The main goal of this project is to assess the reliability of thoracic spinal stiffness. To achieve this, 25 heartily participants (no significant thoracic pain in the past year) and 25 participants with chronic thoracic pain will participate in 2 experimental sessions separated by 24 to 48 hours. These sessions will occur at the Neuromecanic and Motor Control Laboratory of the Universit\u00e9 du Qu\u00e9bec \u00e0 Trois-Rivi\u00e8res (Trois-Rivi\u00e8res, Qu\u00e9bec, Canada). During these sessions, spinal stiffness will be measured at four thoracic spinal levels (T5 to T8) in a randomized sequence via a mechanical device using a servo-linear motor. The device indenter (metallic cylindrical cover with a rubber of 2 cm diameter) will gradually apply 45 N to the spinous process. The muscle activity and displacement data will be recorded during the procedures.