This group includes adults aged 18-65 years without any chronic or acute pathology, no regular medication that could alter autonomic nervous system function, and no low back pain in the past six months.

This group includes adults aged 18-65 years diagnosed with chronic low back pain lasting at least three months, with a minimum pain intensity of 3/10 on the Numerical Pain Rating Scale, and without uncontrolled neurological or cardiac disorders.

This variable represents a time-domain measure of heart rate variability, reflecting parasympathetic nervous system activity and short-term variability in R-R intervals. It is calculated as the root mean square of differences between successive R-R intervals. rMSSD is an established metric to assess vagal modulation and is sensitive to changes in autonomic function.

A non-linear metric derived from the Poincaré plot of heart rate variability, indicating short-term variability in R-R intervals. It is closely associated with parasympathetic activity and vagal modulation.

A non-linear metric derived from the Poincaré plot, representing long-term variability in R-R intervals. SD2 is inversely related to sympathetic activity and reflects the balance between sympathetic and parasympathetic modulation.

This variable is calculated as the reciprocal of SD2, multiplied by 1000. It reflects the level of sympathetic activation, where higher Stress Score values indicate increased stress and reduced parasympathetic activity.

This variable represents the ratio of Stress Score to SD1. It reflects the balance between sympathetic and parasympathetic modulation, with higher values indicating sympathetic dominance, commonly observed in chronic pain conditions.

Represents the average heart rate (beats per minute) recorded during the data collection period. It serves as a general indicator of autonomic activity, providing insights into the balance between sympathetic and parasympathetic modulation.