Fig. 6

The cortisol trajectory and pain trajectory modeling for a group of 100 patients. a Simulated cortisol level for LBP by the HPA model in a group of 100 patients. We keep all same parameters in the HPA model as in Fig. 4 except Kstress, and we choose 100 randomized values for Kstress. The twenty values are randomly generated by computer, the average Kstress is 0.9922, and the variance is 0.2907. Then we simulated the HPA model for each Kstress. The cortisol trajectories vary among patients, from high-amplitude oscillations and low-amplitude ones during the 100-day time period. Each patient’s cortisol data is presented in a with different color. b Average cortisol level for LBP by the HPA model in a group of 100 patients. We also calculated the average daily cortisol-levels to be used to calculate the group-pain trajectory. The level variance is also marked in the group. For the group, the average and variance become stabilized after 60 days. c Pain trajectory calculated from data in Fig. 4a for 100 days of the patients group. The average cortisol level values obtained from HPA model are classified into three different groups, and they present high pain state, low pain state and no pain (healed) state patterns respectively. We take a daily average of cortisol level (24 hourly data points). Subjective thresholds (0.556 and 0.552 in this model) of cortisol level are used to separate the first 100 days into three groups: high-pain day (labeled in the high-pain group in the top) for the days when cort ≫ 0.556, or healed day when 0.556 > cort ≫ 0.552 (labeled in the healed or no-pain group in the middle), and low-pain for cort < 0.552 (in the bottom) for trajectory studies. The simulated pain trajectory is representative for a group of LBP patients who recovered from acute pain in various severities