*3.2. Sex Differences in the Association between Fear-Avoidance and Pain Intensity and the Influence of Affective States*

The crude association between fear-avoidance and pain intensity had a coefficient of 0.17 (95% CI: 0.12, 0.22), *p* = 0.000, indicating that an increase in fear-avoidance of 1 unit was associated with an average pain increase of 0.17. The model that also included the interaction between fear-avoidance and sex showed that the association differed between men and women: the interaction term had a coefficient of 0.18 (95% CI: 0.05, 0.31), *p* = 0.005 (Table 2; model 2). For men, a 1-point increase in fear-avoidance was associated with a −0.02 decrease in pain intensity, whereas for women a 1-point increase in fear-avoidance was associated with a 0.18 increase in pain intensity (Table 2 and Figure 3).

**Table 2.** Unadjusted and adjusted model regarding sex differences in the association between fearavoidance and pain intensity.


Dependent variable: pain intensity; CI = confidence intervals; <sup>a</sup> Adjusted for baseline variables: age, lower leg and the emotions: relaxed \*\*\*, irritated \*\*\* and satisfied \*\*\*; \*\* *p*-value < 0.01; \*\*\* *p*-value < 0.001.

**Figure 3.** Visualization of the interaction between fear-avoidance and sex in relation to pain intensity. Note: the grey area is the 95% confidence interval of the estimates.

In the subsequent model, potential confounders were added. Backward stepwise elimination resulted in a model with the baseline variables 'age' and 'lower leg' and affective experiences 'relaxed', 'irritated', and 'satisfied' included (Table 2; model 5, and Figure 4). By adding the confounders, the association between fear-avoidance and pain intensity in the model with the interaction did not change considerably from a coefficient of −0.02 (95% CI: −0.12, 0.09) with a *p*-value of 0.78 to a coefficient of −0.04 (95% CI:−0.14, 0.05) with a *p*-value of 0.48. Although these three affective experiences all had a significant association with pain intensity, the estimate of the interaction term between fear-avoidance and sex did not change by adding affect to the model (Table 2; model 5).

**Figure 4.** Visualization of the fluctuation of pain intensity, fear avoidance and affective experiences for men and women separately. Note: fear-avoidance and the affective experiences are measured on a 7-point Likert scale and pain intensity is measure on the eleven-point numeric rating scale. Each line represents the average of the 10 beeps per day per variable.
