By the year 2030, up to one in five Americans will be 65 yr and older (40). Pain is a particularly persistent problem in this age group, and it has been estimated that between 40% and 80% of older adults experience pain sufficient to negatively affect daily functioning and overall quality of life (3). Although pharmacological means are regularly used in the management of pain, the increased adverse effects with pharmacological measures associated with aging (3) and the increasing cost of healthcare make it imperative to examine nonpharmacological, cost-effective alternatives for pain relief. One such intervention is exercise, which is frequently used in the management of pain. Although most exercise-training programs in older adults with chronic pain demonstrate reductions in pain and improvements in functional status (20,35), the optimal prescription to relieve pain is not clear. This includes lack of knowledge regarding type, intensity, and duration of exercise to provide pain relief (20,35). Many practitioners feel ill-prepared to adequately prescribe exercise for their older clients because of a lack of information on how to individualize programs to best meet the client’s needs (4), and so it is vital to understand the effect of age on the change in pain perception after exercise.
Sensitivity to a noxious stimulus has been shown to decrease after exercise, a phenomenon known as exercise-induced hypoalgesia (EIH). EIH has been found after exercise of all types, including aerobic exercise, dynamic resistance exercise, and isometric exercise (reviewed by Naugle et al. ). In young healthy adults, the parameters to achieve EIH seem to be specific to the type of exercise performed. Exercise of higher intensity (60%–75% V˙O2max or 200 W) most consistently produces EIH after aerobic exercise (12,19). In contrast, isometric contractions of high intensity (8,21) and low intensity (8,9,21,23,39) induce an EIH response, with the greatest reduction in sensitivity to a noxious stimulus after low-intensity contractions (25%–50% MVC) held for longer duration (8,28).
Our knowledge of EIH is based primarily on studies of young adults, and mostly physically active men. Older adults and women have historically been underrepresented in studies of exercise and pain (1). In the last decade, the pain response after exercise in young women has received more attention in the literature; however, there remains a paucity of information on the effect of aging on EIH. Several studies examining EIH in individuals with chronic pain have included control groups consisting of healthy middle-age adults, with the mean age of control subjects ranging from 34 to 52 yr. Results of these studies show that EIH persists in middle age with reductions in pain sensitivity after both aerobic (11,27) and low-intensity isometric (7,17,24,37) exercise protocols. To our knowledge, no study has examined changes in pain sensitivity immediately after exercise in healthy adults 60 yr and older or the effect of varying the exercise parameters on the magnitude of the EIH response.
There are several factors associated with aging that may affect EIH. For example, older adults experience sarcopenia (a reduction in the number and size of muscle fibers) with greater atrophy of Type II (fast) than Type I (slow) fibers (13,16) as well as a reduced ability to activate endogenous descending inhibitory pathways (25,32,41). Both the recruitment of high-threshold motor units and the activation of descending inhibitory pathways have been implicated as potential mechanisms responsible for EIH (5,8). Thus, the influence of an acute bout of exercise on pain sensitivity may differ in older adults compared with young adults.
Isometric exercise has excellent potential to become an effective pain management tool for a large segment of the population. Individuals of all ages and abilities, including those with limited mobility, can perform isometric exercise. Therefore, the purpose of this project was to determine the effect of isometric contractions that varied in intensity and duration on pain relief in adults older than 60 yr. Because cardiovascular reactivity (22,33) and anxiety (29) have also been implicated in the modulation of pain perception, we recorded anxiety levels and cardiovascular responses to examine their influence on EIH.
MATERIALS AND METHODS
Twelve men (72.4 ± 6.1 yr, mean ± SD) and 12 women (72.0 ± 6.6 yr) participated in this experiment. All subjects were healthy and free of any risk factors that would preclude them from participating in exercise. Subjects were excluded if they reported acute or chronic pain, currently used analgesics or psychotropic medications, or had a score of less than 25/30 on the Mini Mental Status Examination (MMSE) indicative of possible cognitive impairment. The protocol was approved by the institutional review boards at Marquette University and Concordia University Wisconsin.
