Fig. 3

A simulated 2-state pain trajectory, based on the HPA modeling. The cortisol level values obtained from HPA model are classified into two different groups, and they present high-state and low-state patterns. One can use the higher cortisol level to model the high intensity pain, and the lower level to represent low-pain or no-pain. We first take a daily average of cortisol level (24 hourly data points). A subjective threshold (0.1345 in this model; around the average of extreme cortisol values) of cortisol level is used to separate the first 100 days into two groups: high-pain day (labeled in the high group as “cort = 0.13463”) for the days when cort 0.1345, or low-pain day when cort 0.1345 (labeled in the low group as “cort = 0.13442”) for trajectory study. The simulated pain trajectory is representative for a LBP episode that has chronic and intermittent pain