Iniern J . Neuro.~crm<r, 1998, Vol 95, pp. 183-202 Reprints available directly from the publisher Photocopying permitted by license only

$ 1 1998 OPA (Overseas Publishers Association) N.V. Published by license under

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SEX DIFFERENCES IN NATURALLY OCCURRING LEG MUSCLE PAIN

AND EXERTION DURING MAXIMAL CYCLE ERGOMETRY

DANE B. COOK*, PATRICK J. O’CONNOR, STEWART E. OLIVER and YANGMIN LEE**

Depurtment of Exercise Science, University of Georgia

(Received 20 April 1998)

This investigation examined the role of sex in perceptions of leg muscle pain during exercise. Males ( N = 26; age = 23.2 f 3.9) and females ( N = 26; age = 21.9 f 3.5) matched on weekly energy expenditure completed a ramped maximal cycle ergometry test. Leg muscle pain thresholds were determined and pain intensity ratings as well as ratings of perceived exertion were obtained during and after exercise. The power output a t pain threshold was lower in females (129.9 +46.5 watts) compared to males (148.2 f 56.6 watts). Peak power output and peak pain intensity ratings were lower (P < 0.001) in females (21 1.3 f 39.1 watts; 5.5 f 2.9) compared to males (303.6*27.5 watts; 8.5*2.3). A Sex X Relative Intensity (i.e., % peak power output) ANOVA revealed that females reported lower pain ratings at each relative intensity examined ( F = 17.7; d f = 1,50; p < 0.001). The primary conclusion of this investigation i s that females rate naturally occurring leg muscle pain as less intense than males when data are relativized to peak power output.

Keywords: Cycling exercise; musculoskeletal pain; perceived exertion; personality traits

The presence of naturally occurring muscle pain during certain types of exercise in healthy and injury-free individuals is widely recognized yet poorly understood. The perception of naturally occurring muscle pain has been shown to be distinct from the perception of effort during cycling exercise (Cook, O’Connor, Eubanks, Smith and Lee, 1997; Hamilton,

* Corresponding author. 102-A Ramsey Center, Department of Exercise Science, University of Georgia, Athens, Georgia 30602-6554. Tel.: 706-542-438 1; Fax: 706-542-3 148 dcook(j)coe.u- ga.edu

**The Howard Hughes Summer Undergraduate Research Program provided support for Yangmin Lee.

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Killian, Summers and Jones, 1996). These researchers have also shown that pain intensity increases as a positively accelerating function of the power output generated by the individual. This type of pain may reduce a person’s willingness to perform exercise or the ability of working muscles to contract. While understanding naturally occurring muscle pain during exercise has obvious implications for athletes attempting to achieve optimum perfor- mances, it also has potential relevance for numerous, occupational, and health-related concerns as well.

The positive health benefits of regular exercise are well established (Bouchard, Shephard and Stevens, 1994). However, many people do not perform exercise at an adequate frequency, intensity and duration to obtain the health benefits. Although a sedentary lifestyle is determined by multiple factors, it has been suggested that the muscle pain which occurs naturally during intense muscle contractions could contribute to some people’s decision to avoid exercise altogether (Godin, 1994; Hopman-Rock, Kraaimaat, Bijlsma, 1996). Moreover, there is evidence that women, on average, elect to exercise at a lower intensity than men (King et al., 1992). This raises the possibility that sex differences in pain sensitivity or tolerance could contribute to sex differences in exercise behavior.

Laboratory investigations (Bush, Harkins, Harrington and Price, 1993; Fillingim and Maixner, 1995) and literature reviews (Berkley, 1997) have shown that females report pain earlier, exhibit higher pain ratings during a noxious stimulation, and tolerate noxious stimuli for a shorter time than males. The majority of investigations concerning sex differences in pain have employed stimuli (e .g . , cold pressor, electrical stimulation) to which people are infrequently exposed during normal daily living. In contrast, muscle pain experienced during exercise is common, but it has rarely been investigated.