Subjects completed four sessions: one familiarization and three experimental. Sessions were separated by approximately 1 wk, and experimental sessions were randomized. During the familiarization session, subjects signed informed consent and were familiarized to the experimental procedures and the pressure pain device. Pain thresholds and pain ratings induced by the pressure pain device were determined before and after a 30-min quiet rest period. Because the performance of maximal voluntary contractions (MVC) has been found to influence pain reports in young adults (8,21), MVC force was determined during the familiarization session after the completion of the two pain tests. Specifically, a series of three MVC were performed with the left elbow flexor muscles with a 1-min rest between contractions. Subjects were verbally encouraged to achieve maximal force. The highest value of the three maximal efforts was recorded and used for calculation of the submaximal target force to be used in the experimental sessions.
During the experimental sessions, pressure pain perception (i.e., pain threshold and pain ratings) was assessed before and after isometric contractions of the left elbow flexors that varied in intensity and duration. Each experimental session consisted of one of three exercise tasks: 1) three brief MVC separated by a 1-min rest, 2) 25% MVC sustained for 2 min, and 3) 25% MVC sustained until task failure. Measures of blood pressure, HR, and RPE were gathered every 30 s during the sustained exercise tasks. The state portion of the Spielberger State-Trait Anxiety Inventory (STAI) (36) was administered at the start of the session and immediately after each of the two pain tests. A 30-min quiet rest period separated the completion of the first pain test and the initiation of isometric exercise.
Measurement of force during isometric contractions
Subjects were seated upright in an adjustable chair with a padded nylon strap placed vertically over each shoulder to minimize shoulder activity and to stabilize the subject. The left shoulder was placed in slight abduction with the elbow flexed to 90° and the forearm held parallel to the floor in a neutral position midway between pronation and supination. The elbow rested on a padded support, and the forearm, wrist, and hand were placed in a modified wrist–hand–thumb orthosis (Orthomerica, Newport Beach, CA), which was held in place by Velcro straps. The orthosis was rigidly attached to a force transducer (JR-3 Force-Moment Sensor; JR-3 Inc., Woodland, CA) mounted on a custom-designed adjustable support, which measured the forces exerted at the wrist in the vertical direction. A Power 1402 A-D converter and Spike2 software (Cambridge Electronics Design, Cambridge, UK) were used for online recording of the vertically directed forces. The force signal was digitized at 500 samples per second.
During the performance of the submaximal exercise tasks, subjects were required to match the target force as displayed on a monitor. Subjects were verbally encouraged to sustain the force for as long as possible during the contraction to task failure. A subject was deemed to reach task failure when unable to maintain a force within 10% of the target value for three out of five consecutive seconds (8). The same investigator (KJL) visually determined task failure for all subjects.
Pressure pain perception
A custom-made pressure pain device used frequently in the assessment of EIH (8–10) was used to measure pain perception (6). An 8 × 1.5-mm Lucite edge (Romus Inc., Milwaukee, WI) was placed on the dorsum of the right index finger midway between the proximal and the distal interphalangeal joints for 2 min. A 200-g mass was applied to a second-class lever such that a 10-N force (equivalent to a 1-kg mass) was applied to the Lucite edge. During the 2-min test, the subject was asked to press a timing device with their left hand when the pressure from the edge first changed to pain (i.e., pain threshold) and to rate the intensity of pain on a 0–10 numerical rating scale every 20-s throughout the test. The numerical rating scale anchors were 0 = “no pain,” 5 = “moderate pain,” and 10 = “worst pain” (26).
Mean arterial pressure, HR, and RPE
HR and blood pressure were monitored continuously throughout the sustained isometric contractions using an automated beat-by-beat blood pressure monitor (Finapres 2300, Madison, WI). The cuff was placed around the middle finger of the right hand. RPE was determined every 30 s throughout the submaximal exercise tasks using a modified Borg CR10 scale (2). The RPE scale ranges from 1 to 10, with scale anchors of 1 = “Not strong at all” and 10 = “So strong I can’t go anymore.”
