HOEGER BEMENT, MARIE K.; DICAPO, JOHN; RASIARMOS, REBECCA; HUNTER, SANDRA K.
After exercise, pain thresholds increase and ratings of pain decrease, and this change in pain perception is known as exercise-induced analgesia. The greatest change in pain perception after exercise with dynamic contractions is associated with high-intensity (60-75% of maximal oxygen uptake) and longer-duration exercise (>10 min) (24). For example, Hoffman et al. (17) measured pain perception after treadmill running for 10 min at 75% maximal oxygen uptake (V˙O2max), 30 min at 50% V˙O2max, and 30 min at 75% V˙O2max. Pain ratings decreased after the high-intensity and longer-duration exercise only (75% V˙O2max for 30 min). Similarly, weight training that involves lifting heavy loads (i.e., high intensity) during dynamic contractions (three sets of 10 at 75% of one repetition maximum) increased pressure pain threshold and decreased pain ratings (25).
The influence of contraction intensity and duration on exercise-induced analgesia is not fully understood for static (isometric) contractions, which are common to postural and ergonomic tasks. For example, pain perception to a noxious thermal stimulus did not change after isometric exercise of short duration at high or low intensities in men and women (12,33). In contrast, pressure pain thresholds in women (men were not tested) increased after low-intensity isometric contractions of the knee extensor muscles (22% of maximal strength) maintained until failure of the task and after isometric handgrip contractions (30% of maximal strength) sustained for 90 s (27,41). Thus, the dose response of isometric contractions is not defined. Taken together, these results indicate that exercise-induced analgesia may be dependent on either the intensity and duration of isometric exercise performed or the type of experimental pain. In this study, we address the dose response of isometric exercise on pain perception to a mechanical noxious stimulus.
Understanding the duration and intensity of isometric contractions on pain perception has the potential to alter how exercise is incorporated into pain management, especially for men and women with limited mobility. Isometric contractions are relatively easy to perform, to prescribe, and to progress. Thus, the dose response of isometric contractions to produce analgesia in healthy individuals is an important step in the potential application to manage chronic pain conditions.
The purpose of the study was to compare pain ratings and thresholds in adults before and after isometric contractions of varying intensity and duration performed with the elbow flexor muscles. Because there are sex differences in pain perception at rest (36) and before and after exercise (26), we assessed the influence of isometric contractions on pain perception in both men and women. Potential mechanisms were addressed by examining blood pressure and anxiety ratings (26). Pain was assessed using a pressure device on the finger of the nonexercising arm indicating that any changes in pain perception may be mediated by changes in the central nervous system or circulating substances acting on peripheral neurons. We hypothesized, therefore, that the largest exercise-induced changes in pain ratings and threshold would be associated with sustained isometric contractions of long duration and low intensity in both men and women because these contractions involve large adjustments in the central nervous system (10,19). Low-force isometric fatiguing contractions, for example, involve greater impairment of motor output within the central nervous system than high-force, short-duration isometric tasks (43).
MATERIALS AND METHODS
Forty men and women (18-42 yr) participated in this study, which included three experiments. Not all of the subjects completed the three experiments due to scheduling difficulties. Fifteen subjects completed all three experiments. All subjects were healthy with no known neurological or cardiovascular diseases and were not familiar with the protocol before participation in the study. Each subject provided written informed consent, and the protocol was approved by the Institutional Review Board at Marquette University.
General Experimental Protocol
Three different experiments were conducted in this study. The first experiment tested the reliability of the pressure pain device and served as a control. The second experiment assessed if the performance of maximal voluntary contractions (MVC) influenced pain perception. The third experiment assessed the influence of submaximal isometric contractions, which varied in intensity and duration, on pain perception.
Reliability of measuring pain perception was determined using a pressure pain device. Pain thresholds (i.e., when the subject reported they first felt pain) and pain ratings, using a 0-10 numerical rating scale, were assessed twice under resting conditions. Sixteen women (21.9 ± 5.5 yr) and 17 men (20.6 ± 0.5 yr) were assessed 30 min apart, during which time each subject sat in a quiet room. Preliminary data indicated that 30 min was likely required between the two assessments because 15 min was not adequate for recovery from the test in pilot subjects (n = 6).
