In this study, we examined the associations between BMI, sitting, standing and different modes of physical activity (i.e., slow walking, brisk-walking, cycling and high-intensity activity) in a sample of 10–12-year-old children. The results of latent profile analysis suggest that high sitting time invariably coexists with low physical activity time at any intensity. This implies that engaging in high levels of physical activity may potentially serve as a protective factor against the adverse effects of prolonged sitting. However, both normal weight and overweight/obesity can coincide with low physical activity and high sitting time.
The PLS-SEM results revealed associations between different physical activities, sitting time, and BMI following a step-by-step, cascade-like pattern. This pattern begins with an activity type with very light intensity (i.e., standing) and escalates step by step to activities with higher intensities, with connections strengthening as activities progress from lighter to higher intensities. This cascade-like pattern impacts sitting, and decreased sitting time mediates lower BMI. However, this model is effective only when the step-by-step pattern is adhered to, meaning that skipping one step and jumping to a higher intensity activity type negatively affects the model.
To our knowledge, there are no similar studies exploring the association between physical activities at different intensities (i.e., standing, walking, cycling, and high-intensity activity) using thigh-worn triaxial accelerometers with BMI in children. We opted for physical activities as they are simpler to communicate and follow than measures such as moderate-to-vigorous intensity physical activity. However, a few studies have investigated this association using different physical activity intensities in children and adolescents [2, 5, 13, 14].
Recently, Parker et al. (2019) employed self-reported surveys to measure various activities among adolescents, including active travel to school, leisure-time sedentary behavior, sport participation, and demographic variables. Through the application of Latent Class Analysis (LCA), they discerned three distinct typologies: 1) 'physically inactive, highly sedentary', 2) 'highly active and low sedentary', and 3) 'moderately active with high screen time'. These findings align closely with the outcomes of our own study [41]. Their research cohorts collectively demonstrated an escalation in sedentary behavior and a decline in physical activity levels with advancing age, from childhood to adolescence. The authors recommended targeted interventions aimed at reducing sedentary time and promoting increased physical activity [14, 41].
The correlation between physical activity, sedentary time, and adiposity in children and adolescents is considered complex [1]. Biddle et al. (2018) reported a weak association between TV viewing and adiposity in children and adolescents but found inconsistent associations between device-based measures (i.e., accelerometry) of sedentary time and adiposity. A meta-analysis reported a small but significant decrease in BMI when interventions reduced sedentary behavior, particularly in overweight and obese children and adolescents [42]. Some other authors reported a mixed results when considering the relationship between intensity of physical activity and adiposity. Certain studies have cited MVPA as an independent predictor of adiposity in children and adolescents [43, 44], while others have underscored the protective role of light-intensity activities against high adiposity [1, 5]. Biddle et al. 2018 argued that adiposity and sedentary behaviors might be correlated, but this relationship can be confounded by factors such as MVPA, standing, slow walking, sleep, and dietary patterns.
In our study, the latent profile analysis showed that not only normal weight but also overweight children can be either highly active or low active, with high or low sitting time. However, obesity was exclusively associated with high sitting time and low activity time. Biddle et al. recently also demonstrated that the relationship between adiposity and sitting time depends on time spent in total-light intensity physical activity and light-light intensity activities such as standing and slow walking, but not on MVPA or high-light-intensity physical activity in a sample of adolescents [5]. They emphasized the importance of light-intensity physical activity in mitigating the negative effects of prolonged sitting on adiposity in adolescents [5]. Accordingly, several authors have highlighted the necessity of light-intensity physical activity and proposed an integrative model that includes different types and intensities of physical activity, not just MVPA [5–7].
Results of the latent profile analysis indicated that overweight or obese children (class 3) had low standing time and high sitting time. However, class 1, with a high rate of normal weight children, had high sitting time but also high standing time. Furthermore, the PLS-SEM results revealed a direct negative association between standing time and BMI. These findings suggest that overweight and obese children stand less than their normal weight peers. This might occur due to their higher body mass and adiposity, which deter them from standing for extended periods in daily life [1]. This supports the bidirectional association between adiposity and physical activity, in which high adiposity possibly inhibits activity [15]. Moreover, the results of PLS-SEM indicated a direct negative association between standing time and BMI. Still, this association is negatively mediated with higher sitting, suggesting that only high standing can increase sitting time and lead to an increased BMI. Conversely, we observed a negative indirect association (i.e., through physical activities at higher intensities) between standing and BMI. These results suggest that light-intensity physical activity types, such as standing time, are negatively associated with sitting time only when they are part of other more intensive physical activities. However, given the cross-sectional nature of this study, longitudinal studies are required to examine whether standing alone or combined with other physical activity types can alter sitting time or adiposity in children.
Given the likely complex interplay between sedentary time and physical activities, it is crucial to consider the types and intensities of activities that replace sedentary time in interventions [1, 2]. Notably, during waking hours, any reduction in sitting time will primarily result in an increase in light-intensity physical activity types such as standing and slow walking [1], but may also include MVPA [45]. It's been demonstrated that even movements such as increased standing can enhance energy expenditure [46, 47] and positively impact cardiometabolic biomarkers [48].
