Impact of Aerobic Exercise on Pain Modulation in Healthy Adults

Background:

Pain and Its Burden Pain is a prevalent and complex health condition that significantly impacts individuals, families, and society as a whole. Of the 39.4 million adults experiencing pain most days and persisting for over three months, nearly two-thirds report it as "constantly present," with over half describing it as "unbearable and excruciating". Pain can be categorized into physiological and pathological types. Nociceptive and inflammatory pain are physiological responses that serve protective and adaptive functions, whereas pathological pain is considered non-protective and maladaptive.

Central Mechanisms in Pain Modulation Laboratory-based quantitative sensory testing (QST) evaluates sensory function using psychophysical methods, teasing out components of the nervous system that demonstrate loss or gain of sensory function contributing to pain. Pain-specific QST is categorized into static QST and dynamic QST. The static measure of QST is believed to provide simple and unidirectional assessments of static pain sensitivity in local regions and provide insight into peripheral mechanisms. Static QST measures include the determination of thresholds (pain thresholds and pain tolerance) and rating the intensity of the quantified stimulus. Pain processing is a complex system, and static QST provides one point on the scales of sensations. In addition, static QST is limited in capturing the pain modulatory ability of the central nervous system. Therefore, to overcome this limitation, dynamic QST measures come into play.

Dynamic QST provides insight into pain modulatory mechanisms such as pain facilitation or inhibition, or the extent to which the nervous system enhances or suppresses the pain. Dynamic QST assesses the central mechanism by temporal summation (TS) and conditioned pain modulation (CPM). TS assesses the heightened pain perception towards an identical repetitive noxious stimulus (heat or pressure) applied with consistent intensity and frequency (lower than 3 seconds). TS represents pain facilitation and is a behavioral measure of activity at the dorsal horn of the spinal cord. The CPM refers to the endogenous pain modulatory system that assesses the effects of the nociceptive system through the decrease in pain from one stimulus (test stimulus (thermal or electrical or tactile) to one part of the body due to the application of a second pain stimulus (conditioning stimulus (hot or cold bath or ischemia) applied at a distant another body part. CPM represents a pain inhibitory process and is a proxy measure of descending pain inhibition through the spino-bulbar- spinal loop. CPM may be a valuable tool in directing mechanistic-based approaches for treating painful conditions. For example, individuals with painful diabetic neuropathy and poorly functioning CPM have more significant analgesia when given duloxetine, a medication that augments conditioned pain modulation. Additionally, identifying and treating the source of pain is associated with positive changes in CPM. Healthy individuals demonstrate more efficient CPM when compared with people in pain and a systematic review of adults with chronic pain revealed that CPM efficiency improves after a reduction in clinical pain. For example, total hip arthroplasty for treating painful osteoarthritis reduces joint pain and improves post-operative CPM. Collectively, these studies suggest that the CPM test has the potential to predict risk and treatment outcomes for chronic pain, therefore CPM may be considered an essential treatment moderator and mediator. The activation of central mechanisms, particularly endogenous inhibitory systems, is proposed as a fundamental process underlying various pain management interventions. Descending pain inhibitory systems, which are part of central mechanisms, play a crucial role in mediating the analgesic effects of interventions such as manual therapy. Manual therapy stimulates afferent neuronal inputs that activate the central nervous system's descending pathways to inhibit pain. Conditioned Pain Modulation (CPM) shares a similar mechanism, inhibiting pain through descending modulation pathways.

Role of Aerobic Exercise in Pain Inhibition The impact of exercise on pain reduction is well-documented, with initial investigations and subsequent evidence in both animal and human studies. Exercise has proven effective in managing pain associated with various chronic conditions, including fibromyalgia, chronic neck pain, osteoarthritis, rheumatoid arthritis, and chronic low back pain. Additionally, it has been shown to prevent chronic low back pain (CLBP) recurrence and mitigate the progression of chronic musculoskeletal pain conditions. This pain-reducing effect, known as Exercise-Induced Hypoalgesia (EIH), involves several underlying mechanisms. The endogenous opioid system is the most extensively studied mechanism explaining EIH. Another proposed mechanism involves endocannabinoids, which modulate pain through an inhibitory feedback loop that reduces glutamate release at nociceptive synapses. It has been reported that increases in circulating endocannabinoids following exercise were associated with reduced pain perception, as evidenced by pressure pain thresholds (PPT) and decreased thermal heat temporal summation. Acute aerobic exercise has also been shown to enhance Conditioned Pain Modulation (CPM) analgesia in healthy individuals. Studies suggest that higher CPM efficiency correlates with greater EIH. For example, it is found that active participants exhibited higher EIH and more effective CPM after 15 minutes of cycling at 75% VO₂max, compared to inactive participants. Similarly, it has been observed that enhanced CPM analgesia in athletes versus non-athletes, further linking CPM efficiency to physical activity levels. This study aims to investigate the effects of repeated high-intensity aerobic exercise on CPM analgesia in non-athletic populations, addressing gaps in understanding how exercise-induced pain modulation operates beyond athletic contexts.

