Current public health recommendations state that all healthy adults should perform moderate-intensity aerobic activity for a minimum of 150 min·wk−1 or vigorous-intensity aerobic activity for a minimum of 75 min·wk−1 or an equivalent combination in bouts of at least 10 min to promote and maintain health (30). Although it is widely accepted that a physically active lifestyle is important for a variety of health-related outcomes, this recommendation is based primarily on reducing risk for chronic disease and disability. At present, less is known about whether this physical activity prescription is beneficial for other outcomes such as pain.
Pain is an increasing public health concern affecting up to 50% of adults at any given time (15). It is a major symptom associated with many medical conditions and is the most common reason for doctor visits in the United States (5). Furthermore, living with chronic pain is associated with significant psychological distress, and pain has been shown to interfere substantively with a person’s quality of life and general functioning (10).
In patients with chronic pain, there is some evidence for an inverse relationship between physical activity and pain sensitivity. Research has demonstrated that being physically active is associated with a decrease in symptoms and improvements in function for patients with several chronic pain conditions (4,16,18,23,27). Moreover, recent data from our laboratory show that physically patients with active fibromyalgia (FM) are more able to modulate pain than their less active peers (9,21).
Less is known about the effects of regular physical activity behaviors on pain sensitivity in healthy adults. Research looking at the effects of a single bout of exercise has repeatedly demonstrated hypoalgesia for experimental pain stimuli during and after exercise suggesting that physical activity influences pain sensitivity (17). With respect to exercise training, Anshel and Russell (2) showed that sedentary college-age men who participated in a 3-month aerobic exercise program became more tolerant of pressure pain than those assigned to either resistance exercise or an inactive control group. Thus, increasing aerobic activity may influence pain perception. More recently, Andrzejewski et al. (1) examined differences in pain sensitivity within a group of physically active college students. Participants were classified as either moderate or vigorous exercisers on the basis of their responses to a brief self-report questionnaire, and pressure pain thresholds were assessed at several different sites on the body. Results showed that individuals who reported participating in vigorous physical activity had significantly higher pressure pain thresholds than those who re ported participating in moderate physical activity. Thus, the intensity of activity may be related to the degree of pain sensitivity.
Although these studies suggest a link between physical activity and pain sensitivity, they are limited by several important factors. Both lack an objective measure of physical activity, and the homogeneity of physical activity behaviors in their participant groups limits the generalizability of the data. Further, neither addressed the potential negative effects of sedentary time on pain sensitivity. In addition, these studies only examined the two ends of the pain perception continuum, threshold and tolerance, missing the large range of sensitivity that occurs in between. Lastly, these studies treated pain as a unidimensional construct, focusing solely on the intensity aspect and negating the possibility of seeing a relationship between physical activity and the affective dimension of pain.
The present study was designed to extend upon this previous research by assessing the relationship between pain sensitivity and physical activity behaviors in a sample of healthy women with a range of physical activity behaviors. In particular, we hypothesized that 1) participants who met current physical activity recommendations would be less sensitive to pain than their insufficiently active peers and 2) self-report and accelerometer measures of moderate and vigorous activity would be inversely related to both pain intensity and affective ratings to noxious heat and 3) sedentary time would be positively related to these pain ratings.
METHODS
Participants.
Participants were recruited as part of a larger study aimed at determining potential mechanisms of exercise-induced hypoalgesia and were paid $60 for completion of the study. Eligibility criteria included 1) being female, 2) between the ages of 20 and 45, 3) absence of any chronic pain conditions, 4) absence of a current diagnosis of depression, and 5) not currently taking medications that would affect pain sensitivity (e.g., opioids, antidepressants, and cardiovascular medications). Twenty-four healthy women met the criteria and were invited to participate. Three of these declined because of scheduling conflicts, and 21 completed testing procedures.
Procedures.
The institutional review board at the University of Wisconsin-Madison approved all procedures, and all participants signed informed consent documents. Participants reported to the Exercise Psychology Laboratory at the University of Wisconsin for two visits separated by a 7-d physical activity monitoring period. Before the first visit, participants were asked to abstain from nicotine for 2 h, caffeine for 4 h, and alcohol, structured exercise, and pain medications for 24 h. A verbal check was done at the beginning of each session, and all participants indicated they had complied with pretesting instructions.