Some studies have reported the coexistence of high sitting time and high-intensity physical activity among youth [49]. For instance, using cluster analysis, Marshall et al. suggested a coexistence of high sitting time and MVPA [49]. However, the latent profile analysis employed in our study did not confirm the coexistence of high sitting time and/or high BMI and high levels of any physical activity types at different intensities in children, which is according to a recent similar study by Parker et al. (2022) where they used similar methods [41]. Moreover, in our study, we noted that the class with the highest prevalence of overweight and obesity not only had high sitting time but also low standing time, walking, cycling, and high-intensity physical activity time. This finding aligns with the literature showing the connection between obesity, high sedentary time, and low levels of physical activity [5–7, 50–52].
Finally, the results of latent profile analysis revealed a coexistence of low levels of all physical activities at different intensities and high sitting time in children with either high BMI or normal BMI. This suggests that not only overweight/obese children but also a group of normal weight children can exhibit high sitting time accompanied with low physical activity time. Therefore, programs aimed at increasing physical activity time should consider all children, not just those who are overweight and obese [53].
Although we found a direct negative association between high-intensity activity time and BMI, this relationship was mediated by a sequential cascade of activity types from lower to higher intensities and was ultimately moderated by reduced sitting time. Research has shown that focusing solely on high volumes of high-intensity physical activity can lead to unpleasant experiences and feelings [54]. Moreover, prescribing only high-intensity physical activity may result in a compensatory decrease in lower-intensity activities such as standing and slow walking and an increase in sitting time[55]. It may be more practical to reduce sitting time by increasing activities such as standing and walking, which represent the most variable components of daily total energy expenditure [56], to reduce adiposity [2, 57]. Additionally, beginning with low-intensity activities could be more motivational, easier, and more practical than starting from MVPA for preventing obesity and sedentariness in children and adolescents, especially those who are overweight or obese [5, 58–60]. For instance, starting to target sedentary behavior [61] through active pedagogy or height-adjustable desks (i.e., standing lessons) and active breaks, in the school classroom could be practical methods for integrating light-intensity physical activities, thereby increasing energy expenditure while reducing sitting time and adiposity in children and adolescents [47, 61–64].
Our study results, based on PLS-SEM analysis, revealed that the intensity of physical activities directly corresponds with their association with sitting time or BMI. However, we discovered that all physical activities, regardless of their intensity, exhibit either direct or indirect associations with each other, and these connections intensify as activities transition from lighter to higher intensities. For instance, standing showed no connection with brisk walking or high-intensity activities, but it was linked to slow walking. A direct positive association was observed between slow walking and brisk walking, while a negative association was evident with high-intensity activities. These findings emphasize the significance of physical activities of various intensities and their interconnectedness, which ultimately influences sitting time and BMI.
Recently, Dunstan et al. (2021) suggested a "staircase approach," especially for healthy individuals younger than 45 years. This approach advocates for an initial reduction in sitting time, followed by an increase in standing and slow walking, eventually leading to an increase in light-intensity activities and finally, a rise in MVPA [65]. Our findings align with this approach, providing empirical evidence that light-intensity activities seem to serve as a necessary precursor to higher intensities, mediating BMI by reducing sitting time. Removing light-intensity activities could potentially disrupt this model, thereby reducing the effectiveness of interventions aimed at reducing sitting time and BMI.
The PLS-SEM results suggest a mechanistic interplay, where engaging in light-intensity activities may set the stage for higher intensity exercises, akin to the interconnected gears in a machine, each one propelling the next to keep the whole system in motion. (Fig. 3). As intensity levels increase, the relationships between activity types, sitting time, and BMI become stronger. Each intensity level appears to be crucial in maintaining the desired effects on reducing sitting time and BMI. If a step in this cascade, such as brisk walking/cycling, is removed, the associations reverse (i.e., results of supplementary Table 3). This is according to recent study results, where the authors showed that total light intensity physical activity and low light intensity physical activity but not high-light intensity physical activity or MVPA possibly protect deleterious effects of sitting on adiposity in adolescents [5]. This underscores the importance of including a full spectrum of physical activity types to effectively reduce sitting time and BMI (Fig. 3). Overall, the study underscores the value of a balanced, multi-intensity approach to physical activity for positive adiposity outcomes among children. However, longitudinal and experimental studies are needed to validate this model and determine its success in decreasing sitting time and adiposity in children and adolescents.
Strengths and Limitations
The strengths of our study include the use of device-based measures of sitting and physical activities at different intensities and sitting [31, 66], along with the application of person-centered and PLS-SEM analysis methods [23, 24] which have been recommended for studies exploring the association between physical activity, adiposity, and sitting time [5–7, 19]. However, the cross-sectional nature of the study, the use of BMI instead of direct measures of adiposity, and the lack of sleep and dietary patterns data limit our findings.