Muscular Synergies and Behavioral Adaptations of Gait in Pain Anticipation (SYAMAPP)

The purpose of this study is to determine whether the anticipation of pain influences gait control and psychophysiological responses in healthy individuals. Participants will receive a neutral cream described either as potentially painful (experimental group) or inert (control group), and their behavioral and physiological adaptations will be measured during walking tasks.

Pain anticipation can significantly alter motor behavior and physiological responses, even in the absence of actual pain. While previous research has established that conditioned expectations can modify gait patterns, muscle activation, and autonomic responses, the specific locomotor adaptations triggered by pain anticipation in healthy individuals remain insufficiently understood. This study examines how manipulating expectations about a neutral cream influences gait control and psychophysiological markers during walking tasks.

Participants will be recruited through university announcements including posters and emails, with eligibility determined via an online screening questionnaire and verbal confirmation the day of the experience. Healthy adults aged 18-35 who meet the inclusion criteria will participate in a single 90-minute experimental session. Random assignment will place participants into either an experimental group, where they receive the neutral cream with verbal suggestions that it may cause localized pain, or a control group that receives identical application with neutral instructions.

The experimental protocol consists of two primary walking assessments conducted both before and after cream application. First, participants perform free walking trials on a GAITRite mat to measure baseline spatiotemporal parameters including walking speed, cadence, and step length at a comfortable speed until completion of 30 gait cycles. Second, treadmill walking and running trials using a Zebris system evaluate gait transitions through incremental speed changes from 0.5 m/s to 4.1 m/s in increments of 0.4 m/s with 10 second familiarisation time in-between measurements. Both walking tasks are recorded using GoPro cameras to subsequent analysis of changes in joint amplitudes. Throughout these tasks, physiological monitoring captures muscle activity via surface EMG, cardiac responses through ECG and heart rate variability measurements, and electrodermal activity as an indicator of autonomic arousal.

Following the first movement assessments and before the application of the cream, participants complete standardized questionnaires evaluating pain perception using the Brief Pain Inventory and visual analog scales (VAS), fear of movement through the Tampa Scale of Kinesiophobia and Fear-Avoidance Components Scale, and pain catastrophizing via the Pain Catastrophizing Scale.

The cream is applied at the level of the knee, between the two lateral epicondyles of the femur with a 4 cm width, and another VAS is filled in after application. The walking tasks are repeated, measuring the same physiological signs and three more VAS's are administered: one during the treadmill walking task, at the end of the 1.3 m/s speed recording, another during the free walking task, after half of the completed gait cycles, and a final one at the end of the protocol.

The primary outcome measure focuses on changes in the walk ratio, calculated as step length divided by cadence, while secondary outcomes examine alterations in kinematic patterns, muscle activation strategies, and autonomic nervous system responses.

Eligibility Criteria:
Inclusion Criteria:

* Aged 18-35 years
* 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 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