Pain thresholds and pain ratings were measured in 14 women (20.6 ± 1.3 yr) and 13 men (20.4 ± 0.8 yr) before and immediately after performance of three MVC with the elbow flexor muscles. There was a 1-min rest between each MVCs. The MVC task involved a gradual increase in force from zero to maximum over ∼2 s, with the maximal force held for 2-3 s. During performance of the MVC, the force exerted by the elbow flexor muscles was displayed on a monitor, and each subject was verbally encouraged to achieve maximal force. Thirty minutes, during which subjects sat in a quiet room, separated the first pain perception measurement and initiation of the first MVC. Therefore, for experiment 2, the following procedures comprised each session: 1) assessment of pressure pain perception, 2) 30-min rest period, 3) performance ofthree MVC, and 4) reassessment of pressure pain perception.
Pain perception was assessed before and after the performance of three sustained isometric contractions with the elbow flexor muscles that varied in intensity and duration. Eleven women (20.5 ± 1.2 yr) and 11 men (20.4 ± 0.8 yr) participated in this experiment. The three sustained isometric contractions were performed during separate randomized sessions and separated by at least 1 wk. We tested a range of contractions at submaximal intensities and durations to determine the dose effect of static contractions on exercise-induced analgesia. Specifically, the following intensities were chosen because they represented a low- and high-force task and because we were familiar with the performance of these task intensities and the expected mean time to failure in men and women (43). The contractions were as follows: 1) 25% of MVC force sustained until task failure, 2) 25% of MVC force sustained for 2 min, and 3) 80% of MVC force sustained until task failure. Experiment 3 also included measurement of blood pressure, heart rate, and anxiety levels. Therefore, for experiment 3, the following procedures comprised each session: 1) assessment of pressure pain perception, blood pressure, heart rate, and state anxiety levels before and after the isometric contraction and 2) performance of the sustained isometric contraction of the elbow flexors with measurements of heart rate and blood pressure during the contraction. At least 25 min separated the first pain perception measurement and initiation of the isometric contractions. Subjects sat quietly for approximately 30 min to duplicate the 30-min rest period that we established in experiment 1.
For the majority of subjects, the target force for each subject was determined using MVC force values obtained from experiment 2 because the performance of MVC might have influenced pain perception. For those subjects who performed experiment 3 only (n = 7; six males and one female) and not the other experiments, the MVC force used to calculate the target force for the sustained submaximal contractions performed in the subsequent experimental sessions was determined during a separate familiarization session. Reliability of performance of MVC across days is high with less than 5% change in force (21).
The familiarization session involved familiarizing the subjects to the pressure pain device and performing MVCs from which target forces were calculated for the subsequent sessions incorporating sustained submaximal contractions. Subjects were exposed to the pain testing the same way as the subjects in experiment 1, except that reliability was not assessed after 30 min of quiet rest for experiment 3. The MVC were performed during the familiarization session and not during the following exercise sessions to prevent any confounding issues with the changes in pain perception by the performance of the MVC.
State anxiety levels were evaluated using the Spielberg State-Trait Anxiety Inventory (STAI), which has been demonstrated to be a reliable assessment tool (2,40). Anxiety levels were monitored for each subject to determine the association between anxiety and pain perception before and after static contractions in both men and women. The STAI was administered at the start of the experiment and immediately after the two pain tests. We instructed the subjects that the STAI questions pertained to "how you feel right now-at this moment."
