Temporal PA patterns generated from one valid random day of accelerometry data are associated with BMI, WC, obesity, hemoglobin A1c, systolic blood pressure, triglycerides and diabetes but not with the other health status indicators examined. Clinical relevance of differences in mean BMI and WC associated with temporal PA patterns may be contended [54–56]. Therefore, observed mean differences in health status indicators imply that temporal PA patterns may be an important health exposure that holds promise for early detection of lifestyle factors promoting health and disease in the population. Reverse causation in the observed associations cannot be ruled out using the cross-sectional study design, nevertheless, the aim of this study was not to establish causation but to investigate whether developed temporal PA patterns using a novel methodology meaningfully link to health regardless of the direction of this association.
An abundance of research examines the relationship between PA and health. Most studies have focused on categorizing participants based on intensity and frequency of activity [57–59], while others examined daily PA patterns by focusing on distinct time periods when PA was reported such as type of day (weekday vs. weekend) [60], activity phenotypes including “weekend warrior” [61,62], and seasonality [63]. A few studies investigated diurnal patterns of PA (data collected over 5–7 days) and health [64–67]. Distinct temporal PA patterns observed in this study have been detected by two other studies that used k-means and x-means clustering approaches to derive clusters using overall activity measured by metabolic equivalent of tasks (METs) and timing of PA [64,65]. Similarities with the current study include presence of an overall inactive/low activity pattern (Cluster 1) as well as two patterns of higher activity that differed in timing of activity “afternoon engaged/morning engaged” or “moderately active/evening active” in Fukuoka et al. and Neimela et al., respectively (“early/late peaks” in activity in Clusters 4 and 3, respectively).
Cluster 1 was associated with significant higher BMI, WC, and odds of obesity relative to normal weight status compared with Clusters 2 and 3, which demonstrates that a lower activity pattern (with activity counts reaching up to 4.8e4 cph) throughout the day is linked with the most adverse health outcomes as evidenced by prior research [57,68,69]. The fact that this cluster included the highest number of participants (39.2%) is alarming but not surprising as previous literature has documented a high level of sedentary behavior (> 50% of waking time) among U.S. adults [5,70,71]. Moreover, Cluster 1 predominantly includes ages 50–65 years, which is consistent with evidence that activity tends to decline with age [72].
Furthermore, findings of significant lower mean BMI and WC associated with Clusters 2, 3, and 4 compared with Cluster 1 with the lowest activity counts throughout the day as well as significant lower odds of obesity relative to normal weight status associated with Clusters 2 and 3 (higher activity counts throughout the day and late in a day, respectively) compared to Clusters 1 and 4 (low activity counts throughout the day or early in a day, respectively) support previous literature showing that higher activity counts are associated with improved health status [13,14,73], but add new information regarding the timing of these patterns. Additionally, the significant lower mean WC and odds of obesity relative to normal weight status in Cluster 3 (higher PA counts performed between 16:00–21:00) compared to Cluster 4 (higher PA counts performed between 8:00–11:00) is interesting as models controlled for TAC thus, these findings may indicate that observed differences could be explained by temporal differences in these patterns. Contrary to this finding, Fukuoka et al. found that a group with peak MVPA performed in the evening had significantly higher BMI and WC compared to a group with peak MVPA performed at noon [64]. Limited evidence exists regarding the relevance of time of activity through the day in terms of links to health [64–67], so further development of temporal PA patterns may allow additional exploration of time as a potentially important factor. Moreover, the integration of time and counts of activity to clustering along with the findings of clinically meaningful differences in health status, based on distinctive time and count features of activity patterns, indicates that applying a more complex patterning technique to characterize activity through the day, has the potential to unfold the complexity of behavior rather than solely describing PA patterns by sums or labels of maximum activity levels.
Certain socio-demographic characteristics such as those included in this study (Table 1) have been shown to be associated with PA-related differences in health. The temporal PA patterns with higher PA counts (Clusters 2, 3, and 4) were more heavily represented by males compared to females, which corroborates trends observed in two U.S. surveillance systems which revealed that males were significantly more likely to be physically active compared to females [74]. Additionally, the low proportion of participants with PIR level of 0-0.99 included in the clusters with higher PA counts and a respectively high proportion of participants with PIR level of 0-0.99 included in the cluster with the lowest activity counts (Cluster 1) supports findings of an inverse relationship between prevalence of PA and household poverty level [72].
In general, activity counts tended to be lower towards the end of the day (18:00 to 22:00) in all clusters except for Cluster 3. Cluster 3 is characterized by lower activity counts during earlier hours (6:00 to 12:00) with higher counts observed towards the end of the day between 16:00 to 21:00. As this cluster was more heavily represented by ages 20–34 y, perhaps these higher PA counts in the evening may reflect sports activities or going to a gym. On the other hand, Cluster 4 with higher activity counts during early hours (8:00 to 11:00) included a higher proportion of ages 35–49 y, potentially indicating PA during work.
Elements other than intensity and duration of activity such as time of activity can be an important aspect of PA patterns and may describe PA better within the context of lifestyle. Moreover, timing of activity occasions may also be tied to dietary intake and sleep-wake regimens. For instance, an individual with a “night owl” behavior pattern may have a greater evening preference and choose to exercise later in a day compared to one with an “early bird” behavior pattern with morning preference [75]. Therefore, insight into how these various factors interact within a day and as part an overall routine over longer periods of time such as a week, month, or year, may reveal stronger associations to health status compared to when they are considered separately and thus allow for more targeted interventions based on overall lifestyle, work schedules, and family life. Further, the rapid accumulation of data on health behaviors through technology-assisted assessment tools including those targeting dietary and activity patterns will provide additional data for future investigation of whether and how the timing of these activities influences health. Integrating these data will add further knowledge of how daily behavioral patterns may contribute to metabolic dysfunction and chronic disease. Moreover, utilization of complex analytic tools including data-driven methods and traditional methods of epidemiology, to integrate time to behavioral patterns including activity and dietary intake, holds promise to understand how these temporal patterns influence long-term health status.
Strengths of this research include the use of a comprehensive approach to classifying PA exposure that considers the complexity of activity over a 24-hour period rather than examining single activity occasions (i.e., in the morning or evening). In addition, the methodology used in the current study to create temporal PA patterns and compare groups by health status indicators performed similarly in terms of association with health status when compared with the traditional clustering methods based on reported activity occasion (such as by engaging in different activity levels vs. inactivity) [76,77], and the results reveal efficacy that might be enhanced by additional methodology refinement in future studies. The limitations of the study should also be mentioned. One important limitation is the small sample size representing ~ 8% of original sample of participants included in survey years 2003–2006; therefore, study results should be interpreted with caution. Of note, sample size attrition is mostly attributable to the selected age range 20–65 y and the inclusion of health status indicators examined in a fasting subsample of participants (both criteria resulted in loss of ~ 84% of the original sample). Cluster descriptions describe the group and do not represent individuals. Additionally, one valid weekday was used to represent the activity occasions of the participants; though one random valid day has been shown to be sufficient for producing reliable population-level estimates of accelerometer-measured activity [78], patterns of activity could differ based on type of day and may potentially vary more on the weekends compared with the weekdays. Thus, further studies should consider investigation of activity patterns over weekend days. Moreover, accelerometers do not capture all types of activity including static activities (e.g., riding a stationary bike or water activities such as swimming) [39]; therefore, although this is an objective measure of PA, it still may not represent the true activity levels of the U.S. population [61].