Data were analyzed using the Statistical Package for the Social Sciences (version 20; IBM, Chicago, IL) and were screened for outliers (> 3 SD from the mean) and missing data points. Two outliers were identified for state anxiety: one in the MVC session and one in the 25% MVC held to task failure session. Outliers were altered to 1 U greater than the next extreme score (38). Missing pain thresholds as a result of failure to push the timing device were imputed using the prior knowledge method (38). The midpoint between the time of a pain rating of zero and the first pain rating greater than zero was used as the imputed score.
Mixed-design multivariate repeated-measures ANOVA with sex as a between-subjects factor was used in the familiarization session to analyze the effect of repeated pain testing on pain threshold (trial) and pain rating (trial × time) and in the exercise sessions to analyze the effect of isometric contractions on pain threshold (task × trial), pain ratings (task × trial × time), and state anxiety (task × time). To assess for learning effects with the pressure pain device across sessions, the first pain test of the four sessions (familiarization and three exercises) were identified according to lab visit number. Multivariate repeated-measures ANOVA was used to assess for differences in pain threshold (lab visit number) and pain ratings (laboratory visit number × time). For comparisons across time for each of the submaximal exercise tasks, RPE, HR, and mean arterial pressure (MAP) were analyzed at quartiles (start of task, 25%, 50%, 75%, and end task) using mixed-design multivariate repeated-measures ANOVA with sex as a between-subjects factor. When a significant effect was found, simple contrasts and Bonferroni corrected t-tests for post hoc multiple comparisons were used to identify differences. Independent t-tests assessed for sex differences in time to task failure and strength. Bivariate correlations were run to assess for associations between dependent variables with hierarchical regression analyses assessing for predictive relations between sex (step 1) and preexercise pain (step 2) with relative change in pain (absolute change divided by preexercise) after exercise. Pain ratings for the six 20-s time points were averaged for each trial. Averages were used in post hoc testing and in the examination with dependent variables. A P value of <0.05 was used for statistical significance. Data are reported as mean ± SD within the text and mean ± SEM in the figures.
Pain threshold and pain ratings did not change after 30 min of quiet rest (P = 0.31 and P = 0.40, respectively; Figs. 1A and 1B). In addition, no visit order effect was found for baseline pain threshold (P = 0.879, ηp2 = 0.03) or pain ratings (P = 0.980, ηp2 = 0.009), demonstrating no learning effect or effect of session number on baseline pain. Pain ratings increased across the 2-min pain test (time: P < 0.001, ηp2 = 0.82) with no difference between trials (trial × time, P = 0.12). Women had higher pain ratings, averaged during the 2-min pain test (4.2 ± 2.5 vs 2.3 ± 1.7, P = 0.034, ηp2 = 0.19), and a trend for lower pain threshold (41 ± 32 s vs 67 ± 32 s, P = 0.06, ηp2 = 0.15) than men. No trial and sex interaction was identified for either pain threshold (P = 0.52) or pain ratings (P = 0.74). Women did have a steeper rise in pain ratings than men (time × sex: P = 0.04, ηp2 = 0.45).
Exercise Sessions: Pain Threshold and Pain Ratings
Pain threshold increased ˜18.5% and pain ratings decreased ˜19% after exercise (P < 0.001, ηp2 = 0.44 and P < 0.001, ηp2 = 0.48, respectively) and was similar across exercise tasks (task × trial: P = 0.94 and P = 0.55, respectively; Fig. 1C and D). Pre- and postexercise pain reports and relative change in pain reports were averaged for the three exercise sessions because of the absence of task differences. Average scores were used in subsequent analyses.