Perhaps most similar to the naturally occurring muscle pain experienced during exercise is the pain experienced during low intensity muscle contractions performed while blood flow to a limb is occluded. For example, Maixner and Humphrey (1993) investigated pain responses of females and males to dynamic hand grip exercise during occlusion. Initially, handgrip exercise was performed for two minutes at an intensity equal to 30% of a maximal voluntary contraction. A blood pressure cuff was then attached to the upper arm and inflated to 230 mmHg for the purpose of occluding blood flow to the forearm muscles. During occlusion, hand grip exercise was performed every 2 seconds for 20 repetitions and ischemic pain ratings were obtained at two minute intervals for 20 minutes. Pain ratings were higher during this procedure in the female group compared to males.

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SEX & LEG MUSCLE PAIN 185

The purpose of the present investigation was to extend the literature on sex differences in pain by examining whether males and females exhibit differences in naturally occurring leg muscle pain during ramped cycle exercise to maximum tolerance. Based on related research i t was hypothesized that at any given absolute power output females would rate leg muscle pain as more intense compared to males (Noble, Maresh and Ritchey, 1981; O’Connor, Raglin and Morgan, 1996). Also based on related research it was hypothesized, when data were expressed relative to peak power output, that pain intensity ratings for the men would not differ from or would be higher for the women (Eynde and Ostyn, 1986; Koltyn, O’Connor and Morgan, 1991; Noble, Maresh and Ritchey, 1981; Purvis and Cureton, 1981).

METHOD

Subjects

Twenty-six moderately active college-aged females volunteered to partici- pate in the study. Comparisons were made with data from a subset of individuals ( N = 26 medication- and muscle soreness-free males matched on recent physical activity patterns) who previously had completed exercise testing in our labortaory. The mean data for the 26 males reported here have not been published elsewhere; however; individual results from the males in this report have been included in mean data reported previously (i.e., Study 2 and Study 4 in Cook et al., 1997). The females were medication-free except that 27% were taking birth control pills. The investigation was conducted in conformity with the Declaration of Helsinki and prior to participation all subjects signed a consent form approved by the Institutional Review Board for human subjects.

Procedures

All participants reported to the Exercise Psychology Laboratory and completed a medical and a 24-hour history questionnaire which were used to ensure that the subjects were healthy and able to perform a maximal exercise test. Included was a 7-point leg muscle pain scale designed to confirm the participants freedom from resting leg muscle pain/soreness prior to being tested (O’Connor, Morgan and Raglin, 199 I). Those participants reporting resting leg muscle pain prior to testing were rescheduled and completed the

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186 D. B. COOK et a1

testing at a latter date. A structured interview was used to document physical activity patterns over the previous 7-days (Blair, 1984). The procedures for the males and females were identical except that information about menstrual status was obtained from the females only. Female subjects completed a questionnaire that documented birth control usage as well as the beginning of the most recent period of menstrual flow. Those females tested 1 to 14 days after the beginning of the most recent menstruation (n = 14) were classified into a subgroup labeled “follicular phase” while the remaining female participants (n = 8) were placed into a “luteal phase” group.

Maximal Exercise Test

The maximal exercise test was performed on an electronically braked cycle ergometer (Mijnhardt). The seat height and toe clips of the cycle ergometer were adjusted to the desired fit of the subject. While seated, the subjects were given instructions about the pain intensity and perceived exertion ratings that they would report. All tests were performed between 12:OO noon and 5:OO pm to minimize possible diurnal influences. Pedal resistance was controlled by software via an interface cable from a computer to the ergometer. All subjects completed the same exercise test. The protocol began with 1 minute of seated rest followed by 4 minutes of warm up at 25 watts (W). The exercise test began with an initial power output of 50 W. The power output continually increased at a rate of 24 W per minute until exhaustion or the point at which the subjects could no longer keep up with the prescribed minimum pedal rate (50 rpm). Prior to the test, subjects were fitted with a UNIQ Heart Watch set to memory mode in order to assess peak heart rate during the exercise bout.

