Fig. 1

Path diagram of the causal mediation model with a three-variable system. In Path a, PA (independent variable) has a significantly positive relationship with HRV (mediator) in a multiple linear regression model. In Path b, PA (independent variable) was shown as an independent predictor for long-term cardiac death and all-cause mortality (outcome variables) in the multivariable Cox regression model without HRV. In Path c, both PA (independent variable) and HRV (mediator) remained significant to predict the outcome variables using the multivariable Cox regression model. Thus, this mediation analysis confirmed the partial mediation function of HRV. CI: confidence interval; HRV: heart rate variability; HR: hazard ratio; PA: physical activity. * each additional 1% increase in PA. # each additional 1 ms increase in HRV