Depressive symptoms (DS) and low back pain (LBP) pose significant and growing public health challenges, China is no exception(Chen et al., 2022; Lu et al., 2021). The prevalence of DS among Chinese adults (aged 45+ years) was 26 %(Li et al., 2019). Additionally, LBP ranks as the second leading cause of years lost due to disability (YLD) burden disease in China(Wu et al., 2019). Epidemiological studies have provided compelling evidence that DS can lead to long-term adverse outcomes, including functional disability(den Ouden et al., 2013), chronic pain(IsHak et al., 2018), stroke(Robinson and Jorge, 2016), and all-cause mortality(White et al., 2016).

The relationship between DS and incident LBP in middle-aged and older adults is complex and inconsistent. This significant association may depend on the methods used to measure DS and the covariates considered(Pinheiro et al., 2016). Evidence from systematic reviews and meta-analyses supports the long-term association between DS and LBP. Overall, the link between DS and the development of LBP over time is generally positive, regardless of statistical significance. This suggests that individuals with DS are at a higher risk of developing LBP (Pinheiro et al., 2016; Pinheiro et al., 2015; Wong et al., 2022). This relationship might be bidirectional(Yang et al., 2023); for example, LBP could lead to DS(Felicio et al., 2022), while DS is also a risk factor for LBP(Pinheiro et al., 2015). In addition, the view of the comorbid existence of DS and LBP is also widespread(Pinheiro et al., 2016) partly because both conditions share the same brain regions, such as the amygdala and dorsal raphe nucleus, suggesting a shared basis of DS and chronic pain(Zhu et al., 2021).

Many studies investigating the relationship between DS and incident LBP have been constrained by their approach to evaluating DS. These studies typically use binary variable (depressed or not) or continuous variable representing the total score from validated scales like the Center for Epidemiological Studies Depressive Scale (CES-D) (Andresen et al., 1994). This definition of DS does not identify the contribution of specific DS and distinct patterns of DS to LBP. In addition, the coexistence of specific DS within individuals is very common. This is important as specific DS and patterns of DS might have a greater impact on the development of LBP. There's no evidence suggesting that all specific DS equally contribute to LBP(Fried and Nesse, 2015). Thus, it's still unclear whether specific DS and distinct patterns of DS pose different levels of risk for incident LBP. Therefore, it remains unknown whether specific DS and distinct patterns of DS confer differential risk for incident LBP.

To date, the effect of specific DS and patterns of DS on incident LBP is still unclear, which prompted us to examine the longitudinal association between specific DS and patterns of DS and incident LBP among middle-aged and older Chinese adults. We hypothesized that individuals with different DS status, DS scores by quintile, specific DS, and distinct patterns of DS would have different risks of developing incident LBP. Whether the effect of DS on the risk of incident LBP differed by age and sex was examined because existing evidence supports that DS and incident LBP are more common in female and middle-aged and older female adults, respectively(Vlaeyen et al., 2018; Wu et al., 2019).