Pain Assessment

Pain intensity was assessed using a category scale with ratio properties. The category scale was devised by making minor modifications to Borg’s 0 to 10 category-ratio scale for measuring perceived exertion (Borg, 1982). The modifications consisted of slight rewording of the verbal anchors based on the Pain Perception Profile (Tursky, Jammer, Freidman, 1982). The pain intensity scale ranged from 0 (no pain at all) at 10 (extremely intense pain, almost unbearable). With this scale, if the subjective intensity increases above 10, the subject is free to choose any number larger in proportion to 10 that describes the proportionate growth of the sensation. All participants were tested using the same scale which is presented in Figure 1. Our initial

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SEX & LEG MUSCLE PAIN

Pain Intensity Scale

No pain at all

Very faint pain (just noticeable)

Weak pain

Mild pain

Moderate pain

Somewhat strong pain

Strong pain

Very strong pain

Extremely intense pain (almost unbearable)

Unbearable pain

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FIGURE 1 Pain scale employed in the investigation.

work with the instrument provided evidence for the reliability and validity of this tool as well as it usefulness in quantifying pain during an exercise test (Cook et al., 1997). All subjects were asked to read the following instructions carefully regarding the pain scales.

Category Scale Instructions

You are about to undergo a maximal exercise test. The scale before you contains the numbers 0 to 10. You will use this scale to assess the perceptions of pain in your legs during and after the exercise test. In this context, pain is defined as the intensity of hurt that you feel. Don’t underestimate or overestimate the degree of hurt you feel, just try to estimate it as honestly and objectively as possible.

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188 D. B. COOK et al.

The numbers on the scale represent a range of pain intensity from “very faint pain” (number 1 /2) to “extremely intense pain-almost unbearable” (number 10). When you feel no pain in your legs, you should respond with the number zero. When the pain in your legs becomes just noticeable, you should respond with the number 1/2. If your legs feel extremely strong pain that is almost unbearable, you should respond with the number 10. If the pain is greater than 10 respond with the number that represents the pain intensity you feel in relation to 10. In other words, if the pain is twice as great then respond with the number 20.

Repeatedly during the test you will be asked to rate the feelings of pain in your legs. When rating these pain sensations, be sure to attend only to the specific sensations in your legs and not report other pains you may be feeling (e.g., seat discomfort).

It is very important that your ratings of pain intensity reflect only the degree of hurt you are feeling in your legs both during and after exercise. Do not use your ratings as an expression of fatigue (i.e., inability of the muscle to produce force) or relief that the exercise task is completed.

In summary you will be asked to: (a) provide pain intensity ratings in your legs only; (b) give ratings as accurately as possible; and (c) not under-or over- estimate the pain, but simply rate your pain honestly. You should use the verbal expressions to help rate your sensations.

Subjects were verbally instructed that they would be repeatedly asked to rate the intensity of the pain in their legs, and that they should point to the number that corresponds to that intensity. Additionally, subjects were instructed to remember to depress a button next to the scale when the pain in their legs was just noticeable (pain threshold). The button stopped a timer (unreadable to the subject) that was started at the time exercise began and indicated the time that pain threshold was reached for each subject.

Pain ratings were obtained every minute during exercise until leg muscle pain threshold was reached, at this point ratings were obtained every 30 seconds until the participants was unable to maintain the minimum pedal rate. At this point, the test ended and the subjects immediately stopped moving their legs while continuing to expire into the mouthpiece for three minutes. Pain ratings were obtained every 15 seconds during this recovery period.

