It is well established that participation in regular resistance exercise (RE) results in significant improvements in muscular strength and muscular hypertrophy, which can, in turn, enhance physical functioning and human performance (13,20). Additionally, with growing recognition of its potential therapeutic value, RE is frequently incorporated as an important aspect of many physical therapy and orthopedic rehabilitation programs. Pain experienced in response to exercise can impede the progress of well-designed training programs, and such adverse responses also may undermine one's motivation to adopt and maintain the exercise prescription. Accordingly, developing a more comprehensive understanding of the pain sensitivity in response to RE participation is an important consideration in the design of both exercise training and rehabilitation programs.
Mounting empirical evidence suggests that exercise is associated with alterations in the perception of pain (16,23). Specifically, findings from this line of inquiry suggest that single episodes of high-intensity (≥75% of maximal aerobic capacity) aerobic exercise are accompanied by a hypoalgesic response (i.e., diminished pain sensitivity) shortly after the cessation of activity. This hypoalgesic response has typically been characterized by increases in pain thresholds (i.e., the point at which an individual perceives a stimulus to be painful) and increases in pain tolerances (i.e., point at which an individual is not willing to further endure noxious stimulation). In addition to the favorable alterations in pain threshold and pain tolerance, ratings of pain intensity have also been found to be lower after higher-intensity episodes of running (7) and cycling (19). Much of the extant research has focused on changes in pain perception after acute aerobic exercise (7,16,19,23). However, given that many individuals are unable or unwilling to engage in high-intensity aerobic activity, it is important to determine whether alternative modes of exercise can also alter the perception of pain.
Unfortunately, knowledge of the extent to which other forms of physical activity, such as RE, influence pain sensitivity is limited at the present time. Few studies have examined alterations in pain perception after acute RE, and findings of this research are equivocal. For example, Bartholomew et al. (1) found that although pain tolerances increased significantly after RE, pain thresholds remained unchanged. By contrast, Koltyn and Arbogast (17) found that acute RE resulted in significant postexercise increases in pain thresholds and reductions in pain ratings. Although it is unclear why the results for pain thresholds were different between the 2 studies, it is possible that methodological differences between the 2 studies may explain some of the inconsistencies. Different RE protocols, for example, were used in the 2 studies. The RE in the Bartholomew et al. (1) study consisted of 20 minutes of circuit weight training at a self-selected intensity, whereas the RE in the Koltyn and Arbogast study (17) consisted of 45 minutes of lifting 3 sets of 10 repetitions at 75% of each individual's 1-repetition maximum (1RM). In addition, it seems that exercise was performed throughout the day in both studies, and it is unclear whether the time of day that exercise was performed in these studies had an influence on pain sensitivity.
Many physiological variables shown to interact with pain sensitivity (e.g., blood pressure, substance P, beta-endorphin) have been found to exhibit a circadian rhythm (21). Additionally, diurnal variations in mood states associated with altered pain sensitivity have also been reported (3). Consistent with these findings, it has been hypothesized that there may be diurnal rhythms in pain perception as well. However, research examining diurnal variations in experimentally induced pain have yielded inconsistent results (18). Because of the limited number of investigations examining hypoalgesic responses to RE and the mixed findings evident in studies of diurnal variations in pain perception, additional research is needed to clarify these relationships. If acute RE results in diminished pain sensitivity, it is important to determine whether this hypoalgesic response is dependent on the time of day that exercise is performed. Determining what factors may influence alterations in pain sensitivity accompanying acute exercise participation has important implications for the design of rehabilitative exercise programs. Research examining diurnal variations in the perception of pain in response to acute RE could foster a more comprehensive understanding of how to effectively design and deliver rehabilitative RE sessions. Therefore, the purpose of this investigation was to examine changes in pain perception after acute RE and also to determine whether alterations in pain sensitivity are influenced by the time of day that RE is performed.
Experimental Approach to the Problem
To examine differences in pain perception after acute RE, 21 recreationally trained men who volunteered for the study were asked to complete a 1RM testing session and 2 subsequent bouts of RE. Each RE session consisted of completing 3 sets of 10 repetitions at 75% 1RM for 4 exercises. The 2 RE sessions were performed at different times of day on separate days. Specifically, 1 RE session was performed between 6:00 and 8:00 am, and the other RE session was completed between 6:00 and 8:00 pm. Assessments of pain threshold and pain ratings were obtained during a 2-minute exposure to a pressure-pain stimulus immediately before and after (1 and 15 minutes post exercise) RE performed in the morning and evening. Additional physiological (blood pressure) and psychological (state anxiety) variables previously shown to interact with pain sensitivity were also assessed before and after acute RE performed in the morning and evening.
