Our study showed that relative time spent on MVPA during leisure reduces the risk of LTSA while relative time spent on MVPA during work increases the risk. Beneficial association between MVPA at leisure and LTSA was only observed for the low work MVPA group. These results support the existence of the ‘PA paradox’.
At leisure, more time spent on MVPA relative to other physical behaviors (sedentary, stand, LIPA and time in bed) was significantly associated with lower risk of LTSA (z=-2.26, P = 0.02). For example, reallocating 20 minutes to MVPA from other leisure behaviors was associated with 20% lower risk of LTSA. This observation of a beneficial association of PA at leisure with LTSA is in accordance with the results of existing studies using self-reported measures of PA (50) and not applying the CoDA approach (7). The potential mechanisms behind benefits of leisure time PA could be the increase of both improving health and physical capacity (51, 52), making the workers better perform their work tasks. Overall, we observed that reallocating just a little duration, say 5 minutes, to MVPA from other behaviours seem to lower risk of LTSA. Increasing a little duration of MVPA (defined as time spent fast walking, stair climbing, running, and cycling) could be feasible for many workers and can be facilitated by modifying the structural environment (eg, more bike lanes) or work environment (eg, work tasks offering restitution, likely giving energy and motivation to workers to perform leisure MVPA (53)) making it more MVPA friendly for workers. A slight lowered risk for LTPA can have enormous effects on reducing economic costs for companies and the society, as well as for the individual since LTSA often leads to unemployment and further aggravation of health and life-situation crisis (54, 55).
At work, more time spent on MVPA relative to other physical behaviors was positively associated with LTSA (z = 2.27, P = 0.02). For example, reallocating 20 minutes to work MVPA from other behaviors was associated with 15% higher LTSA risk. No previous studies on the association between work PA and LTSA have used technically measured PA, like accelerometers, and a CoDA approach with prospective register-based LTSA. Thus, we cannot directly compare the estimates of our study with previous studies (3, 7). Nevertheless, the overall finding of an increased risk for future LTSA with higher levels of work PA is in line with the some studies based on self-reports (3, 7). The potential mechanism behind our finding could be that work MVPA is influenced by different constraints and comprise different characteristics than leisure PA (56). Work PA is performed mainly to complete working tasks and compared with leisure, there is limited possibility of tailoring the duration, intensity, and variation of the PA according to the individual needs and preferences. Because of these constraints, the work PA can lead to excessive exertion and fatigue without sufficient possibility for recovery (57), which over time can increase risk of impaired health and LTSA (58, 59).
We also observed that our results were robust when the analyses were adjusted for SES indicators. Studies testing the PA paradox have been criticized for not adjusting for or removing the socioeconomic confounding (9). To address this limitation, we performed the analyses without and with adjustment for a proxy measure of SES (three categories: white-collar, blue-collar-skilled, and blue-collar-unskilled) and even stratified the analyses on these categories. We still observed the PA paradox even after these adjustments and stratifications based on SES, confirming that PA paradox exist independent of SES of workers.
When we stratified the workers in three groups, low, moderate and high work MVPA, the negative association between relative leisure MVPA and LTSA was clear among those with low work MVPA. This negative association was weak and non-significant among those with low and moderate work MVPA. These results show that leisure MVPA is of particular importance among those with low work MVPA, for example administrative workers or the drivers. Conversely, too much work MVPA time seem to reduce the beneficial effect of leisure MVPA.
Strengths and limitations
The main strengths of the study are the thigh-worn accelerometery-based physical behaviors data, shown to be highly reliable and valid (32, 60). Another strength is the CoDA approach applied for the analyses of the study, handling the compositional structure of time-use data of PA (46, 61). Additionally, this study adjusted for remaining physical behaviors (sedentary behaviours, standing and LIPA and time in bed) within 24 hours. Another strength was the usage of national register data with valid prospective measures of LTSA (37). Finally, the opportunity to remove/adjust for possible SES confounding when testing the PA paradox supports our main findings.
One limitation of the study is the relatively small study sample (n = 929) based on workplace recruitment, limiting generalizability of the results of the study to the general population. Thus, similar future studies should be conducted on larger and more representative working populations. We used proxy measure of education and type of work indicating workers’ SES. Therefore, a better measure of SES confounding [such as data from national registers on household income, job group, and education (Statistics Denmark (62))] are needed in the future to confirm these findings. Similar future studies should also focus on testing the PA paradox in relation to other outcomes such as mortality.
LTSA is an early precursor for impaired health with an extensive economic burden on workplaces and society (23, 24). Moreover, LTSA can have enormous consequences for the individual workers as LTSA is a strong predictor of premature exit from the labor market (19, 21) and mortality (63). Because PA at work and leisure are modifiable factors, the findings of the present study can be of importance for better prevention of LTSA with systemic interventions at both work and leisure environments.