During the first visit, participants completed several questionnaires to assess mood, health, and well-being including the POMS, the State–Trait Anxiety Inventory, and the Short Form-36 Health Survey (22,26,32). Psychophysical heat pain testing was then conducted to assess each participant’s range of pain sensitivity. Participants were exposed to a total of 14 thermal stimuli presented in a random order and separated by 1-min intervals. Stimuli consisted of seven temperatures, each presented twice, ranging from 43°C to 49°C in 1°C increments for 10 s each. The baseline temperature for all heat testing procedures was 35°C, and temperature increased at a rate of 8°C·s−1. All heat stimuli were applied to the thenar eminence (the part of the palm just below the thumb) of the right hand using a Medoc TSA-2001 thermal sensory analyzer with a 900-mm2 Peltier thermode (Medoc Advanced Medical Systems, Minneapolis, MN). After each heat stimulus, participants were asked to rate the pain intensity and pain unpleasantness of each temperature using the Gracely Box SL category ratio pain rating scales (0–20) (12). Standardized instructions were given to all participants for use of the rating scales. Briefly, they were instructed that pain intensity is defined as how much the stimulus hurts and that unpleasantness is defined as how bothersome the stimulus is and that they should attempt to differentiate these two dimensions of pain. They were also instructed that each number on the scales represents a category of sensation that is ordered according to its intensity or unpleasantness and that the verbal anchors (e.g., “mild pain,” “slightly distressing”) should be used to help determine the level of pain intensity or unpleasantness experienced in response to each heat stimulus.
After completion of pain sensitivity testing, participants were issued an ActiGraph™ GT1M accelerometer (Pensacola, FL) to objectively measure physical activity. The ActiGraph™ is an electromechanical device designed to measure acceleration forces generated by the movement of the wearer. The monitor is attached to an elastic belt and worn on the hip. Study participants were asked to wear the accelerometer during waking hours, with the exception of water-based activities, for the 7 d in between their first and second testing sessions. Participants were also asked to complete a daily physical activity log including wake-up time, the time(s) the monitor was put on and taken off during the day, any time spent participating in water-based physical activity such as swimming, and bedtime. A verbal check was done on the second day of testing to confirm that the information on the log sheet was complete and accurate.
The second visit occurred 1 wk after the first and included return of the accelerometer and completion of the long form of the International Physical Activity Questionnaire (IPAQ). The IPAQ is a widely used and validated 7-d recall questionnaire designed to assess physical activity in a variety of domains including leisure time, domestic and gardening, work-related, and transportation-related physical activities during 1 wk (3).
Data processing and analysis.
Data were processed and analyzed as follows. Standard criteria for inclusion of accelerometer data were applied—at least 10 h of valid wear time for a minimum of three weekdays and one weekend day (29,31). In-house software was used to process this information to calculate the average number of minutes per day spent doing sedentary, light, moderate, and vigorous physical activity. Cutpoints for accelerometer counts per minute were as follows: sedentary = <100, light = 101–760, moderate = 761–5724, and vigorous = >5725 (11,19,20). Data from the IPAQ were used to calculate MET-minutes per week spent in the domains listed above as well as minutes spent doing moderate-intensity (3–6 METs) and vigorous-intensity (>6 METs) activities on the basis of the standardized scoring guidelines (available at www.ipaq.ki.se/ipaq.htm). Average pain intensity and unpleasantness ratings were calculated for each of the seven temperatures presented during heat pain testing.
All statistical analyses were conducted using SPSS 19.0 (IBM Corp., Armonk, NY). To address the potential association between meeting current physical activity recommendations and pain sensitivity, participants were classified into two groups: meets recommendations and insufficiently active. Independent-samples t-tests were conducted to assess group differences in mood, health, and well-being and physical activity measures. Group differences in pain ratings for intensity and unpleasantness were analyzed using separate 2 × 7 repeated-measures ANOVAs with group and temperature as independent variables and average pain intensity and unpleasantness ratings as dependent variables. Effect sizes (Cohen d) were also calculated to determine the magnitude of the group differences in pain ratings at each temperature. Lastly, to address the relationship between physical activity/sedentary time and pain sensitivity, correlation coefficients (Spearman ρ) were calculated between average minutes per day spent in moderate, vigorous, and sedentary behaviors and average pain intensity or unpleasantness ratings. Spearman ρ was used because of the nonnormal nature of the physical activity data. Significance was set at α = 0.05 for all analyses.
