Self-reported physical activity predicts pain inhibitory and facilitatory function

Abstract

The prevalence estimates of chronic pain among adults in the United States may be as high as 40%, affecting approximately 100 million adults (15). Pain increases physical disability (32), reduces quality of life, and is costly to both the individual experiencing pain and the nation (15). Alarmingly, a recent study reported that the national cost of pain exceeds the cost of the nation’s priority health conditions (e.g., cardiovascular disease, neoplasms, endocrine, nutritional and metabolic diseases), with costs ranging from $560 to $635 billion annually (15). Clearly, a need exists for effective methods to prevent and treat chronic pain. A rapidly growing body of evidence suggests that exercise may be a viable means to aid in the prevention of chronic pain and reduce ongoing pain symptoms in chronic pain populations. Indeed, data from observational studies (23), randomized controlled trials (19), and laboratory studies suggest a relationship between levels of physical activity and chronic pain (12, 26). For example, RCTs demonstrate that systematic aerobic exercise reduces pain symptoms and improves physical function in multiple chronic widespread (19) and regional pain conditions (17). Additionally, recent laboratory studies have showed that subjective and objective measures of physical activity are negatively related to suprathreshold pain sensitivity of painful heat stimuli in FMS patients (13, 26). A few studies have examined the relationship between pain sensitivity and physical activity in healthy adults. Most recently, Ellingson and colleagues found that greater vigorous physical activity as measured by accelerometers was significantly related to reduced pain intensity and unpleasantness ratings to noxious thermal stimuli in healthy, younger women (12). Similarly, Adrzejewski et al. revealed that pressure pain thresholds at a variety of skeletal muscles sites were higher in younger adults who reported engaging in vigorous physical activity compared to those who reported participation in only moderate physical activity (3). Anshel and Russel demonstrated that an aerobic training intervention increased pressure pain tolerance compared to a control group with no exercise training (4). While these studies suggest a link between physical activity and pain sensitivity, it is not known whether levels of physical activity are related to the functionality of pain modulatory processes. Importantly, several studies have shown that regular physical exercise exerts beneficial effects on several biological mediators (e.g., serotonin, endogenous opioids) of pain inhibition and facilitation (1,14,34). Pain is modulated by complex endogenous systems that both facilitate and inhibit pain. Alterations in the function of these systems have been implicated in multiple chronic pain conditions (24) and in older adults (31). Several sophisticated tests of pain modulatory mechanisms exist in the pain literature. Dysfunction of pain facilitation has often been assessed by the method of temporal summation. This procedure consists of the administration of short-duration repeated noxious stimuli of a constant intensity and measuring the consequent increase in pain as an indirect method of evaluating sensitization of the central nervous system (11). Endogenous pain inhibition has typically been assessed by a “pain inhibits pain” model termed conditioned pain modulation (CPM) or a model called offset analgesia (offset). CPM is the central inhibition of pain in a local area by a second pain that can be experienced anywhere in the body (38). Offset is an inhibitory temporal sharpening mechanism characterized by a pronounced reduction in perceived pain intensity evoked by slight decreases in noxious temperatures compared to those of equal magnitude increases (16). To date, no studies have investigated whether individuals who are more physically active exhibit enhanced descending pain modulatory function using the tests of temporal summation of pain, CPM, or offset analgesia. The purpose of this study was threefold. We sought to determine whether self-reported levels of physical activity in healthy adults predicted 1) pain sensitivity to heat and cold stimuli, 2) pain facilitory function as tested by temporal summation of pain, and 3) pain inhibitory function as tested by CPM and offset analgesia. We hypothesized that participants who reported relatively greater levels of physical activity would exhibit less pain sensitivity to thermal stimuli, reduced temporal summation of pain, and greater inhibition of pain via CPM and offset analgesia.