Exercise and Pain in Non-Hispanic Blacks and Whites

Racial discrepancy in the experience of pain is well documented within the literature. Non-Hispanic Blacks (NHB) report greater amounts and severity of clinical pain compared to non-Hispanic Whites (NHW) in a variety of clinical conditions. NHB also experience greater pain-related symptoms and disability, regardless of descriptive factors such as age, gender, and socioeconomic status or clinical factors such as comorbidities and duration of disease. Further, NHB tend to be medically undertreated for pain, regardless of nearly identical reporting of pain level and severity compared with NHW. Not only is adequate pain treatment infrequent, but many pain treatments are less effective in NHB than in NHW, commonly resulting in less reduction of pain severity in this population.

In comparison to NHW, NHB report greater sensitivity to several types of pain, including heat pain, cold pressor pain, ischemic pain, and electrical stimulation. NHB show increased temporal summation, implying central nervous system hypersensitivity and hyperexcitability to noxious (painful) stimuli. Specifically, regarding experimental heat pain, NHB differ minimally in pain threshold or ratings of pain intensity compared to NHW, but perceive greater unpleasantness of pain and show a lower pain tolerance. The physiological mechanisms underlying this racial discrepancy remain unknown.

Experimental pain assessment is valuable in understanding the experience of pain between individuals. Pain is an individual experience and differs from one to another. However, differences in experimental pain perceived may estimate the clinical pain one would experience. Studies show that greater experimental pain sensitivity is associated with greater experience of day-to-day pain in healthy adults and can predict higher levels of clinical pain in adult patients.

While affective (subjective) pain perception has been the focus of the majority of pain assessment studies, in 2008 Campbell et al. investigated the nociceptive flexion reflex (NFR), an involuntary and objective component of the pain response. The NFR is a spinal-mediated reflex that follows activation of nociceptive A-delta afferents by noxious stimuli. This study shows the NFR threshold in NHB is decreased compared to NHW, implying a physiological difference between these racial groups that may contribute to the discrepancy in pain perception. Skin blood flow via vasodilation, another physiological measure, has been shown to increase in response to noxious thermal stimuli in order to distribute heat away from a singular point in the cutaneous environment and prevent damage. Assessing changes in skin blood flow in response to noxious thermal stimuli may further elucidate physiological differences between races that affect pain perception.

It is important to investigate more effective prevention or treatment methods for pain in NHB. Exercise-induced hypoalgesia (EIH) is characterized by decreased pain sensation following physical exercise. Specifically, EIH leads to decreased sensitivity to noxious (painful) stimuli, increased pain thresholds, increased pain tolerance, and decreased pain ratings. EIH has been shown to occur following several types of exercise, such as acute bouts of submaximal, continuous aerobic exercise and acute bouts of isometric hand-grip exercise. Lending to the knowledge of racial discrepancy in pain perception, a study by Umeda et al. in 2016 showed NHB had a smaller magnitude of EIH compared to NHW following isometric hand-grip exercise, but racial differences in EIH remain incompletely understood.

It is suggested that hypoalgesia following exercise may be due to an overlapping of pain perception and cardiovascular mechanisms. For example, pain regulation and blood pressure control are associated with the same brain stem nuclei. Further, hypertensive individuals have shown reduced sensitivity to noxious (painful) stimuli compared to normotensives, and acute pharmacological elevations in blood pressure have been shown to alter pain perception. Therefore, it is important to investigate alterations in pain perception following both acute (which should elicit elevations in blood pressure) and chronic (which should elicit overall declines in blood pressure) exercise.

Several studies have investigated the effects of submaximal exercise on various types of pain and it is typical to see moderate-intensity continuous (MIC) exercise incorporated into pain management programs (generally 30 minutes of 70% VO2max exercise). However, two studies in particular display data indicating that as workload increases, pain thresholds increase in a stepwise manner. Other studies show EIH occurs more consistently following exercise at higher intensities (> 70% VO2max), indicating that high intensity interval training (HIIT) may produce more beneficial results in affective and/or objective responses to pain than MIC exercise. Several studies have looked at affective pain perception following acute submaximal bouts of aerobic and isometric exercise. Those that have investigated acute HIIT have only examined affective responses. The investigators do not know of any studies that have looked at the acute and chronic effect of MIC exercise and HIIT on affective and objective pain perception in NHB and NH There are three main purposes of this study, and they are as follows: 1) to assess objective and subjective responses to pain in NHB and NHW, 2) to assess the use of acute versus chronic exercise in pain management for NHW and NHB, and 3) to assess the use of MIC exercise versus HITT in pain management for NHW and NHB. The investigators hypothesize that 1) NHB will show decreased skin blood flow response to noxious stimuli compared to NHW prior to exercise training, 2) NHB will report greater pain ratings than NHW prior to exercise training, and 3) chronic HIIT will mitigate the differences in skin blood flow and pain ratings between NHB and NHW.