Perceived Exertion

Ratings of perceived exertion (RPE) were obtained during and following exercise using a standardized 6 to 20 scale (Borg, 1978). Explicit previously described written and oral instructions were provided on the use of the RPE scale (Cook et al., 1997). This scale has been widely used for research and clinical application and there is a significant corpus of data to support its

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SEX & LEG MUSCLE PAIN 189

validity (Noble and Robertson, 1996). Ratings of exertion were obtained prior to each pain rating.

Metabolic Measurements

During the exercise test measurements of oxygen consumption (VOz), carbon dioxide production (VC03, and expired ventilatory volume (Ve) were obtained every 20 seconds using a metabolic cart (Sensormedics, model 2900). The flow meter was calibrated by making multiple comparisons to a three liter piston syringe. 0 2 and C 0 2 analyzers were calibrated automatically by the metabolic cart’s software following the presentation of high span and low span gases. The high span gas mixture contained -26.0% O2 and -4.0% C 0 2 while the low span mixture contained - 16.0% O2 and -0% C02. The Scholander technique was used to verify the concentration of 0 2 and C02 in the tanks containing high and low span gas mixtures (Scholander, 1947).

Personality

In order to examine the potential role of personality on pain responses to exercise the participants completed the Eysenck Personality Questionnaire (EPQ) (1 3 ) . The EPQ provides measures of extroversion, neuroticism, psychoticism, and conformity. Based on Eysenck’s personality theory and prior research showing personality correlates of pain (e.g., Eysenck and Eysenck, 1985; Morgan and Horstman, 1978; Spangler and Reynolds, 1991) it was hypothesized that a moderate negative correlation would exist for both females and males between extroversion and pain ratings during exercise, and that a moderate positive correlation would be found between neuroticism and pain ratings during exercise for both groups.

Statistical Analysis

The magnitude of sex differences was determined by computing the effect size as defined by Cohen’s d (Cohen, 1988). Pain threshold and peak pain rating data were analyzed using independent t-tests. Two-way repeated measures ANOVAs were used to examine main effects for Sex and Exercise Intensity as well as interaction effects. ANCOVAs were employed to examine main and interaction effects after partialling out the effects of selected potential confounding factors. Pearson correlations were used to examine the relationships between the 0- 10 pain ratings and the RPE measures. Data are reported as mean * SD.

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RESULTS

Subjects' Characteristics

Females were characterized by a mean age of 21.9f3.5yrs, height of 165.4 f 7.0 cm, and weight of 59.5 f 8.6 kg. Males were characterized by a mean age of 23.2f4.0yrs, height of 180.6&5.4cm, and weight of 78.9 f 9.4 kg. As expected, the males possessed higher maximal aerobic power than the females, and both groups exhibited values that were within the range on normal on this variable. The seven-day interview data indicated that both groups were moderately active; the mean energy expenditure for females being 268.1 f 31.2 kcal.kg-'wkp' and for males was 268.6 f 40.4 kcal.kgp'wk-'.

Pain Threshold Data

Pain threshold data for females and males are presented in Table 1. The magnitude of the group differences in measures of exercise intensity at leg muscle pain threshold were small ( d l 0.40) and not significantly different between groups.

High variability in pain threshold was found for both groups. Pain threshold for female subjects occurred at an absolute power output that ranged from 25 to 232 watts and a relative power output that ranged from 10% to 90% of peak power output. Pain threshold for male subjects occurred at an absolute power output that ranged from 66 to 322 watts and a relative power output that ranged from 24% to 95% of peak power output.

Pain, Effort and Metabolism During Exercise

Pain responses at selected submaximal absolute exercise intensities are shown in Figure 2. The data are limited to exercise intensities that all 52

TABLE I female ( N = 26) and male ( N = 26) participants

Absolute and relative (% peak) measures of exercise intensity at pain threshold for

Females Males Effect %Peak mean (2Z SD) mean (fSD) Size(d) Q s

Power Output (watts) 130.0 (f46.5) 148.2 (h56.6) 0.35 61.5% 48.8% VO2 (ml.kg-'min-') 23.2 (& 8.3) 20.8 ( 2 Z 8.8) 0.28 63.6% 47.9% Perceived Exertion 12.0 (h 2.7) 10.9 (5 2.8) 0.40 64.2% 55.6%

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SEX & LEG MUSCLE PAIN 191

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Power Output (W) FIGURE 2 146, & 158 watts for male ( N = 26) and female ( N = 26) subjects.

