epoch length should be the same that was used to validate the cut-points.24
The purpose of the present study is to systematically examine the device-based adherence to the aerobic part of the updated 24-h movement guideline in Finland. The new guideline for adults aged 18 to 64 years combines recommendations for the PA, sedentary behavior, and sleep across the whole day (Fig. 1). Also, a scheme for assessing population-based adherence to the aerobic part of the PA guidelines is outlined.