Twenty-one recreationally trained men (mean = 21.4 years; SD = 2.5) volunteered to participate in the present study. Each participant was physically active and reported regular RE participation during the past year. The sample size provided an a priori statistical power of 0.80 to detect differences in pain perception responses after RE performed in the morning and evening (24) based on an alpha value of 0.05, a high correlation among the repeated measurements (r = 0.80), and the expectation of a medium effect size (Cohen d) accompanying the hypoalgesic effect of acute exercise as documented in previous research (17,19). Before participation in the study, each volunteer read and signed an informed consent document that had been previously approved by the university's human subjects institutional review board.
Each volunteer was requested to refrain from caffeine or alcohol consumption for at least 12 hours and to abstain from exercising for 24 hours before participating in the RE sessions. Adherence to these procedures was verified by self-report at the beginning of each RE session. On arriving at the laboratory for the initial testing session, the participants completed a battery of questionnaires, which included a 24-hour history questionnaire that assessed participants' sleeping, activity, and eating behaviors during the previous day; a medical history questionnaire; the State-Trait Anxiety Inventory (STAI ); the Morning Evening Questionnaire (14); and the Eysenck Personality Questionnaire (EPQ ). The state anxiety and select personality characteristics (i.e., extraversion and neuroticism) tapped by the EPQ were included because it has been reported that pain perception is, in part, mediated by psychological factors (22). After the completion of the questionnaires, each participant completed a 1RM test for the seated chest press, leg press, torso-arm pull-down, and overhead press exercises. All exercises were performed using Med-X resistance training machines (Med-X, Ocala, Fla). The maximum tests were conducted using previously validated procedures (26) and were used to calculate the 75% 1RM workload used during the submaximal RE sessions.
After the 1RM testing session, participants completed 1 RE session between 6:00 and 8:00 am and 1 session between 6:00 and 8:00 pm. The RE sessions were presented in a counterbalanced order, and bouts were completed on separate days at least 72 hours apart to allow for adequate recovery. Each RE session lasted approximately 45 minutes in duration and comprised 4 exercises that were completed for 3 sets of 10 repetitions at 75% 1RM. Exercises were performed in the following order: leg extension, torso-arm pull-down, chest press, and overhead press. Repetitions were completed using deliberate, controlled motion of the load. To ensure appropriate form on each repetition, exercise technique was monitored by a trained investigator. A 2-minute recovery interval was maintained between each exercise and also between each set throughout the RE sessions. The load, volume, and rest intervals incorporated in this RE protocol were specifically designed to be consistent with commonly employed contemporary resistance training guidelines for enhancing muscular strength and hypertrophy.
Pain thresholds and pain ratings were assessed immediately before and after (1 and 15 minutes) each RE session. The pain stimulus consisted of applying pressure to the forefinger for 2 minutes using the Forgione-Barber pressure pain stimulator. The Forgione-Barber stimulator produced experimentally induced pressure pain through the application of a 3000-gm force to the middle digit of the left forefinger. Previous research has demonstrated that this procedure produces a painful sensation but does not cause short-term or long-term tissue damage or trauma (17,19,22). Selected physiological (blood pressure) and psychological (state anxiety) variables that have been previously linked with altered pain sensitivity were also assessed before exposure to the pain stimulus, before and after RE.
It is important to acknowledge that control sessions were not included in the present study. Although consideration was given to the inclusion of control sessions in the morning and evening, results from 2 previous studies directly comparing the influence of acute exercise and control conditions on experimentally induced pressure pain revealed that hypoalgesia occurred after exercise but did not occur after quiet rest (17,19). Therefore, given that (a) the primary purpose of the present study was to directly compare time-of-day differences in the hypoalgesic response to acute RE, (b) previous findings have demonstrated that quiet rest control conditions do not alter sensitivity to experimentally induced pain, and (c) it was desirable to minimizing the amount of pain testing that participants completed, a control condition was not included in this study design.
Pain threshold was defined as the amount of time that elapsed from the initial exposure of the Forgione-Barber pressure pain stimulus to the point at which the participant perceived the stimulus to be painful. Measurement of pain threshold was obtained by instructing the participants to press a button attached to a timing device at the exact moment when they begin to perceive the stimulus to be painful. The amount of elapsed time was recorded by the timing device that was concealed from the participants' view throughout the session. This procedure has been shown to be a valid and reliable index of pain sensitivity in previous research (15,17-19).