Mean pain intensity ratings (ISE) at absolute power outputs of 98, 110, 122, 134,

subjects were able to complete. Repeated measures ANOVA revealed significant main effects for Sex [F(1,50) = 7.3, p = 0.0091 and Exercise Intensity [F(,,250) = 45.6, p < 0.0011 as well as the Sex X Exercise Intensity interaction [F(5.250) = 4.9, p = 0.0011. The figure illustrates that while sex differences in pain ratings did not exist at low power outputs as the absolute exercise intensity increased the pain ratings increased at a faster rate for the females compared to the males.

Pain responses expressed relative to each individual’s peak power output are shown in Figure 3. Repeated measures ANOVA revealed significant Sex [F(1,50) = 10.4, p = 0.0021 and Exercise Intensity [F(4,200) = 228.7, p < 0.0011 main effects as well as a significant Sex X Exercise Intensity interaction [F(4,200) = 6.3, p < 0.0011 . When the data were expressed relative to peak power output, females rated leg muscle pain as less intense than males and the pain ratings increased at a slower rate for the females compared to the males. Two separate ANCOVA’s, one using peak power output [F(1,49) = 8.7, p = 0.0051 and the other using peak RPE [F(1,49) = 8.8, p = 0.0051 as the covariate, failed to eliminate the main effect for Sex.

Figure 4 illustrates RPE as a function of absolute exercise intensity. Repeated measures ANOVA revealed significant Sex [F(1,50) = 1 1.3, p = 0.0021 and Exercise Intensity [F(5,250) = 261.3, p < 0.0011 main effects

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Percent Peak Power Output

FIGURE 3 power output for male ( N = 26) and female ( N = 26) subjects.

Mean pain intensity ratings (+SE) as a function of relative percentage of peak

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98 110 122. 134 146 158

Power Output (W) FIGURE 4 110, 122, 134, 146, & 158 watts f6r male (A' = 26) and female ( N = 26) subjects.

Mean ratings (*SE) of perceived exertion (RPE) at absolute power outputs of 98,

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SEX & LEG MUSCLE PAIN 193

as well as a significant Group X Exercise Intensity interaction [F(5,250) = 6.7, p = 0.0011. The figure illustrates that females reported the exercise as being more effortful at every absolute power output and that the RPE ratings increased at a faster rate for the females compared to the males.

Figure 5 shows the RPE data at 6O%, 70%, SO%, 90% and 100% of peak power output. Repeated measures ANOVA revealed significant Sex [F(, ,so) = 10.4, p = 0.0021 and Exercise Intensity [F(4,200) = 437.4, p < 0.0011 main effects, but a non-significant interaction [F(4,200) = 1.5, p = 0.221. Females rated the exercise as less effortful at every relative exercise intensity compared to the males.

Both pain and RPE increased as a function of exercise intensity and these constructs were moderately related for both the female: [6O% (Y = 0.34); 70% (Y = 0.41); 80% (Y = 0.55); 90% (Y = 0.49); 100% (Y = 0.68)] and male:

(v = 0.65)] samples. [60% (Y = 0.26); 70% (Y = 0.49); 80% (Y = 0.48); 90% (Y = 0.42); 100%

Peak Exercise Data

Peak values are presented in Table 11. Peak pain ratings for males and females averaged 8.5 (*2.2) and 5.5 (&2.9), respectively. Peak RPE for

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Percent Peak Power Output FIGURE 5 percentage of peak power output for male ( N = 26) and female ( N = 26) subjects.

