Muscle Synergies in Pain and Pain Anticipation

Participants will be recruited through university announcements. Eligibility will be determined by a screening process that assesses prior experience with capsaicin cream. Healthy adults aged 18-35 who meet the inclusion criteria will be enrolled and assigned to experimental Group ("Prior Experience"): Individuals who report a prior painful experience with capsaicin cream.

Participants will complete a first session consisting on a walking assessment on a treadmill. The speed will be incrementally increased from 0.8 m/s to 1.1 m/s. Throughout this task, muscle activity will be recorded via electromyography (EMG), and ground reaction forces will be captured by force platforms embedded in the treadmill.

Following the initial walk, participants will complete a set of standardized questionnaires to establish baseline scores for pain perception (Visual Analog Scale, VAS), fear of movement (Tampa Scale of Kinesiophobia, TSK), and pain catastrophizing (Pain Catastrophizing Scale, PCS).

Then, a capsaicin cream will be applied to the knee, specifically in the area between the lateral epicondyles of the femur, in a 4 cm wide band. Participants will immediately fill out the VAS to report any pain sensations. The walking assessment is repeated, measuring the same physiological signals (EMG and ground reaction forces).

The participants will come to the laboratory a second time to measure the same outcomes but, instead of applying capsaicin again, they will receive an inert cream presented as capsaicin (neutral massage cream MEDICAFARM). The procedure will be repeated.

The primary objective is to determine whether a prior painful experience with capsaicin causes individuals to generalize that pain memory to a neutral sensation, leading to different outcomes than in those who only have a negative expectation. We will specifically investigate the magnitude of this effect on muscle synergies.

Inclusion criteria

• Aged 18-35 years old
• Healthy participant with no self-reported diagnosed neurological, musculoskeletal, or cardiovascular disorders
• Body Mass Index (BMI) < 30
• Ability to walk unaided for at least 10 minutes without major physical limitations

Exclusion criteria

• Presence of pain at the time of testing
• Self-reported diagnosed condition affecting mobility
• Known allergy to any component of the study cream
• Allergy to EMG electrode adhesives
• Inability to stand unassisted for more than 1 minute
• Inability to walk/run at the required speed and distance
• Weight exceeding 150 kg (due to treadmill limitations)
• Recent intake (prior intervention) of:
• Analgesics (within 6 hours)
• Cigarettes (within 6 hours)
• Caffeine (within 2 hours)
• Skin lesions at the cream application site
• Pregnancy
• Failure to understand instructions
• Daytime pain reported by participant on day of experimentation