Measurement of Force during Isometric Contractions
For experiments 2 and 3, subjects performed isometric contractions with the elbow flexor muscles of the left arm. Subjects were seated upright in an adjustable chair, with the left arm abducted slightly and the elbow resting on a padded support. The elbow joint was flexed to 90° so that the forearm was horizontal. The force at the wrist was directed upward when the elbow flexor muscles were activated voluntarily. Two nylon straps were placed vertically over each shoulder to restrain the subject and to minimize shoulder movement. The hand and forearm were placed in a modified wrist-hand-thumb orthosis (Orthomerica, Newport Beach, CA), which was held in place with Velcro straps. The forearm was placed midway between pronation and supination. The orthosis was rigidly attached with a metal clamp to a force transducer. The forces extended by the wrist in the vertical direction were measured with the force transducer (JR-3 Force-Moment Sensor; JR-3 Inc., Woodland, CA) that was mounted on a custom-designed adjustable support. The forces detected by the transducer were recorded online using a Power 1401 A-D converter and Spike2 software (Cambridge Electronics Design, Cambridge, UK). The force exerted in the vertical direction was graphically displayed on a 19-inch monitor located 1.5 m in front of the subject. The force signal was digitized at 500 samples per second.
Submaximal Isometric Sustained Contractions
During the sustained submaximal isometric contractions, each subject was required to match the vertical target force as displayed on the monitor. For the contractions sustained until failure (25% or 80% MVC), the subjects were verbally encouraged to sustain the force for as long as possible. Task failure was determined when the force declined by 10% of the target value for three out of five consecutive seconds.
Pressure Pain Perception
A pressure pain device, similar to that used in previous exercise-induced analgesia research, was used to measure pain perception (17,26). This device was modified from the Forgione and Barber (13) strain gauge stimulator. A 200-g mass was applied to a second-class lever at a distance from the axis to ensure a 10-N force (equivalent to a 1-kg mass) on the finger. The force at the finger was applied through a Lucite edge (8 × 1.5 mm; Romus Inc., Milwaukee, WI) and was placed on the right index finger midway between the distal and the proximal interphalangeal joints for 2 min (Fig. 1). During the 2-min period, each subject was asked to press a timing device when they first felt pain (i.e., pain threshold represented in seconds) and to rate their pain intensity in 20-s intervals starting at the beginning of the pain test using a numerical rating scale (0-10). Therefore, a higher pain threshold indicated that the subject first reported pain at a later time point than individuals with a lower pain threshold. The scale contained the following anchors: 0 = no pain, 5 = moderate pain, and 10 = worst pain (30). A numerical rating scale was used versus a visual analog scale because subjects did not have the use of their hands to mark on a sheet. Specifically, the pressure pain device was placed on their right index finger, and orthosis was applied to their left forearm to perform the isometric contractions. The subjects were given no feedback of time during the 2-min period of pain measurement.
FIGURE 1-Pressure pa...Image Tools
Mean Arterial Pressure (MAP) and Heart Rate
Blood pressure and heart rate were monitored during the sustained submaximal contractions (experiment 3) using an automated beat-by-beat blood pressure monitor placed around the middle finger of the nonexercising hand (Finapres 2300, Madison, WI). MAP and heart rate were analyzed for the 25% isometric contractions (held to task failure and the 2-min task) only due to a poor signal during recording of many of the 80% MVC tasks. MAP and heart rate recorded during the contractions were analyzed by comparing ∼15-s averages at 25% intervals of the duration (i.e., 0%, 25%, 50%, 75%, and 100%). For each interval, the blood pressure signal was analyzed for the mean peaks [systolic blood pressure (SBP)], the mean troughs [diastolic blood pressure (DBP)], and the number of pulses per second (multiplied by 60 to determine heart rate). MAP was calculated for each ∼15-s average with the following equation: MAP = DBP + 1 / 3(SBP − DBP) (8). MAP and heart rate were not recorded during the application of the pressure pain device during any of the three experiments because during preliminary data collection, subjects reported that it was too distracting to their determination of pain perception.
Data are reported as means ± SD within the text and displayed as means ± SE in the figures. For experiments 1 and 2, repeated-measures ANOVA (multivariate for some variables) was used to analyze pain threshold (trial) and pain ratings (trial × time), with sex as a between-subject factor. For experiment 3, multivariate repeated-measures ANOVA was used to analyze the following variables: pain threshold (task × trial), pain ratings (task × trial × time), and STAI (task × trial × time), with sex as a between-subject factor. Post hoc analysis using paired t-tests was done to assess the exercise-induced change in pain ratings at the six time points (20-120 s) during the 2-min pain test. Additional analysis was conducted with the female subjects using independent t-tests to compare the changes in pain ratings after isometric contractions with the reliability data. MAP and heart rate were not analyzed for the 80% MVC task during experiment 3, so separate repeated-measures ANOVA (mixed design) were used for comparisons across time for MAP during the 25% MVC tasks (start, 25%, 50%, 75%, and end of the fatiguing contraction) with sex as a between-subject factor. Pearson correlations were calculated to determine associations between dependent variables. A level of P ≤ 0.05 was used for statistical significance.