Mean ratings (hSE) of perceived exertion (RPE) as a functlon of relative

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TABLE I1 Peak data for female ( N = 26) and male ( N = 26) participants

Females Males Effect mean (fSD) mean (fSD) size ( d )

Peak Pain (0- 10) 5.5 (3.0) 8.5 (2.2) 1.2* Peak RPE (6 - 20) 18.7 (1.4) 19.6 (0.9) 0.8* Peak V 0 2 (ml.kg-'rnin-') 36.5 (6.1) 43.4 (6.4) 1.1* Peak Power Output (watts) 211.3 (39.1) 303.6 (27.5) 2.8* Peak Heart Rate (beat min-') 180.3 (10.1) 185.0 (1 1.6) 0.4

*Effects significantly different at p 5 0.008

males and females averaged 19.6 (hO.9) and 18.7 (* 1.4), respectively. Independent t-tests revealed significant differences between groups for all peak data analyzed with the exception of peak heart rate.

Recovery Data for Pain, RPE and V 0 2

Pain intensity, RPE, and VOz responses during recovery are presented in Figures 6 - 8 . Significant Time main effects were found for all three variables. Sex main effects were found for VOz ( F = 9.7, p = 0.003) and pain ratings ( F = 10.0, p = 0.003). The Sex X Time interaction was also

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Time (see)

FIGURE 6 ( N = 26) subjects.

Mean pain ratings as a function of recovery time for male ( N = 26) and female

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FIGURE 7 ( N = 26) and female ( N = 26) subjects.

Mean ratings of perceived exertion as a function of recovery time for male

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Time (sec) FIGURE 8 male ( N = 26) and female ( N = 26) subjects.

Mean oxygen consumption (ml.kg-'min-') as a function of recovery time for

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significant for both V 0 2 ( F = 2.6, p = 0.007) and pain ratings (F(12,600) = 3.6, p < 0.001). All three variables were characterized by a fast initial decline followed by a slower secondary decline.

Influence of Menstrual Cycle and Birth Control Use

No significant differences were observed between the follicular and luteal groups on peak power output, peak RPE, or peak pain ratings. These two groups also did not differ on pain responses to exercise whether expressed in absolute watts or as a percentage of peak power output. There was no effect of birth control usage on any of the pain or exertion responses to exercise.

Personality

The female and male groups possessed mean personality scores that were within the published norms for college-aged individuals (Eysenck and Eysenck, 1975). Mean (fSD) extroversion, neuroticism, psychoticism and conformity scores for the females versus the males, respectively, were as follows: 14.3f4.0 vs. 15.9f3.9; 10.4f4.8 vs. 9.2f4.9; 1 . 4 f 1.3 vs. 3.8 f 2.7; and 8.0 f 3.5 vs. 5.9 f 3.3. Strong correlations within each group (n = 26) were not found between the personality traits measured and pain ratings during exercise. Predicted moderate negative correlations between extroversion and pain ratings (6O%, 70%, 80%, 90% and 100%) were not observed for either the male (range of r = 0.01 to 0.29) or female (range of r = -0.03 to 0.13) group. Predicted moderate positive correlations between neuroticism and pain ratings were not observed for the females (range of r = -0.09 to 0.14). However, such correlations were found for the males at the 60% ( r = -0.41) and 70% (r = -0.36) exercise intensities. Conformity scores were unrelated to pain ratings during exercise, and an unpredicted moderate positive correlation was observed between psychoticism scores and pain ratings at peak exercise (Y = 0.41) for the females.

DISCUSSION

No sex differences in leg muscle pain threshold during cycle ergometry were found, in part, due to the large variability in pain threshold. The large variability and lack of sex difference in pain threshold is consistent with

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SEX & LEG MUSCLE PAIN 197

prior related research (Cook et ul., 1997; Ellermier and Westphal, 1995; Fillingim and Maixner, 1995; Maixner and Humphrey, 1993).