Women reported lower pain thresholds (P = 0.01, ηp2 = 0.27) and higher pain ratings (P = 0.004, ηp2 = 0.31) than men. Women experienced greater reductions in pain ratings after exercise than men (trial–sex: P = 0.003, ηp2 = 0.34), which was unaffected by task (trial–task–sex: P = 0.12). Post hoc analyses revealed that women reported a reduction in pain ratings (23%, P = 0.001, Cohen’s d = 0.45), whereas men had no significant change (9%, P = 0.28, Cohen’s d = 0.08; Fig. 2B). There was a trend for an interaction between trial and sex for pain threshold with women reporting greater increases in pain threshold than men; however, this failed to reach statistical significance (P = 0.06; Fig. 2A).
To assess for relations of preexercise pain sensitivity and change in pain when controlling for sex, correlation and hierarchical regression equations were completed. There was a negative correlation between preexercise pain threshold and relative change in pain threshold (r = −0.59, P = 0.001). Hierarchical regression analysis revealed that sex explained 29% of the variance in relative change in pain threshold, F(1, 22) = 10.49, P = 0.004. The addition of preexercise pain threshold to the equation failed to produce a significant change in F (P = 0.07). Although the overall regression model remained significant, F(2, 21) = 7.66, P = 0.003, adjusted R2 = 0.37; neither sex (P = 0.126) nor preexercise pain threshold (P = 0.072) were found to provide a unique contribution to the model, indicating a high degree of interdependence between these two variables. No predictive association of sex (P = 0.213) or sex and preexercise pain ratings (P = 0.307) with relative change in pain ratings was identified.
Strength and Time to Task Failure
Men were stronger than women (P < 0.001, Cohen’s d = 2.8), and the MVC force was negatively associated with time to task failure (r = −0.419, P = 0.042) such that stronger individuals reached task failure more quickly. Time to task failure was similar between men and women for the submaximal contraction (610 ± 176 s vs 800 ± 371 s, respectively, P = 0.13, Cohen’s d = 0.66). Strength (r = −0.636, P = 0.001), but not time to task failure (r = 0.392, P = 0.06), was associated with the relative change in pain threshold such that stronger individuals experienced smaller threshold increases. Neither strength (P = 0.35) nor time to task failure (P = 0.72) was related to relative change in pain rating.
A time and task interaction (P = 0.04, ηp2 = 0.39) and a main effect for task (P = 0.01, ηp2 = 0.35) were found for anxiety. Simple contrasts showed both effects involved differences between the MVC and the 25% MVC to task failure sessions, with the interaction occurring between the second (after the preexercise pain test) and third (postexercise) State-Trait Anxiety Inventory administration. Post hoc comparisons indicated subjects reported a nonsignificant decrease in state anxiety after exercise in the MVC session (P = 0.62) and a small increase in anxiety after exercise to task failure (23.4 ± 5.2 vs 25.6 ± 6.1, P = 0.01, Cohen’s d = 0.39). The average state anxiety across the MVC session was slightly less than during the 25% MVC to task failure (23.6 ± 5.1 vs 24.8 ± 5.9, P = 0.003, Cohen’s d = 0.21). Anxiety before and after the first pain assessment in each session was not different (P > 0.05), indicating that the pain test itself did not induce changes in state anxiety. In addition, no main effect for sex (P = 0.17) was found nor was there a sex and time interaction (P = 0.49).
MAP, HR, and RPE
MAP, HR and RPE were analyzed at quartiles (start of contraction, 25%, 50%, 75%, and end contraction) for each of the submaximal tasks. All three increased over time during the 25% MVC × 2 min task (P < 0.001) and 25% MVC to task failure (P < 0.001) sessions. There was no difference between the men and the women for either MAP or HR at onset of exercise and no main effect of sex or interaction between sex and time for any of the three variables (P > 0.05). No significant correlations were identified between change in MAP, HR, or RPE with change in pain reports in either submaximal session.
We examined EIH after isometric exercise of elbow flexor muscles at various durations and intensities in healthy older adults. The main findings of the study were as follows: 1) older adults experienced EIH, which was similar across all three tasks (3 MVC, 25% MVC held for 2 min, and 25% MVC held to task failure), and 2) both older men and women experienced increases in pain threshold, but only older women experienced reductions in pain ratings. To assess whether reductions in pain were a manifestation of repeated pain testing (30), we examined pain sensitivity before and after 30 min of quiet rest. There was no change in pain reports, thus the pain testing itself did not induce the reduction of pain after isometric exercise. Furthermore, the absence of a visit order effect when comparing baseline pain reports indicates that pain sensitivity was not affected by exposure to the pain device.