RESULTS
Twenty-one women (mean age = 30.0 ± 5.8 yr) completed all testing procedures. Of the 21 women, 10 were married and 18 were employed at least part-time. All participants were college educated and reported being in good health at the time of testing (see Table 1 for participant characteristics and Table 2 for physical activity data). Twelve participants met current physical activity recommendations, and nine did not. There was not a significant difference in wear time for the accelerometer between groups. There were no significant group differences in mood or health-related assessments, with the exception of vitality, which was significantly higher in those meeting current physical activity recommendations (P < 0.0001). However, vitality was not significantly related to any of the primary outcome variables (P > 0.05).
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TABLE 1: Participant characteristics.
TABLE 2: Physical activity behaviors.
In partial support of our primary hypothesis, results from the repeated-measures ANOVAs demonstrated that those who met physical activity recommendations during the 7-d physical activity monitoring period had significantly lower pain unpleasantness ratings in response to noxious thermal stimuli than their insufficiently active peers (F1,19 = 7.30, P = 0.014; Fig. 1). There was not a significant difference for average pain intensity (F1,19 = 3.14, P = 0.092). However, effect sizes were moderate to large for both intensity and unpleasantness for all temperatures above 43°C (Table 3). Correlational analyses demonstrated a significant relationship between vigorous physical activity measured with accelerometry and both pain intensity (ρ = −0.44, P = 0.02) and pain unpleasantness (ρ = −0.52, P = 0.01; Fig. 2). There were no significant relationships between pain ratings and accelerometer measures of moderate or sedentary activity or any of the self-reported physical activity variables (Table 4).
TABLE 3: Effect size calculations demonstrating the magnitude of group differences in pain ratings at each temperature.
TABLE 4: Physical activity and pain sensitivity.
FIGURE 1: Pain intensity and unpleasantness ratings at each temperature for participants who met recommendations compared with those who did not.
FIGURE 2: Relationship between accelerometer measures of average minutes of vigorous physical activity per day and average pain intensity and pain unpleasantness ratings. Average pain intensity and unpleasantness ratings represent the overall average across all seven temperatures presented.
DISCUSSION
Our results provide preliminary evidence that meeting current physical activity recommendations is associated with a decreased sensitivity to pain in women. In support of our primary hypothesis, participants who met this prescription were less sensitive to pain than their insufficiently active peers, demonstrated by their significantly lower pain unpleasantness ratings. Although there was not a statistical difference in pain intensity, moderate to large effect sizes for both dimensions of pain sensitivity suggest that the differences were meaningful. Results from the correlational analyses suggest that participation in vigorous physical activity accounted for the association between meeting guidelines and decreased sensitivity to pain. Lower intensity activities did not seem to be related to pain in this sample.
Previously, Anshel and Russell (2) showed that untrained men who participated in an aerobic exercise training program became more tolerant of pain. More recently, Andrzejewski et al. (1) found that self-reported participation in regular vigorous physical activity was related to higher pain thresholds. Our results extend upon these studies by demonstrating that a relationship exists between physical activity and pain sensitivity in a sample of young to middle-age women with a variety of physical activity behaviors. These results are based on accelerometer measures of physical activity, and consequently, graded intensities of physical activity and sedentary time could be objectively quantified and compared. Further, we assessed pain sensitivity using standard psychophysical methods. This allowed us to examine relationships between physical activity behaviors and a range of pain stimuli and to determine that participation in vigorous activity was associated with pain-related benefits, particularly a reduction in the affective dimension of the pain experience.