As predicted, females reported higher leg muscle pain ratings than males at selected submaximal power outputs during cycle ergometry (e.g., 122 to 158 watts) and they reported lower pain ratings during submaximal cycling when the data were expressed relative to each participant’s peak power output. Interpretation of these observations requires the consideration of a number of features of the experiment.

Peak Exercise Test Results

V02pcak, RPEPeak and peak power output results for the females and males were consistent with expected values based on prior research with females and males similar in age and physical activity history (Blair, 1984; McArdle, Katch and Katch, 1996; Noble and Robertson, 1996). The average peak RPE was above 18 for both groups; RPE values of 18 and above have been used as one criterion that a “maximal” exercise test has been achieved (American College of Sports Medicine, 1995). Males consistently achieve higher VOZpeak and peak power output values compared to females (McArdle, Katch and Katch, 1996). These expected sex differences occur largely because males have a larger amount of muscle mass, more hemoglobin, and larger oxygen transport organs (e.g., lungs, heart).

Peak pain ratings were found to be higher for the males than the females. Several factors were likely to have contributed to the higher peak pain ratings for the males. On average, the male subjects produced nearly 100 watts more power output than female subjects, and since the exercise test protocol was identical for the men and women the males performed nearly 4 more minutes of exercise than the females. It may be that the higher power output by the males resulted in greater deformation of nociceptive mechanoreceptors (type I11 afferent fibers). Concomitantly, the increased exercise time may have allowed for a greater accumulation of noxious metabolites within the exercising muscle resulting in greater nociceptive chemoreceptor stimulation (type TV afferent fibers). Alternatively, the larger amount of muscle mass required to achieve the higher power output may have been associated with a larger number of nociceptors being stimulated. It seems most likely that all these factors interacted to produce a greater afferent signal which ultimately contributed to higher peak pain ratings for the males.

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Pain and Exertion Ratings at the Absolute Exercise Intensities

Although sex differences in pain ratings did not exist at the lowest absolute power outputs (ie., 98 and 110 watts), as the absolute exercise intensity increased the pain ratings were higher and increased at a faster rate for the females compared to the males. Physiological factors related to exercise capacity, such as the amount of muscle mass and VOZpeak, are likely to have contributed to these sex differences in pain ratings. At any given absolute power output the average female was required to use a greater percentage of the total available muscle fibers compared to the average male. This, combined with the lower maximal aerobic capacity for the females, means that the females were likely to have generated energy using anaerobic metabolic mechanisms at a lower absolute power output compared to the males. The concentration of noxious biochemicals, such as hydrogen ions and bradykinin, are increased in the muscle as a by-product of anaerobic metabolism (Stebbins, Carretero, Mindroiu and Longhurst, 1990). Although muscle or blood concentrations of noxious biochemicals were not measured in the present investigation, it is likely that these factors played a role in the sex differences in pain ratings observed when the data were analyzed in relation to the absolute exercise intensity.

Ratings of perceived exertion were higher for the females compared to the males at all six absolute exercise intensities examined, including the low power outputs (Le., 98 and 110 watts). These findings are in agreement with previous research examining exertional ratings in groups of males and females (Noble, Maresh and Ritchey, 1981; OConnor, Raglin and Morgan, 1996). The reasons for the sex difference in perceived exertion are not clear, but are likely to be related to the fact that on average the males possessed a greater capacity than the females. Consequently, at any absolute power output the females were exercising at a higher intensity relative to their maximal capacity.

