Fig. 4

A simulated cortisol level data and pain trajectory for acute pain. a A simulated cortisol level of acute pain over the 100-day period. Here, we present a case of simulated cortisol dynamics that is with acute pain that quickly subsides in 1–2 weeks. We modify a HPA model with a reduced adreno-corticotrophin hormone degradation rate (Vs4 is reduced from 15 to 0.907 µg days), while others remain at a normal level. The cortisol level quickly returns to a flat level in a, after a few initial oscillations. b Simulated 3-state pain trajectory calculated from simulated cortisol data in a. The 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. Again, we first take a daily average of cortisol level (24 hourly data points). Subjective thresholds (0.23986 and 0.2985 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) for the days when cort ≫ 0. 23986, or healed day when 0.23986 > cort ≫ 0. 23985 (labeled in the healed or no-pain group), and low-pain for cort < 0. 23985 for trajectory studies. The simulated pain trajectory is representative for an acute LBP episode and the corresponding pain trajectory will converge quickly to the healed or no-pain state