Vigorous physical activity is often painful (8). Reduced affective responses to pain stimuli may be an adaptation to repeated muscle pain exposure, resulting in a preserved ability to sense and discriminate pain but reducing the distressing or unpleasant qualities of the stimulus. Different relationships between pain intensity and unpleasantness and physical activity behaviors are biologically plausible given the separate pathways involved in processing the sensory (spinothalamic tract) and affective (spinomesencephalic tract) dimensions of pain (33). Thus, physical activity may influence affective pain networks (e.g., anterior cingulate cortex, insula, periaqueductal gray) preferentially over sensory pain networks (e.g., thalamus, sensory and motor cortices) (25). However, recent neuroimaging data from our laboratory suggest that both sensory and affective pain networks are related to physical activity behaviors (9,21). Moreover, the present study found moderate to large effect sizes for comparisons of ratings between physical activity groups for both intensity and unpleasantness. Consequently, it is likely that we had insufficient power to detect the effect for intensity due to our small sample size.
Contrary to our hypothesis, sedentary behavior did not seem to have a deleterious effect on pain sensitivity. Although the negative consequences of a sedentary lifestyle have been widely reported (13,28), to our knowledge, this is the first study to investigate the relationship between sedentary behavior and pain. It is possible that pain sensitivity is unrelated to sedentary behavior. However, our sample for this study consisted of a group of healthy women with a relatively large amount of time spent in moderate and vigorous physical activity and lower amounts of sedentary time than much of the general US population (24). Consequently, these results may not hold true for a more sedentary group with less overall physical activity. Additional study is warranted to see if pain sensitivity is related to sedentary time in individuals with a more sedentary lifestyle.
Contrary to our hypothesis, our self-report measure of physical activity was also unrelated to pain sensitivity. A recent study comparing the long form of the IPAQ and accelerometer data in a population-based sample of healthy adults found only low to moderate correlations between these two physical activity measures (14). The authors concluded that although the IPAQ is a valid measure of physical activity, it likely overestimates moderate and vigorous activity and underestimates sitting time. This is consistent with our results showing higher amounts of physical activity and lower amounts of sitting time than the accelerometer data demonstrates. The lack of relationships between self-reported physical activity and pain sensitivity suggests that actual rather than perceived levels of physical activity are important for pain processing and highlight the importance of using objective measures of physical activity when possible in physical activity research.
LIMITATIONS AND CONCLUSIONS
This study included a relatively small sample of healthy women. Consequently, results cannot necessarily be generalized to men or to those who are experiencing chronic pain conditions. Moreover, the cross-sectional nature of the study design does not allow for causality to be determined. Although it is possible that participation in vigorous physical activity leads to a decrease in sensitivity to pain, it is also possible that those who are less pain sensitive choose to participate in higher intensity physical activities. Studies with a larger sample of both male and female participants and with a prospective design that includes manipulation of physical activity behaviors are needed to better determine the nature and direction of the relationship between physical activity and pain.
Results from this study have many potential applications including aiding our understanding of why exercise functions as a treatment for those with chronic pain conditions and providing a rationale for including physical activity assessment in pain research. Exercise has increasingly been prescribed as a way to relieve symptoms and avoid disability associated with conditions such as fibromyalgia and chronic low back pain (4,6). However, the mechanisms that underlie symptom improvement with exercise are unknown. Future research aimed at assessing the effect of exercise training on chronic pain could include psychophysical pain assessment to determine whether changes in pain sensitivity to experimental pain stimuli are associated with symptom improvement. For studies examining pain sensitivity in healthy individuals, assessing and accounting for physical activity behaviors using objective measures of physical activity are warranted, especially when subgroup comparisons are of interest (e.g., gender differences). Further, in studies comparing patients with chronic pain and healthy individuals, matching controls on physical activity is important to ensure that results reflect differences attributable to the disease state that are not simply a consequence of deconditioning (7).
In summary, our study shows that meeting physical activity recommendations is associated with a reduction in pain sensitivity. In addition to the well-established exercise-induced hypoalgesic response, the data suggest that regular physical activity may have a cumulative effect on central pain processing. If confirmed, these results would add to the list of numerous benefits associated with leading a physically active lifestyle.
There was no funding received for this study.
There were no conflicts of interest during this study for any of the authors.
The results of the present study do not constitute endorsement by the American College of Sports Medicine.
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