Several studies have shown pain relief after training programs of aerobic or dynamic resistive exercise in older adults with a variety of chronic pain conditions, including lower back pain, osteoarthritis, myofascial pain, and osteoporosis (reviewed by Koltyn ). The present study adds to the literature by showing an acute change in pain sensitivity after a single session of isometric exercise in healthy older adults. Although previous studies have shown a reduction in pain after isometric contractions, they have examined either young (8,9,21,23,39) or middle-age (7,17,24,37) healthy adults. To our knowledge, this is the first study to examine this response in adults older than 60 yr.
In young adults, EIH is dependent on the type of isometric contraction. Both high and low-intensity isometric contractions decrease pain; however, the greatest decrease in pain occurs after contractions of lower intensity (25%–50% MVC) held to task failure (8,28). Previously, we hypothesized that the task specificity in young adults was due to activation of high-threshold motor units (8). Specifically, pain decreased with maximal intensity contractions and submaximal contraction held to task failure, tasks when high-threshold motor units are activated. No reduction in pain was seen with submaximal contractions held for short duration (22%–37% of time to task failure) when few high-threshold motor units are recruited. The lack of task specificity in older adults may be due to the age-related atrophy of Type II fibers. However, a reduction in pain after the 2-min exercise task suggests that fiber-type recruitment does not fully explain EIH after isometric contractions. There are likely several mechanisms responsible for EIH (5,18), and high-threshold motor unit recruitment may act as an additive factor in young, but not older adults.
Few studies have examined sex differences in EIH after isometric exercise. Our data indicate that only older women reported a reduction in pain ratings after exercise, and a trend for women to have greater increases in pain threshold was also found. These findings are similar to those of Koltyn et al. (21) in young adults after an isometric handgrip contraction at 40%–50% MVC, where young women but not men experienced increased pain thresholds and decreased pain ratings. In contrast, Umeda et al. (39) found EIH magnitude to be similar between young men and women. Women, however, reported greater preexercise pain sensitivity than men both in this study and by Koltyn et al. (21), whereas no sex difference in preexercise pain sensitivity was identified by Umeda et al. (39). Thus, higher preexercise pain sensitivity may be associated with greater EIH. To assess this possibility, we conducted hierarchical regression analyses controlling for sex. The failure of preexercise pain to make a unique contribution to the prediction of relative change in pain threshold when controlling for sex, along with the lack of an association between preexercise and relative change in pain ratings, indicates that preexercise pain sensitivity alone does not explain the sex difference in EIH magnitude.
Alterations in anxiety and interactions between the cardiovascular and pain regulatory systems have the potential to affect the EIH response. Higher levels of state or trait anxiety (29), for example, have been shown to be associated with heightened pain sensitivity, and it has been suggested that there is an interaction between the cardiovascular and the pain regulatory systems (22,33). To address the possible influence of anxiety and the cardiovascular exercise response on our findings, we examined state anxiety before and after exercise as well as MAP, HR, and RPE during exercise. Our findings showed average state anxiety to be somewhat less during the MVC session than the 25% MVC to task failure in part due to a small increase in state anxiety after the fatiguing submaximal contraction. These differences were slight, separated by only 1–2 U with small effect sizes (Cohen’s d 0.22 and 0.39, respectively), and do not seem to have influenced the EIH response as reductions in pain were similar across all three tasks. No associations were found between change in pain reports with change in HR, MAP, or RPE, a finding consistent with prior reports in young adults (8,39). Despite having comparable levels of state anxiety and similar cardiovascular responses to exercise, older women experienced greater reductions in pain ratings after exercise than older men. Collectively, these results indicate that neither state anxiety nor cardiovascular reactivity mediate the EIH response.