Participants rated the intensity of the pain stimulus at 15-second intervals during the 2-minute exposure to the pressure pain stimulus using a 0-10 pain rating scale (5). The even numbers of the scale had the following verbal anchors: 0 = no pain, 2 = uncomfortable, 4 = very uncomfortable, 6 = painful, 8 = very painful, 10 = extremely painful (almost maximum). Participants were instructed to rate the intensity of the pain they felt using whole or half numbers from the scale. Pilot research indicated that this pain rating scale correlated significantly with other pain intensity measures including a visual analog scale (15) (r = 0.85) and Borg's (4) CR10 pain rating scale (r = 0.95). Additionally, this method for assessing pain ratings in response to acute exercise has been shown to demonstrate acceptable validity and reliability in previous research (17,19).
Blood pressure was measured in the right arm with a Marshall Arteriosound Automated Blood Pressure Monitor. Two measures were obtained consecutively at each pre- and postexercise assessment interval, and the average of these 2 assessments was recorded as the outcome value that was used in data analysis.
State anxiety was also measured at each assessment interval before and after an RE session using the 20-item state scale of the STAI (27). The SAI has well-established psychometric properties and has been used extensively in prior acute exercise research.
Personality Traits and Morningness-Eveningness Orientation
Selected personality traits proposed to interact with pain perception were also assessed. Specifically, the EPQ (8) and Morningness-Eveningness Questionnaire (MEQ ) were employed. The EPQ is a 90-item scale that taps 3 personality traits of extraversion, psychoticism, and neuroticism. The EPQ has been shown to demonstrate adequate psychometric properties (8). The MEQ is a 19-item scale that assesses an individual's general orientation and/or preference for morning and evening activity. Total scores on the MEQ subsequently correspond to 1 of 5 morningness-eveningness types: definite morning, moderate morning, neither, moderate evening, or definite evening.
Separate repeated-measures analyses of variance (ANOVAs) were used to test for differences in the outcomes after RE in the morning or evening. Specifically, pain threshold, blood pressure, and state anxiety were analyzed with 2 (time of day: morning and evening) × 3 (time: baseline, post-1, and post-15) repeated-measures ANOVAs. Pain ratings were analyzed with a 2 (time of day: am and pm) × 3 (time: baseline, post-1, and post-15) × 8 (ratings: every 15 seconds) repeated-measures ANOVA. The traditional F tests were computed using the Hunydt and Feldt conservative degree-of-freedom adjustments when the assumption of sphericity was violated. Tukey post hoc analysis was used to determine the location of significant mean differences. Effect sizes (d) accompanying selected mean differences were obtained by dividing the mean difference by the pooled SD. Finally, bivariate correlation analyses were conducted to examine the relationship between changes in pain threshold and selected physiological and psychological outcomes.
Descriptive statistics are presented in Table 1 as mean ± SD and in Figures 1 and 2 as mean ± SE. Results from the MEQ indicated that, of the 21 participants, 3 were classified as “definite evening” types, and 7 participants were “moderate evening” types. Additionally, 11 participants were identified as “neither” type, and no participants were classified as “definite morning” or “moderate morning” types.
Analysis of pain threshold revealed a significant trial main effect (p < 0.003). Post hoc analysis revealed that, when collapsed across time of day, pain threshold increased significantly (d = 0.52) from baseline to 1 minute after RE. However, pain threshold reported 15 minutes post exercise was not significantly different from the baseline value (d = 0.16). Alterations in pain sensitivity previously have been linked with changes in blood pressure and anxiety (7,8). Accordingly, bivariate correlation analyses were conducted to examine the relationship between changes in pain threshold and changes in blood pressure and state anxiety. The results demonstrate that changes in pain threshold were not significantly related to changes in either systolic blood pressure (r = 0.17; p > 0.26) or state anxiety (r = −0.22; p > 0.15) at 1 minute post exercise. Additionally, the relationships between pain threshold and selected personality variables assessed by the EPQ (14) were also examined. The results revealed that neither extroversion (r = 0.26; p > 0.18) nor neuroticism (r = −0.08; p > 0.87) were significantly correlated with baseline pain threshold. Results for pain threshold are summarized in Figure 1.
Analysis of pain ratings yielded a significant rating main effect (p < 0.001) as well as a significant trial × rating interaction (p < 0.025). Post hoc analyses demonstrated that, when collapsed across time of day and trial, pain ratings increased during the 2-minute exposure to the pressure pain stimulus. Decomposition of the trial × rating interaction revealed that pain ratings, when collapsed across time of day, decreased significantly (d = −0.40) from baseline to 1 minute after RE. However, pain ratings reported 15 minutes post exercise were not significantly different from baseline values (d = −0.13). The results for pain ratings are summarized in Figure 2.
