Pain Perception and Exercise-Induced Hypoalgesia Across Circadian Rhythms

Exploring Pain Perception Through Exercise-Induced Hypoalgesia Across Circadian Rhythms for Healthy Participants

The goal of this randomized crossover clinical trial is to investigate whether exercise-induced hypoalgesia (EIH) is influenced by circadian rhythm in healthy adults aged 18-40 years.

The main questions it aims to answer are:

• Does the time of day (morning vs. evening) influence changes in pain perception following exercise?
• Do chronotype, sex, or stimulation site (bony vs. muscular) alter the magnitude of this effect?

Participants will:

• Complete questionnaires
• Undergo quantitative sensory testing
• Attend both morning and evening sessions in randomized crossover order

Exercise-induced hypoalgesia (EIH) is a temporary reduction in pain sensitivity following physical activity. High-intensity functional training (HIFT) has been shown to produce EIH, yet the influence of circadian rhythms on this response is not well established. Circadian timing regulates physiological processes, including pain modulation, and may contribute to variability in exercise-related analgesia.

This randomized crossover trial will examine whether time of day (morning vs. evening) affects EIH in healthy adults. It will also explore the relationship between an individual's chronotype, the efficacy of EIH and the difference in magnitude between men and women, the difference in effect between bony and muscular landmarks for pressure stimulus application.

Null Hypothesis (H0) : Circadian rhythm has no significant influence on the efficacy of exercise-induced hypoalgesia (EIH), as measured by changes in pain perception before and after high intensity functional training at different times of day (morning vs. evening).

Alternative Hypothesis (H1): Circadian rhythm significantly influences the efficacy of exercise-induced hypoalgesia (EIH), with differences in pain perception before and after high intensity functional training depending on the time of day (morning vs. evening).

Each participant will complete both morning and evening sessions in randomized order, separated by ≥72 hours. Participants who completed the morning sessions will cross over to the evening sessions and vice versa. Within each session, they will perform a 12-minute supervised deep breathing control condition and a 12-minute HIFT intervention, separated by a 30-minute rest.

Inclusion Criteria:

• Healthy participants
• age 18 to 40 years
• good level of English that allow the enrolled participant to provide informed consent and complete the questionnaires provided
• being categorized as "moderate" in the International Physical Activity Questionnaire (IPAQ)

Exclusion Criteria:

• inability to report pain intensity reliably
• consumption of narcotics or tobacco products
• regularly taking pain medications and analgesics
• pregnant or planning to get pregnant
• surgery within the last 12 months
• acute or chronic pain conditions
• uncontrolled hypertension
• cardiovascular diseases, pulmonary diseases, neurological diseases with significant changes in somatosensory and pain perception, metabolic diseases, serious systemic diseases or conditions that restrict normal daily activities, inflammatory conditions (e.g. rheumatoid arthritis), or orthopedic injuries
• intolerable pain during the pain perception test
• serious psychiatric conditions (e.g., schizophrenia and bipolar disorder) or psychological disorders (e.g., depression) that may affect pain thresholds (16).
• Severe sleep disorders and circadian rhythm disturbances
• BMI > 30
• serious fatigue or delayed-onset muscle soreness (9) at least 24 h after exercise sessions or a sudden change in activity for more than one week