Although the underlying mechanisms for EIH are not clear, the finding of a reduction in pain at a site distant to the exercising muscles suggests central or systemic mechanisms are involved. One such mechanism is the activation of endogenous inhibitory pathways via conditioned pain modulation (CPM) (5). CPM involves attenuation of pain sensitivity in one body area in the presence of a conditioning painful stimulus elsewhere (i.e., “pain inhibits pain”). Older adults have been found to have reduced efficiency of the CPM response (25,32,41). If CPM is responsible for EIH, older adults would therefore be expected to experience a lower magnitude of EIH than young adults. An examination of relative change in pain in young adults after a low-intensity isometric contraction held to task failure, exercise likely to be perceived as painful and thus act as a conditioning stimulus, revealed that the young adults reported a 48% increase in pain threshold (Cohen’s d = 0.60) and 24% decrease in pain ratings (Cohen’s d = 0.50) (8). In contrast, our older subjects reported a 20% increase in pain threshold (Cohen’s d = 0.26) and 20% decrease in pain ratings (Cohen’s d = 0.26) using the same exercise protocol and pain induction method. Although speculative at this time as muscle pain induced during exercise was not assessed in either study, it is plausible that age-related changes in CPM may account for these differences in EIH effect size. Future studies are clearly needed to examine the possible role of CPM in EIH and its relation to changes in the exercise response with aging.
The finding that older adults can obtain equivalent pain relief from several different isometric contraction dosages has important implications for exercise prescription in the management of pain in adults, especially for older women. Practitioners and their older clients may have more flexibility in choice of isometric exercise parameters and greater ability to individualize and vary a home exercise program to best meet the client’s current needs, abilities, and preferences. Young adults on the other hand may require greater specificity of exercise prescription to achieve maximal clinically important reductions in acute pain. Reductions in pain ratings of 15% are considered to be minimally clinically significant (i.e., the smallest magnitude of change correlating with patient perception of an overall improvement in pain) in patients with chronic musculoskeletal pain (34). Older adults in this study reported reductions in pain in both pain ratings and pain thresholds of ˜19%, and greater reductions have been found in young adults with the same exercise protocol (8). Thus, isometric exercise has the potential to induce clinically relevant pain relief for adults of all ages. Importantly, because men did not demonstrate a change in pain ratings after exercise, these data suggest that older men may have less pain relief than older women after isometric exercise.
The present study is the first to examine the dose–response relationship of isometric exercise and pain reduction in older men and women. It does have some limitations. We examined the EIH response in healthy older adults without pain. As musculoskeletal pain complaints increase with age, our subjects may not be representative of older adults with persistent ongoing or chronic pain conditions. Future studies are needed to determine whether the acute pain response after exercise in adults older than 60 yr is altered by the presence of such a condition. We also did not examine the duration of the hypoalgesic response after exercise cessation. It is not known if EIH duration after exercise is similar for older adults as for younger adults. Finally, differences in strength cannot be ruled out as having an effect on pain relief after isometric exercise. It is well established that men are usually stronger than women (15), and these strength differences persist with aging (14,31). Despite exercising at different absolute forces, our protocols were designed so that men and women reached the same physiological end point with 25% MVC to task failure contractions; hence, the reduction in MVC was similar. In addition, although both men and women experienced increases in pain thresholds and a relation between strength and relative change in pain threshold was found, no relation was found with change in pain ratings despite a robust sex difference in magnitude of EIH. Future studies are needed to clarify the potential influence of strength on EIH.
In contrast to young adults who receive the greatest pain relief with isometric contractions of low-moderate intensity held for long duration (8,28), we have shown that isometric contractions of varying intensities and durations induce similar reductions in pain in healthy older adults. Although both older men and women experienced increases in pain threshold, only older women experienced decreases in pain ratings. These age and sex differences demonstrate that older healthy men and women have different exercise requirements than young adults for the reduction of pain.
This work was supported in part by a Concordia University Wisconsin Intramural Research Grant to KJL.