Pain Ratings at the Relative Exercise Intensities

Because there is a linear relationship between muscle strength and muscle size (Ikai and Fukunaga, 1968) one technique that can be used to minimize the role of sex differences in muscle mass and peak power output on the pain responses to exercise is to express the data relative to each person’s peak power output. This technique has been found to eliminate or minimize various sex differences that exist during acute exercise including cardiovas- cular, metabolic, and hormonal responses (e.g., Cureton and Sparling, 1980;

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SEX & LEG MUSCLE PAIN 199

Demello, Cureton and Boineau, 1987; Zwiren, Cureton and Hutchinson, 1983). Thus, potential differences in noxious biochemicals produced as a by- product of exercise are minimized by analyzing the data as various percentages of each person’s peak power output (e.g., 6O%, 70%, etc. . .).

After relativizing on peak power output, it was found that the females reported lower pain ratings compared to the males. The exact reason for the sex difference in pain ratings cannot be determined from the design of the present investigation. Our analyses suggest that this sex difference was not attributable to sex differences in selected personality traits. However, females cycling at the same relative exercise intensity as males (e.g., 60%) are producing a lower absolute power output (in watts) because their peak power output is less than the males. Thus, the absolute pressure on mechanically sensitive nociceptors is likely to be less for the females compared to the males.

The finding of lower pain ratings for females is in contrast to previous research examining pain ratings during noxious stimulation (Berkley, 1997; Ellermier and Westphal, 1995; Feine, Bushness, Miron and Duncan, 1991). It is difficult to make direct comparisons between the present and previous work because the current analysis has relied upon relativizing the noxious stimuli to each participant’s maximal, and this procedure is not always appropriate when employing experimental pain procedures such as heat, cold or electrical stimulation. Also, Cook et al. (1997) showed no relationship between ratings of naturally occurring muscle pain and pain ratings in response to noxious pressure. This underscores the difficulty in generalizing from one type of noxious stimulus to others with regard to sex differences in pain ratings.

Menstrual Cycle and Pain Responses

It should be recognized that the methods employed to assess menstrual cycle status are less precise than other available methods such as the use of hormonal measures. Also, the menstrual cycle analyses involved relatively small and unequal sample sizes. Despite these limitations, our analyses suggested that birth control use and the phase of the menstrual cycle at the time of testing did not greatly influence the results of the present investigation. Previous research examining the effects of menstrual phase on pain sensitivity have been equivocal. For example, Tedford, Warren and Flynn ( I 977) reported that in normally menstruating females aversion thresholds for electric shock were highest during the ovulatory phase. In

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200 D. B. COOK et a/.

contrast, Goolkasian (1980) reported that women with normal menstrual periods exhibited increased pain sensitivity to heat during the ovulatory phase. Although not the primary aim of the present investigation, examination of menstrual phase, based on subjective reports of most recent menstruation, revealed no differences in pain ratings in the follicular versus luteal phases. In the absence of any objective measures of phase (e.g., measures of temperature or hormonal changes) combined with the relatively small number of participants per group in these subanalyses it is premature to speculate on possible reasons regarding the lack of a large effect.

Pain Ratings During Recovery

Pain ratings during recovery were characterized by a fast initial decline (e.g., the first 30 seconds) followed by a slower secondary decline (e.g., 30- 180 seconds) for both groups. Higher peak pain ratings achieved by the males likely contributed to their ratings remaining higher than the females during recovery. Both groups reported leg muscle pain during recovery despite the fact that muscle contractions had ceased. These data indirectly suggest that the abatement of leg muscle pain during recovery from exercise is linked to the removal of noxious biochemicals from the leg muscles.

Overall Summary

The main finding of the present investigation was, that when expressed relative to peak power output, females report leg muscle pain as less intense than males during maximal cycling exercise. The mechanisms regarding this difference are not clear, but the greater exercise capacity (e.g., power output and exercise time) of the males is a likely contributor. We believe that leg muscle pain experienced during exercise warrants continued investigation in part because traditional methods of producing laboratory pain lack generalizability, and since a deeper knowledge of muscle pain during exercise may help in understanding diverse phenomena such as the limits of human athletic performance, postexercise analgesia, and why some people enjoy and adhere to vigorous exercise programs while others do not.

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