Analysis of systolic blood pressure yielded a significant trial main effect (p < 0.001). Post hoc analysis revealed that, when collapsed across time of day, systolic blood pressure increased significantly from baseline (d = 0.68) to 1 minute after RE. Analysis of diastolic blood pressure yielded a significant trial main effect (p < 0.03). Post hoc analyses demonstrated that, when collapsed across time of day, diastolic blood pressure decreased significantly relative to the baseline value at 15 minutes after RE. The results for systolic and diastolic blood pressure are summarized in Table 1.
Analysis of state anxiety revealed that the main effects for time of day (p > 0.11) and trial (p > 0.66), as well as the time of day × trial interaction (p > 0.86), were nonsignificant. The state anxiety results are summarized in Table 1.
Determining what factors may influence alterations in pain sensitivity accompanying acute exercise participation has important implications for the design of rehabilitative exercise programs. The findings of the present investigation demonstrate that acute RE results in similar postexercise hypoalgesic responses irrespective of whether the exercise is performed in the morning or evening. Specifically, compared with baseline values, pain threshold was significantly higher and pain ratings were significantly lower at 1 minute after RE performed in both the morning and the evening. The observation of diminished pain sensitivity is consistent with results reported by Koltyn and Arbogast (17), who have documented a hypoalgesic response after acute RE. However, the results of this study also extend previous findings by demonstrating that hypoalgesia after RE is not influenced by the time of day that RE is performed.
Although hypoalgesic responses to RE did not differ as a function of the time of day when exercise was performed, it should be recognized that these results can only be generalized to the times of day employed in the present investigation. The participants in this study completed RE between 6:00 and 8:00 in the morning and between 6:00 and 8:00 in the evening. These times were chosen to enhance ecological validity because many individuals involved in regular exercise training frequently exercise either before or after completing their usual daily tasks. Furthermore, the exercise sessions were limited to times when individuals would be customarily awake. It is possible that diurnal variability may be observed if different times of day were studied, and future studies should examine whether pain sensitivity after exercise is influenced by other testing times. Additional research is also needed with other groups of subjects, including individuals who are characterized as “morning” types. None of the individuals who participated in this study were characterized as “morning” types. Three of the participants were identified as “definite evening” types, 7 participants were “moderate evening” types, and 11 participants were identified as “neither” type. Therefore, the results from this study cannot be generalized to individuals who are characterized as “morning” types.
Although an increase in pain threshold and a decrease in pain ratings were evident at 1 minute after RE, changes in each measure of pain sensitivity were found to return to baseline levels by the 15-minute postexercise assessment. Therefore, the hypoalgesic response after RE did not persist for as long as changes in pain sensitivity documented in some prior studies incorporating aerobic exercise. For example, Droste and colleagues (7) found that pain thresholds increased during exercise and subsequently demonstrated a gradual return to baseline levels at approximately 1 hour after exercise. It is currently unclear why the time course of the hypoalgesic response in the present study differed from the pattern documented previously. However, it is possible that the continuous nature of aerobic exercise vs. the intermittent nature of RE contributes to the divergent hypoalgesic responses. In support of this contention, previous animal model research indicates that by manipulating whether exercise was intermittent or continuous, it was possible to elicit either naloxone-reversible or naloxone-insensitive hypoalgesia after exercise (11). These findings suggest that there may be multiple analgesia systems (opioid and nonopioid) and that manipulation of exercise session parameters may result in differential hypoalgesic responses.
It also should be noted that elevations in systolic blood pressure were found at 1 minute after RE. There is evidence indicating a potential link between pain sensitivity and blood pressure. For example, the same brainstem nuclei are associated with pain regulation and blood pressure control (6,12). Furthermore, common neurotransmitters (e.g., monoamines) and neuropeptides (e.g., opioids) are involved in both functions (6,12,23). There is also evidence indicating that individuals with hypertension exhibit reduced sensitivity to noxious stimulation relative to normotensives (6), and pharmacological elevations of blood pressure have been found to be associated with alterations in pain sensitivity (2). Blood pressure is typically elevated during exercise; however, only a limited amount of research has been conducted examining the interaction between exercise, hypoalgesia, and blood pressure. Interestingly, although increases in pain threshold and systolic blood pressure were observed at 1 minute post exercise, pain threshold was not significantly correlated with systolic blood pressure. There seem to be conflicting findings in the literature with regard to whether blood pressure is correlated with pain perception. It is unclear why results have been inconsistent, but possible factors that may have contributed to the equivocal findings include 1) the fact that a variety of pain stimuli have been used, 2) variations in the timing of blood pressure and pain perception assessments, 3) various protocols for measuring blood pressure and pain perception (e.g., pain threshold vs. pain tolerance), and 4) a wide variation in sample sizes. Further research is needed to clarify the relationship between blood pressure and pain sensitivity.
