Discussion
The key finding of this study is that the time-use composition consisting of sleep, SB, LPA and MVPA was associated with the frequency and intensity of LBP. Specifically, our compositional isotemporal substitution analyses showed that: (1) reallocating time from SB or LPA to sleep is associated with lower frequency and intensity of LBP; (2) reallocating time from MVPA to any of the remaining 24-hour movement behaviours and from SB to LPA is associated with lower average LBP intensity and (3) reallocations of time in the opposite directions were associated with unfavourable LBP outcomes. These reallocations of time may be meaningful from clinical and public health perspectives.
The favourable associations of reallocating time to sleep with the frequency and intensity of LBP found in our study are in accordance with findings of previous studies that did not use CoDA.14 24 This suggests that LBP sufferers may benefit from getting additional sleep, and that the potential importance of sleep for the management of LBP should be considered when developing and/or updating clinical guidelines. Our finding also supports the recommendation concerning sleep that has recently been included in the Australian clinical guidelines for the management of LBP.11 According to our analyses, additional sleep time was associated with relatively small differences in LBP (1–6% lower LBP intensity for 30 min/day higher sleep time). However, even small differences can have an important impact, especially at the population level.25 It should be noted that the predictions were made from the mean composition with 7.9 hours/day of sleep, which is already within the recommended duration for adults (ie, 7–9 hours/day).26 It might be that the associations between reallocations of time to sleep and LBP outcomes would be stronger among individuals whose sleep duration is insufficient.
Our finding that MVPA was unfavourably associated with the intensity of LBP might be seen as inconsistent with the previous literature showing a beneficial association between physical activity and LBP.12 27 28 However, most previous literature reporting a favourable relationship has focused only on leisure-time MVPA,12 27 28 while little is known about the relative contribution of total MVPA (ie, MVPA as part of work, transportation, leisure and household) to LBP. Previous literature on occupational and household MVPA has usually reported unfavourable relationships with LBP.28 29 Similarly, a recent study that used CoDA and included LBP sufferers found that reallocating time to leisure-time MVPA was associated with a lower risk of long-term sickness absence, while reallocating time to occupational MVPA was associated with a higher risk.30 Given that MVPA in our study included all domains, it may be that unfavourable association between MVPA as part of work and LBP28–30 dominated over favourable association between leisure-time MVPA and LBP.12 27 28 Our findings challenge the general advice to be physically active to decrease the severity of LBP and support the provision of more specific recommendations with the distinction between different domains and intensities of physical activity. Future studies in time-use epidemiology are needed to explore the relative importance of domain-specific MVPA on a variety of LBP outcomes (eg, intensity, disability, chronicity).
Our finding that reallocating time to LPA (from other wake-time behaviours) was favourably associated with the intensity of LBP (and vice versa) is in accordance with previous research.13 31 For example, a study that used CoDA found that reallocating time from SB to standing and walking (two common types of LPA) was associated with a lower intensity of LBP.31 Furthermore, according to our analyses, additional time spent in LPA was associated with relatively small differences in LBP intensity (1–6% lower LBP intensity for 30 min/day higher LPA). It seems that substantially larger reallocations of time towards LPA would be needed to observe a clinically meaningful difference in LBP. Given that both SB and LPA usually constitute a large proportion of daily time (in our sample each of the two behaviours constituted around 30% of daily time), such large reallocations from SB to LPA might be feasible for LBP sufferers.
We did not find a significant association between the 24-hour movement behaviour composition and the occurrence of LBP, which is in accordance with one previous study that used CoDA and reported no statistically significant differences in MVPA, LPA, standing, SB and time in bed between LBP sufferers and their asymptomatic peers.30 However, previous reviews have shown that short sleep is associated with an increased risk of LBP.32 Some (but not all) previous reviews have also suggested that low MVPA33 and high SB34 are associated with increased risk of LBP. Our findings may differ from previous findings due to differences in the definition of LBP or differences in the sample demographics. In our study, LBP was defined as experiencing LBP on at least one occasion in the past year, while, for example, some previous studies were focused exclusively on ‘clinically relevant’ LBP.32 33
The differences associated with isotemporal substitutions were either pointing in the same direction for both frequency and intensity of LBP, or at least one of the two relationships was not statistically significant. Given that a non-significant relationship is not evidence of no relationship (the statistical inference tests only allow us to reject the null hypothesis but not to accept it), none of the relationships can be considered as ‘conflicting’. Nevertheless, it is interesting that all types of isotemporal substitutions were significantly associated with the intensity of LBP, while only 4 out of 12 possible types of isotemporal substitutions were significantly associated with the frequency of LBP. The reason for this might be the difference in the sensitivity of the response scales used to assess these two outcome variables. While the frequency of LBP was assessed on a 6-point ordinal scale, the intensity of LBP was assessed on a scale from 0 to 100. Future studies should consider alternative options for assessing LBP frequency, as this may affect the findings on its relationships with movement behaviours.
Strengths and limitations
The key strengths of this study were as follows: (1) a relatively large sample of LBP sufferers and (2) the use of CoDA in accordance with the recent methodological developments in the field of time-use epidemiology. The study also had some limitations. First, due to the cross-sectional design of the study, we could not draw conclusions about causality. Movement behaviours can influence LBP, but it can also be that LBP influences movement behaviours, or that the relationship is bidirectional.35 Second, our hypothetical reallocations of time were performed around the mean composition in our sample. It may be that the findings would be different in study populations with a different mean composition. Also, predictions from our reallocation analysis cannot be generalised to clinical LBP populations, because our study was conducted among participants from the general adult population, most of whom did not have clinically important LBP. Third, reliability of physical activity, SB and sleep estimates is generally somewhat lower when assessed by questionnaires, compared with device-based measures.36 37 Imperfect reliability of physical activity, SB and sleep estimates may have attenuated their relationships with the measures of LBP in our study. This means that the true relationships between physical activity, SB and sleep with LBP are likely to be stronger than the ones found in our study. Fourth, there was a mismatch between the reference period for the assessment of LBP (ie, prior year) and movement behaviours (past week). For some participants, it is possible that the periods of their LBP were preceded by or coincided with a time-use composition that is very different from the one recorded in our study. However, past week is a common reference period in the assessment of movement behaviours using self-reports38 and accelerometers,36 37 because assessing movement behaviours across 1 week may provide valid estimates of individual’s habitual activity.39 Fifth, DABQ can be used to assess not just overall but also domain-specific physical activity and SB. However, only 10% of our participants reported engaging in occupational MVPA and the rest of the sample reported 0 min/day of occupational MVPA. The compositional isotemporal substitution analysis cannot be used if the composition includes zeros.22 While zero replacement methods are available,22 the use of imputation methods is generally not recommended if the proportion of zeros in the dataset is very high, as in our study.40