Regression analysis
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REGRESSION
The association between the preoperative pain-related psychological factors and MVPA and SB at 6 and 12 months after surgery was analysed with a linear model regression analysis using the PROC MIXED procedure with an unstructured covariance matrix, performed in SAS version 9.4 (SAS Institute, Cary, NC, USA). This procedure accounts for the correlated nature of within-person repeated measures and uses all available data to estimate model parameters and is particularly suitable for longitudinal data where not all participants have the same number of observations.A total of six regression models were performed using this procedure. Three models had relative time spent in SB at 6 and 12 months post-surgery as the dependent variable, with each model including one of the predictors separately. The remaining three models had relative time spent in MVPA at 6 and 12 months post-surgery as the dependent variable, with each model also including one of the predictors separately. The confounders were the same for all regression models. The regression models were controlled for the preoperative value of the dependent variable (e.g. the models with relative time spent in MVPA at 6 and 12 months as the dependent variable were controlled for preoperative time spent in MVPA). The regression models’ assumptions were evaluated by checking multicollinearity between independent variables, as well as assessing the linearity between predictors and outcome, normality of variance and homogeneity of variance. A predictor with a p-value ≤ 0.05 was considered statistically significant.