Research Gaps and Future Directions:

Although Exercise-Induced Hypoalgesia (EIH) and its relationship with pain sensitivity have been extensively studied, several critical gaps remain. Specifically, the mechanisms underlying the modulation of pain through repeated high-intensity aerobic exercise in non-athletic populations are not fully understood. Furthermore, the role of psychosocial factors, such as optimism, pain catastrophizing and marginalization, in mediating central pain modulatory responses (CPM) has received limited attention. Future research should explore the long-term effects of high-intensity exercise perceived as painful on central pain pathways in clinical population and how these effects differ across diverse sociocultural contexts. Investigating these interactions will advance our understanding of pain processing and provide foundational data for developing personalized interventions targeting chronic pain management.

Specific Aims:

Aim 1: To determine the extent to which repeated exposure to high-intensity aerobic exercise modifies pain sensitivity.

Hypothesis: Pain sensitivity will improve (less sensitive) in the multiple exercise session group compared to the single exercise session group.

Aim 2: To explore the mediation effect of sociocultural factors on the relationship between repeated high-intensity aerobic exercise exposure and pain sensitivity.

Hypothesis 2: Sociocultural factors will mediate the relationship between repeated high-intensity aerobic exercise exposure and pain perception.

Impact and Significance:

This investigation into pain inhibitory mechanisms has the potential to significantly advance our understanding of chronic pain pathophysiology and identify novel therapeutic targets. By elucidating the neurobiological processes underlying pain perception and modulation, this study addresses critical clinical questions regarding changes in pain sensitivity and the influence of psychosocial factors on pain perception following nervous system training. The primary innovation of this proposal lies in its comprehensive assessment of central pain mechanisms through QST in conjunction with continuous stimulation via high intensity aerobic exercise. This approach, combined with the evaluation of sociocultural constructs such as ethnic optimism, pain catastrophizing, and marginalization, offers a unique perspective on pain processing. By bridging the gap between neurophysiological mechanisms and sociocultural influences on pain, this research has the potential to significantly impact both our theoretical understanding of chronic pain and clinical practice in pain management. The insights gained could inform more targeted and effective interventions, ultimately improving the quality of life for individuals suffering from chronic pain.

Research Plan:

The study in this proposal will be based on psychophysical data and self-reported questionnaires using methods approved by the University of Texas El Paso Institutional Review Board. This prospective experimental study involves healthy participants who will undergo baseline assessments, including psychological questionnaires and psychophysical approaches to measuring individual sensitivity and endogenous pain modulation. Immediately after an intervention period, participants will be reassessed. All assessments will be performed by an evaluator who will be blind to psychological measure data and group assignment.

Participants:

Forty individuals will be recruited by posted fliers approved by the IRB on the University of Texas El Paso campus and the El Paso community.

Inclusion criteria: a) pain-free; b) between 18-50 years old. Exclusion criteria: Participants will not be excluded based on race or gender but will be excluded if they meet any of the following a) non-English speaking; b) systemic medical condition is known to affect sensation; c) regular use of prescription pain medication to manage pain; d) current or history of chronic pain condition; e) currently using blood thinning medication; f) any blood clotting disorder such as hemophilia; g) any contraindication to the application of ice or cold packs, such as uncontrolled hypertension, cold urticaria, cryoglobulinemia, paroxysmal cold hemoglobinuria, and circulatory compromise; h) involved in vigorous physical activities like heavy lifting, digging, aerobics or fast bicycling i) Any lung or respiratory condition that impairs exercise capacity, including but not limited to asthma.

Recruitment and Informed Consent Procedure:

Prospective participants who express interest in the study will be encouraged to contact the research team directly to initiate the screening process. This preliminary screening will include the administration of the Physical Activity Readiness Questionnaire (PAR-Q: 32) to determine eligibility for participation.

The Principal Investigator or a designated Co-Investigator will conduct the screening either via telephone or email, following an Institutional Review Board (IRB)-approved script. This interaction will serve to provide potential participants with a comprehensive overview of the study, including its purpose, procedures, potential risks, and benefits. Additionally, the investigator will address any questions or concerns the participant may have. As part of the screening, the PAR-Q: 32 will be administered in the form of a series of structured questions designed to assess the participant's readiness for physical activity and ensure their suitability for the study. This questionnaire may be completed verbally over the phone or through written communication via email. Participants are not required to disclose specific details regarding which questions apply to them. Instead, they only need to inform the investigator if one or more of the questions are relevant to their condition. This approach maintains participant privacy while ensuring that only eligib