Evaluation of Psychophysical Responses After Mechanical Stimulation: a Reliability Study

Pain is a global health problem involving physiological, psychological, and contextual dimensions that complicate its objective and reproducible evaluation, even in well-controlled environments. Acute mechanical pain, particularly pain associated with surgical incisions or cutting lesions, is of special clinical relevance. It constitutes a central component of postoperative pain and is frequently observed in non-suicidal self-injury contexts. Specifically, in individuals with Borderline Personality Disorder (BPD), over 80% report the use of cutting as an emotional tension regulation strategy. Given the complexity of evaluating this phenomenon, standardized Quantitative Sensory Testing (QST) is essential to determine the state of the somatosensory system in both patients and experimental models. QST measures the intensity at which a stimulus produces a specific sensory perception. However, traditional mechanical QST modalities (e.g., von Frey filaments, pinprick) have a critical limitation: strong stimuli can provoke actual tissue damage. To address this, a non-injurious sharp blade model was developed, allowing the reproduction of the initial sensory characteristics of incisional pain without causing injury (such as sharp pain, primary, and secondary hyperalgesia). While this model has been successfully tested in healthy volunteers and BPD patients, its manual application depends heavily on the operator, leading to involuntary fluctuations in speed, force, and duration. To solve this, a robotic system (Poux et al., 2025) can apply these controlled mechanical stimuli, increasing precision, controlling the applied force, and allowing exact synchronization with electrophysiological recordings. The recent incorporation of an interchangeable head featuring the non-injurious blade model enables the first-ever exploration of non-injurious sharp mechanical stimulation applied via robotics.

The primary objective of this protocol is to evaluate the reliability of psychophysical responses to non-injurious sharp stimuli applied by the robotic system.

The sample size was determined based on the desired precision for estimating the repeatability of the system. Specifically, the objective was to estimate the within-subject standard deviation. The precision with which we can estimate the standard deviation depends on both the number of subjects N and the number of observations per subject M (Bland, 2010) with a 95% confidence interval and a margin of error of ±25% was proposed, from which it is determined that the participation of N = 31 volunteers and M = 2 observations per volunteer is required. To account for a potential 5% dropout rate, the final recruited sample size is N = 33. The volunteers undergo two identical sessions: a baseline session and a re-evaluation session, separated by an interval of at least 48 hours.

During each sessions, the following procedures will be conducted:

Pain Catastrophizing Scale (PCS): A 13-item self-administered questionnaire (scored on a 0-4 Likert scale) to measure negative and exaggerated coping related to pain experiences, specifically assessing hopelessness, magnification, and rumination.

Muscle Stiffness: Non-invasive evaluation using a portable device (MyotonPRO) applying brief (15 ms) mechanical stimuli to automatically calculate muscle tone, stiffness, and elasticity in the forearm.

Pinprick Pain Threshold: Measured using a customized system

Robot-Controlled Blade Thresholds: The robotic system will apply the non-injurious blade (4 mm x 0.1 mm) to the ventral forearm at a constant speed of 5 mm/s. Following a familiarization stage, participants will use a push-button to indicate their Pain Threshold (when the tactile sensation becomes distinctly painful) and Tolerance Threshold (when the pain becomes unbearable or reaches the 5000 mN safety limit).

Conditioned Pain Modulation (CPM): After establishing baseline blade pain thresholds, participants will submerge their dominant hand up to the wrist in a cold water bath maintained below 4°C for up to two minutes. During immersion, participants will continuously rate their pain on a computerized Visual Analog Scale (VAS) from 0 to 100. Immediately after withdrawing the hand, the robotic blade thresholds will be re-evaluated to measure the descending pain inhibition system.

Inclusion criteria

• Age between 18 and 60 years.
• No history of chronic pain, neuromuscular disorders, or any disease that affects the nociceptive system and prevents its evaluation under normal conditions.
• Willingness and ability to fully understand the content and scope of the experiment and to comply with its instructions.
• Having signed the informed consent.

Exclusion criteria

• Pregnancy.
• History of chronic pain, neuromuscular disorders, or any disease that affects the nociceptive system and prevents its evaluation under normal conditions.
• History of addictive behavior, defined as abuse of alcohol, cannabis, opioids, or other drugs.
• History of mental illnesses.
• Presence of fever, tuberculosis, malignant tumors, infectious processes, or acute inflammatory processes.
• Use of analgesics within 24 hours prior to participation in the experiment.
• Skin lesions, scars, or dermatological conditions in the area of stimulus application.
• Lack of cooperation.