No conflicts of interest are declared by any of the authors. The results of this study do not constitute endorsement by the American College of Sports Medicine.
1. Beery AK, Zucker I. Sex bias in neuroscience and biomedical research. Neurosci Biobehav Rev. 2011; 35 (3): 565–72.
2. Borg GA. Psychophysical bases of perceived exertion. Med Sci Sports Exerc. 1982; 14 (5): 377–81.
3. Cavalieri TA. Management of pain in older adults. J Am Osteopath Assoc. 2005; 105 (3 Suppl 1): S12–7.
4. Christmas C, Andersen RA. Exercise and older patients: guidelines for the clinician. J Am Geriatr Soc. 2000; 48 (3): 318–24.
5. Cook DB, Koltyn KF. Pain and exercise. Int J Sport Psychol. 2000; 31: 256–77.
6. Forgione AG, Barber TX. A strain gauge pain stimulator. Psychophysiology. 1971; 8 (1): 102–6.
7. Ge HY, Nie H, Graven-Nielsen T, et al. Descending pain modulation and its interaction with peripheral sensitization following sustained isometric muscle contraction in fibromyalgia. Eur J Pain. 2012; 16 (2): 196–203.
8. Hoeger Bement MK, Dicapo J, Rasiarmos R, Hunter SK. Dose response of isometric contractions on pain perception in healthy adults. Med Sci Sports Exerc. 2008; 40 (11): 1880–9.
9. Hoeger Bement MK, Rasiarmos RL, DiCapo JM, et al. The role of the menstrual cycle phase in pain perception before and after an isometric fatiguing contraction. Eur J Appl Physiol. 2009; 106 (1): 105–12.
10. Hoeger Bement MK, Weyer A, Hartley S, Drewek B, Harkins AL, Hunter SK. Pain perception after isometric exercise in women with fibromyalgia. Arch Phys Med Rehabil. 2011; 92 (1): 89–95.
11. Hoffman MD, Shepanski MA, Mackenzie SP, Clifford PS. Experimentally induced pain perception is acutely reduced by aerobic exercise in people with chronic low back pain. J Rehabil Res Dev. 2005; 42 (2): 183–90.
12. Hoffman MD, Shepanski MA, Ruble SB, Valic Z, Buckwalter JB, Clifford PS. Intensity and duration threshold for aerobic exercise-induced analgesia to pressure pain. Arch Phys Med Rehabil. 2004; 85 (7): 1183–7.
13. Hunter SK, Brown DA. Muscle: The primary stabilizer and mover of the skeletal system. In: Neumann DA, editor. Kinesiology of the Musculoskeletal System: Elsevier; 2010. p. 47–76.
14. Hunter SK, Critchlow A, Enoka RM. Influence of aging
on sex differences
in muscle fatigability. J Appl Physiol. 2004; 97 (5): 1723–32.
15. Hunter SK, Enoka RM. Sex differences
in the fatigability of arm muscles depends on absolute force during isometric contractions. J Appl Physiol. 2001; 91 (6): 2686–94.
16. Hunter SK, Thompson MW, Ruell PA, et al. Human skeletal sarcoplasmic reticulum Ca2+ uptake and muscle function with aging
and strength training. J Appl Physiol. 1999; 86 (6): 1858–65.
17. Kadetoff D, Kosek E. The effects of static muscular contraction on blood pressure, heart rate, pain ratings and pressure pain thresholds in healthy individuals and patients with fibromyalgia. Eur J Pain. 2007; 11 (1): 39–47.
18. Koltyn KF. Analgesia following exercise: a review. Sports Med. 2000; 29 (2): 85–98.
19. Koltyn KF. Exercise-induced hypoalgesia
and intensity of exercise. Sports Med. 2002; 32 (8): 477–87.
20. Koltyn KF. Using physical activity to manage pain in older adults. J Aging
Phys Act. 2002; 10: 226–39.
21. Koltyn KF, Trine MR, Stegner AJ, Tobar DA. Effect of isometric exercise on pain perception and blood pressure in men and women. Med Sci Sports Exerc. 2001; 33 (2): 282–90.