Experiment 1: Reliability
Pain threshold and pain ratings did not change after 30 min of rest (P = 0.67 and 0.25, respectively). All subjects completed the entire 2-min pain test. However, women had significantly lower pain thresholds compared with men during the 2-min application of the pressure pain stimulator (31.5 ± 23.5 vs 54.5 ± 35.9 s; P = 0.04; Fig. 2A) and higher pain ratings (5.0 ± 2.9 vs 3.0 ± 2.8; P = 0.009; Fig. 2B). There was an increase in pain ratings during the 2-min pressure assessment for both men and women (P = 0.0001; Figs. 2B-2D). Furthermore, there was an interaction of trial and sex (P = 0.04) because women reported slightly lower pain ratings in the second trial whereas men had no change (Figs. 2C and 2D). There was no interaction of trial and sex for pain threshold because both men and women had no change in pain threshold after 30 min of quiet rest (P = 0.85).
FIGURE 2-Pain thresh...Image Tools
Experiment 2: MVC
No change in MVC force was observed across the three MVC trials for men and women (men: 304 ± 73, 311 ± 66, and 305 ± 66 N; women: 175 ± 27, 180 ± 29, and 178 ± 33 N) indicating that there was no measurable fatigue across the trials. Men, however, were stronger than the women (P < 0.001).
There was an increase in pain threshold for both men (40.8 ± 26.5 to 46.3 ± 27.1 s) and women (22.2 ± 15.6 to 34.6 ± 27.7 s) after the performance of three MVCs (P = 0.03; Fig. 3A). Pain ratings decreased (P = 0.03) after the MVC for both men and women, with significance at the following time points: 40 s (P = 0.0001), 60 s (P = 0.02), and 80 s (P = 0.02; Fig. 3B). All subjects completed the entire 2-min test. Consistent with the first experiment, there was a main effect of sex. Women reported higher pain ratings (P = 0.005) than men before and after the completion of the MVCs. There was no main effect of sex for pain threshold (P = 0.09). There was no interaction of trial and sex for pain thresholds (P = 0.40) or pain ratings (P = 0.12), indicating that men and women had similar increases in pain threshold and decreases in pain ratings.
FIGURE 3-Pain thresh...Image Tools
Experiment 3: Sustained Submaximal Contractions
Pain threshold and pain ratings.
The greatest increase in pain threshold occurred after the 25% MVC held until task failure exhibited by a task and trial interaction (P = 0.008). Similarly, a large decrease in pain ratings occurred after the 25% MVC held until task failure compared with the other two tasks (25% MVC sustained for 2 min and 80% MVC sustained until failure; task × trial: P = 0.04; Fig. 4). The differences were only significant between the 25% MVC held to task failure and the 25% held for 2 min and failed to reach significance between the 25% and the 80% MVC held to task failure (P = 0.07). All subjects completed the entire 2-min pain test.
FIGURE 4-Average pai...Image Tools
Pain thresholds and pain ratings did not change after the 25% MVC sustained for 2 min (P = 0.09 and 0.38, respectively; Figs. 3C and 3D). There was an increase in pain threshold for both men and women after the 25% MVC held until task failure (P = 0.001; Fig. 3E). Pain ratings decreased after the 25% MVC held until task failure (P = 0.0001) with significance at 40 s (P = 0.008), 60 s (P = 0.0001), 80 s (P = 0.0001), 100 s (P = 0.010), and 120 s (P = 0.02; Fig. 3F). Pain threshold was similar before and after the 80% MVC task (P = 0.26; Fig. 3G). Pain ratings, however, decreased after the 80% MVC task (P = 0.04), with significance at 40 s (P = 0.03) and 60 s (P = 0.02; Fig. 3H).