Several limitations of the present investigation should be acknowledged when evaluating the results. First, the sample comprised healthy young men who had been engaging in regular resistance training for a year before participating in the study. Accordingly, it would be inappropriate to generalize these findings to other population subgroups or to men differing in health status or resistance training experience. Second, because the present study employed an experimentally induced pressure pain stimulus, it also is unclear whether RE results in similar hypoalgesic responses to other types of experimentally induced pain or whether alterations in the perception of endogenous, naturally occurring muscle pain occur with acute RE. Therefore, although acute RE resulted in a hypoalgesic response to experimentally induced pressure pain in this investigation, it cannot presently be determined whether similar exercise-induced hypoalgesic responses would be observed during exposure to other forms of endogenous or external pain stimuli. Consequently, replication of these findings in more diverse samples and using alternative types of pain stimuli is needed. Similarly, the effects of RE on the perception of clinical pain in individuals burdened with chronic pain (e.g., arthritis) are not known. There is, however, some research indicating possible diurnal patterns in peak pain intensity associated with osteoarthritis and fibromyalgia (2,9), but it is unknown whether alterations in clinical pain after exercise exhibit diurnal variations. Recent evidence also demonstrates that pain sensitivity during experimentally induced pain is significantly related to self-reported clinical pain in osteoarthritis patients (10). In light of this relationship, additional investigations of the effect of acute RE on experimentally induced pain of patients with chronic disease are warranted, and the results of such studies could have valuable implications with regard to determining the efficacy of incorporating RE in the treatment of chronic pain. Finally, in the absence of a control condition, the possibility that experimental artifacts (i.e., regression to the mean, effects of multiple testing, etc.) may have contributed to the observed hypoalgesic responses cannot be discounted. Nonetheless, given that the patterns of change in pain responsivity were not linear across the 3 assessments, it is unlikely that regression to the mean or multiple exposures to the pain stimulus account for the present results. Future studies examining changes in pain perception after RE should incorporate random assignment of participants into both exercise and control conditions to control for the potential influence of such behavioral artifacts.
In summary, it is concluded that acute RE results in diminished pain sensitivity to an experimentally induced pressure pain stimulus, and this hypoalgesic response is observed independent of whether the exercise is performed in the morning or evening. Furthermore, because pain threshold and pain ratings obtained at 15 minutes after exercise were not different from baseline values, the present findings suggest that the hypoalgesic response that emerges after acute RE is relatively transient. Finally, although acute RE also resulted in a postexercise increase in systolic blood pressure, a physiological outcome that has been linked with changes in pain sensitivity previously, pain perception and blood pressure were not significantly correlated in the present investigation. This finding indicates that changes in sensitivity to experimentally induced pressure pain after acute RE may not be dependent on increases in systolic blood pressure. These results underscore the fact that exercise-induced hypoalgesia is a complex phenomenon that may not be attributable to any single underlying determinant, and they also suggest that further research attempting to delineate the mechanisms responsible for alterations in pain sensitivity after acute RE are warranted.
The results of the present study suggest that acute RE performed at 75% 1RM results in alterations in the perception of experimentally induced pressure pain irrespective of whether the exercise session is performed in the morning or evening. Pain perception can influence both acute exercise performance and subsequent motivation to initiate or maintain regular RE. Accordingly, these findings may have important implications for RE prescription. One implication is that acute RE of sufficient intensity can produce meaningful increases in pain threshold and pain tolerance. Participants reported significantly higher pain thresholds and lower pain ratings after acute RE. Therefore, RE may be a valuable mode of exercise to include in structured rehabilitation programs where pain is often a limiting factor in exercise or human performance. However, given that the decreased pain sensitivity observed in this investigation returned to baseline levels within 15 minutes post exercise, practitioners who promote the beneficial pain responses of acute RE among participants and/or patients should also inform them of the potentially transient nature of such acute effects. A second relevant implication is that the favorable changes in pain perception after RE were observed irrespective of the time of day that exercise was performed. Thus, from a prescription standpoint, there presently does not seem to be an optimal time of day to obtain the observed effects, and sessions prescribed during the morning or evening can be expected to be equally effective in producing alterations in the perception of pain.
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