22. Koltyn KF, Umeda M. Exercise, hypoalgesia and blood pressure. Sports Med. 2006; 36 (3): 207–14.
23. Kosek E, Ekholm J. Modulation of pressure pain thresholds during and following isometric contraction. Pain. 1995; 61 (3): 481–6.
24. Kosek E, Ekholm J, Hansson P. Modulation of pressure pain thresholds during and following isometric contraction in patients with fibromyalgia and in healthy controls. Pain. 1996; 64 (3): 415–23.
25. Lariviere M, Goffaux P, Marchand S, Julien N. Changes in pain perception and descending inhibitory controls start at middle age in healthy adults. Clin J Pain. 2007; 23 (6): 506–10.
26. McCaffery M, Pasero C. Pain: Clinical Manual. 2nd ed. St. Louis (MO): Mosby; 1999. p. 795.
27. Meeus M, Roussel NA, Truijen S, Nijs J. Reduced pressure pain thresholds in response to exercise in chronic fatigue syndrome but not in chronic low back pain: an experimental study. J Rehabil Med. 2010; 42 (9): 884–90.
28. Naugle KM, Fillingim RB, Riley JL 3rd. A meta-analytic review of the hypoalgesic effects of exercise. J Pain. 2012; 13 (12): 1139–50.
29. Okawa K, Ichinohe T, Kaneko Y. Anxiety may enhance pain during dental treatment. Bull Tokyo Dent Coll. 2005; 46 (3): 51–8.
30. Padawer WJ, Levine FM. Exercise-induced analgesia: fact or artifact? Pain. 1992; 48 (2): 131–5.
31. Peiffer JJ, Galvao DA, Gibbs Z, et al. Strength and functional characteristics of men and women 65 years and older. Rejuvenation Res. 2010; 13 (1): 75–82.
32. Riley JL 3rd, King CD, Wong F, Fillingim RB, Mauderli AP. Lack of endogenous modulation and reduced decay of prolonged heat pain in older adults. Pain. 2010; 150 (1): 153–60.
33. Ring C, Edwards L, Kavussanu M. Effects of isometric exercise on pain are mediated by blood pressure. Biol Psychol. 2008; 78 (1): 123–8.
34. Salaffi F, Stancati A, Silvestri CA, Ciapetti A, Grassi W. Minimal clinically important changes in chronic musculoskeletal pain intensity measured on a numerical rating scale. Eur J Pain. 2004; 8 (4): 283–91.
35. Singh MA. Exercise comes of age: rationale and recommendations for a geriatric exercise prescription. J Gerontol A Biol Sci Med Sci. 2002; 57 (5): M262–82.
36. Spielberger CD, Gorsuch RL, Lushene PR, Vagg PR, Jacobs AG. Manual for the State-Trait Anxiety Inventory (Form Y). Palo Alto: Consulting Psychologists Press; 1983. p. 36.
37. Staud R, Robinson ME, Price DD. Isometric exercise has opposite effects on central pain mechanisms in fibromyalgia patients compared to normal controls. Pain. 2005; 118 (1–2): 176–84.
38. Tabachnick BG, Fidell LS. Using Mulitvariate Statistics. 4th ed. Boston: Allyn and Bacon; 2001. p. 966.
39. Umeda M, Newcomb LW, Ellingson LD, Koltyn KF. Examination of the dose–response relationship between pain perception and blood pressure elevations induced by isometric exercise in men and women. Biol Psychol. 2010; 85 (1): 90–6.
40. Vincent GK, Velkoff VAU.S. Census Bureau. The Next Four Decades: The Older Population in the United States: 2010 to 2050. Washington (DC): U.S. Department of Commerce, Economics and Statistics Administration, U.S. Census Bureau; 2010. p. 14.
41. Washington LL, Gibson SJ, Helme RD. Age-related differences in the endogenous analgesic response to repeated cold water immersion in human volunteers. Pain. 2000; 89 (1): 89–96.