Further analysis with the women indicated that there was a greater change in pain ratings after the 25% and 80% MVC held until task failure compared with the reliability data (experiment 1; P = 0.043 and 0.046, respectively). The average change in pain ratings for the women after 30 min of quiet rest was 8%, whereas the average change in pain ratings after the 25% and the 80% MVC tasks held until failure was 25% and 23%, respectively.
There was an overall sex effect for pain ratings (P = 0.03), but not pain threshold (P = 0.35), with women reporting higher pain ratings than men. The sex effect in pain ratings was common to all tasks. Although the data failed to show statistical significance, these data suggest that women may have experienced greater exercise-induced decreases in pain ratings than men (trial × sex: P = 0.08). There was no interaction between trial and sex for pain threshold (P = 0.40). Additionally, there was no interaction between task, trial, and sex for the pain ratings (P = 0.41) or the pain threshold (P = 0.26).
Time to Task Failure
Women sustained the 25% MVC held until task failure for a longer duration than men (538 ± 177 s vs 326 ± 69 s, respectively, P = 0.001). Because both sexes sustained the contraction until failure for the 25% MVC, pain perception was assessed at the same magnitude of fatigue (i.e., at task failure). Therefore, the 25% MVC task held for 2 min represented 22% and 37% of the duration for the 25% MVC held until task failure for the women and men, respectively. For the 80% MVC held until task failure, the time to task failure was similar for men (26 ± 6 s) and women (29 ± 10 s, P = 0.38).
There was no association between time to task failure for the 25% MVC task with the change in pain threshold or pain ratings. For the 80% MVC task, there was no association between time to task failure with the absolute or percent change in baseline pain ratings.
There was no change in state anxiety levels after the first pain assessment before all three sustained contractions in experiment 3, indicating that the pain test did not induce measurable changes in anxiety. However, state anxiety levels increased (P = 0.001) after each of the sustained contractions (25% MVC 2 min: 31.2 ± 9.2 vs 32.9 ± 8.6; 25% MVC task failure: 28.6 ± 8.0 vs 31.3 ± 9.4; 80% MVC task failure: 29.1 ± 8.8 vs 31.0 ± 9.8). There was no main effect of sex and no interactions of sex and trial (P > 0.05).
MAP and Heart Rate
MAP and heart rate were analyzed during the 25% MVC sustained for 2 min and the 25% MVC sustained until task failure at 25% time intervals. MAP increased during the 25% MVC sustained for 2 min (i.e., pressor response; P = 0.003). There was a sex effect because men exhibited higher MAP than women during the contraction (P = 0.003). Resting MAP was similar for the men (83.2 ± 12.9 mm Hg) and women (79.4 ± 7.6 mm Hg). The time and sex interaction was not significant (P = 0.09). In contrast, heart rate did not change during the 25% MVC sustained for 2 min.
MAP and heart rate increased during the 25% MVC sustained until task failure (P < 0.0001 and 0.0001, respectively; Fig. 5A). For MAP, there was a main effect of sex because the men had higher MAP than women throughout the fatiguing contraction (P < 0.0001). Additionally, MAP increased at a greater rate for men than women indicated by an interaction of sex and time (P = 0.04; Fig. 5A). Heart rate increased similarly for the men and women during the fatiguing contraction sustained until failure of the task (Fig. 5B). There was no association between the changes in MAP during the 25% MVC sustained until failure and the absolute or percent change in baseline pain threshold or pain ratings. There was, however, a negative correlation between the changes in MAP and time to task failure for the 25% MVC task; those subjects with a briefer time to task failure had a greater change in MAP (P = 0.05).
FIGURE 5-MAP (A) and...Image Tools
The purpose of the study was to compare pain perception of men and women before and after isometric contractions that varied in intensity and duration performed with the elbow flexor muscles. The main findings of this study were the following: 1) pain levels induced by a pressure device on the finger decreased after brief maximal contractions and sustained submaximal isometric contractions (25% and 80% MVC tasks held until failure) for men and women but not for the 25% MVC task held for 2 min; 2) the greatest decrease in baseline (preexercise) pain perception to a pressure stimulus occurred after a low-intensity submaximal contraction sustained to failure; and 3) women exhibited higher pain ratings compared with men before and after isometric contractions.
Pain perception was assessed using a pressure device on the finger of the nonexercising arm over a 2-min interval. Therefore, any analgesia reported after the contractions by the men and women was due to systemic mechanisms influencing peripheral and/or central neural circuits, although previous research demonstrating greater exercise-induced analgesia in the exercising limb compared with contralateral and distal body parts suggests that there are both systemic and local mechanisms involved (28). To assess if our measurements of pain perception were influenced by repeated assessments, we determined if any change in pain reports occurred 30 min apart. We found no change in pain threshold after the 30-min rest, although some of the subjects reported low levels of pain before pushing the timing device. This occurrence was consistent before and after 30 min of quiet rest. Pain ratings were similar for men after the 30-min rest, and women had a small reduction in pain ratings. Additional analysis of the women verified that there were significantly greater reductions in pain ratings after the isometric contractions held until task failure than quiet rest. Thus, the changes in pain ratings after the isometric contractions were due to the exercising stimulus and not due to repeated pain assessments.
Exercise-induced analgesia occurred after isometric contractions, and the greatest change in pain threshold and pain ratings occurred at failure of the low-force fatiguing contraction. Pain threshold increased and pain ratings decreased after the 25% MVC sustained until failure. Furthermore, this was the only contraction that decreased peak pain ratings, with significant decreases reported at the end of the 2-min pain test. In contrast, when the 25% MVC was sustained for 2 min, we did not observe an analgesic response. Kosek et al. (27,28) also reported a decrease in pain after both arm and leg isometric contractions sustained to failure at ∼ 22% MVC of the quadriceps and with a 0.5- or 1.0-kg weight attached to the respective limb. Low-intensity contractions must be performed for a long duration (greater than 2 min) to produce hypoalgesia, after which it is not related to time to task failure. Thus, there is likely a duration threshold for hypoalgesia to occur.
In addition to the low-intensity contraction, pain ratings and pain threshold were altered after three brief MVC (3 s duration and 1 min apart), and pain ratings were altered after an 80% MVC sustained to failure. Fatigue, which is demonstrated by a reduction in the force-generating capacity of the muscle (15), was not apparent during performance of the MVC because the force produced was similar across the three trials. Therefore, muscle fatigue does not need to be induced for analgesia to occur. Our results indicate that short-duration contractions need to be performed at high intensities to elicit an analgesic response.
High-intensity contractions including MVC and the 80% MVC task involve recruitment of high-threshold motor units. Furthermore, during a submaximal contraction sustained until failure (25% MVC until task failure), higher-threshold motor units are progressively recruited as the already-active motor units become fatigued so as to sustain the required force (11,16). Two minutes may have not been long enough to recruit high-threshold motor units during the 25% MVC task. Taken together, our results suggest that high-threshold motor units need to be recruited during isometric contractions to induce a significant hypoalgesic response. The duration threshold for the low-intensity task to produced hypoalgesia, therefore, may be related to the motor units involved in the contraction.
Women were more sensitive than men to pain before and after all the isometric contractions. Other studies have reported that women are more sensitive to a noxious stimulus than men at rest (36) and before and after isometric exercise (26). The exact mechanisms are not known; however, several psychological and physiological factors have been proposed. These include acute hormonal fluctuations (i.e., menstrual cycle), blood pressure, heart rate, body composition, cortisol secretion, and personality characteristics (3,4,9,36,37). We specifically addressed whether anxiety levels and the cardiovascular response of men and women explained the sex differences in pain perception. There was no association, however, between these variables and pain perception in men and women. Other issues that may influence the sex difference include the type of experimental pain stimulus (36) and finger size. Typically, men have a larger index finger than women and so this may be a contributing factor to the sex differences in pain perception with the method of testing used in this study.
Despite these sex differences in pain perception, the magnitude of exercise-induced analgesia was similar. After the 25% MVC task held for 2 min, there was no change in pain threshold or pain ratings for either sex, but when sustained until failure (25% MVC sustained until failure), both men and women reported significant analgesia. Previous research studies have shown that women may experience greater exercise-induced analgesia than men. Koltyn et al. (26) found that women, but not men, reported a decrease in pain after a 40-50% MVC contraction sustained for 2 min. Koltyn et al. used a higher intensity of contraction and tested pain perception on the exercising appendage, which is more likely to produce greater analgesia compared with contralateral and distal limbs (28). Comparisons of the studies indicate that sex differences in exercise-induced analgesia after isometric contractions may be dependent on the testing site as well as intensity of the contraction.
Changes in anxiety levels have been associated with or implicated in modulating levels of pain (14,32). After all the three submaximal isometric contractions, state anxiety levels increased similarly for men and women. Furthermore, only two of the three isometric submaximal contractions exhibited an increase in pain threshold and/or decrease in pain ratings, but all three tasks demonstrated the same increase in state anxiety. Direct comparison of people with different anxiety levels and pain is required to specifically assess the relationship between anxiety and exercise-induced analgesia. Our study, however, indicated that anxiety levels do not likely play a role in exercise-induced analgesia and corroborates other studies that demonstrate no relation between anxiety levels and pain (1,5,35).
We assessed MAP and heart rate before and during the two low-intensity tasks. Previous studies have shown a relationship between cardiovascular measures and pain perception (6,29,34). MAP increased during the low force isometric fatiguing contractions: this increase is known as the pressor response. Sex differences in the pressor response are primarily due to activation of groups III and IV afferents due to excessive metabolite accumulation in the active muscle (23,31,38). Consistent with these findings, we found that the pressor response was associated with time to failure and was greater for the men than the women during the contractions as found previously when men were stronger than women (18,20). There was no association, however, between exercise-induced changes in MAP and changes inpain threshold or pain ratings after the low-intensity tasks. Other studies have also shown that increases in blood pressure during isometric contractions (10-50% MVC) were not related to pain intensity after submaximal isometric exercise (22,26). Thus, we found no evidence that pain perception measured after exercise was associated with or modulated by the change in MAP during the low-force submaximal contractions in men and women.
Our study demonstrates that isometric contractions are capable of eliciting analgesia, using a mechanical noxious stimulus, in healthy men and women after long-duration and high-intensity, short-duration isometric contractions. Because we assessed pain perception immediately after exercise only, we do not know how long lasting the analgesia effect occurs in either men or women. Consequently, future studies need to examine the efficacy of isometric contraction in producing long-duration changes in pain perception and to assess if similar results are obtained with other types of experimental pain.
The efficacy and the dose response of isometric contractions as a pain reduction treatment in clinical populations are unknown. Postsurgical patients with moderate pain levels required a 20% reduction in pain intensity for a minimal improvement in pain during opioid titration (7). Similarly, the minimal clinically important difference in chronic musculoskeletal pain intensity occurred with a 15% reduction in the numerical rating scale (39). Our study showed that for healthy adults, there was a 20% reduction in pain ratings after the low-intensity, long-duration task, indicating that the performance of an isometric contraction produced a clinically meaningful difference in pain intensity. The use of isometric contractions as a therapeutic tool for patients with a clinical pain condition warrants further investigation because an individual's response to exercise can be different if pain is present (22,27,42).
In conclusion, pressure-induced pain levels decreased after brief maximal contractions and sustained submaximal fatiguing isometric contractions, which involve recruitment of high-threshold motor units for both men and women. Exercise-induced increases in pain threshold and decreases in pain ratings were greatest after a low-intensity submaximal contraction sustained to failure. Women were more sensitive to pain than men before and after isometric contractions. Exercise-induced changes in pain perception, however, were not associated with blood pressure during the contraction or related to anxiety levels. This study indicates that men and women can achieve decreases in pain with isometric contractions that are not localized to the working muscle and therefore systemic. Integration of isometric contractions should be explored in the management of painful conditions.
This study was supported by an award from the Arthritis Foundation to MKHB. The results of the present study do not constitute endorsement by ACSM.
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