Sex-related hormonal influences on pain and analgesic responses

Considerable evidence indicates sex-related differences in pain responses and in the effectiveness of various analgesic agents. Specifically, females are at greater risk for experiencing many forms of clinical pain and are more sensitive to experimentally induced pain relative to males. Regarding analgesic responses, nonhuman animal studies indicate greater opioid analgesia for males, while a limited human literature suggests the opposite. Though the mechanisms underlying these effects remain unclear, the influence of gonadal hormones on nociceptive processing represents one plausible pathway whereby such sex differences could emerge. The present article reviews the complex literature concerning sex steroid effects on pain responses and analgesia. First, nonhuman animal research related to hormonal effects on nociceptive sensitivity and analgesic responses is presented. Next, human studies regarding gonadal hormonal influences on experimental pain responses are reviewed. Several potential mechanisms underlying hormonal effects on nociceptive processing are discussed, including hormonal effects to both peripheral and central nervous system pathways involved in pain transmission. Finally, based on these findings we draw several conclusions and make specific recommendations that will guide future research as it attempts to elucidate the magnitude and importance of sex-related hormonal effects on the experience of pain.

Low intensity vagal nerve stimulation lowers human thermal pain thresholds

The effect of vagal nerve stimulation (VNS) on thermal pain sensation was studied in eight subjects who had vagal nerve stimulators surgically implanted for purposes of seizure control. Prior to their involvement in the study, all subjects had the intensity of their VNS (30 Hz, 0.5 ms, 1.0-2.75 mA) adjusted upwards until achieving their desired clinical effect of reduced seizures. Thermal pain thresholds were determined using a Medoc TSA-2001 with a thermode applied to the skin of the forearm. During VNS at settings 100% of those used clinically to control their seizures, subjects showed a statistically significant decrease in their thermal pain threshold of 1.1+/-0.4 degrees C. Acute effects of graded VNS on thermal pain thresholds were determined in seven of the subjects after cessation of chronic VNS. Two thermal threshold measurements were obtained while the subject received sham stimulation (0 mA intensity), during tactile control stimulation and during 30 s of VNS at intensities approximately 33, 66 and 100% of the settings utilized to control their seizures. Tactile control stimulation was provided by electrical stimulation of the skin of the ankle with the intensity adjusted by the patient to match the intensity of any sensations felt in the neck during VNS. Subjects were not aware of the settings employed. Their stimulator was adjusted with each trial and an ascending/descending ordering of intensity was utilized with an inter-trial interval of 2 min. Thermal pain thresholds were significantly decreased in relation to tactile control stimulation at all intensities of VNS tested with the greatest effect occurring at the 66% level. Subjects were also monitored non-invasively and hemodynamic responses to VNS were determined. No significant alterations in hemodynamic variables were observed. The findings of this human study are consistent with experiments in non-human animals which demonstrate a pro-nociceptive effect of low intensity VNS.

Sex-dependent effects of reported familial pain history on recent pain complaints and experimental pain responses

Previous research has demonstrated that both sex and familial pain history can influence clinical pain, and sex is known to affect experimental pain responses. However, the potential interactive effects of sex and family history on pain-related symptoms and experimental pain have not been investigated. This experiment examined recent pain complaints and laboratory pain responses as a function of sex and reported family history of pain in 212 (122 female, 90 male) young adults. All subjects completed questionnaires regarding family history of pain, recent pain experiences, and psychological measures of hypervigilance. Then, warmth detection thresholds, heat pain thresholds and heat pain tolerances were determined. Results revealed sex-dependent influences of familial pain history on recent pain complaints and experimental pain responses. Specifically, a positive family history of pain was associated with increased reports of pain over the previous month and poorer general health as well as enhanced sensitivity to thermal stimuli among females but not males. Higher levels of hypervigilance accounted for some of the family history effects on recent pain complaints but not experimental pain measures. Potential mechanisms underlying these effects of family history among females are discussed.

The relationship of sex and clinical pain to experimental pain responses

Considerable research indicates increased experience of clinical pain among females relative to males, and females also demonstrate enhanced responses to experimentally-induced pain. However, previous research has not investigated the relationship between clinical and experimental pain responses in healthy females and males. This experiment examined recent clinical pain as well as thermal pain thresholds and tolerances in 209 (117 female, 92 male) healthy young adults. All subjects completed questionnaires concerning pain-related symptoms over the previous month and subsequently underwent thermal pain assessment. Females reported a larger number of pain sites and greater health care utilization over the month preceding the experimental session, and females also exhibited greater sensitivity to thermal stimuli. In addition, females above the median on the number of pain episodes demonstrated greater thermal pain sensitivity compared to females below the median, but thermal pain responses did not differ as a function of clinical pain among males. The differences remained significant after correcting for psychological variables including hypervigilance and sex role expectancies. These results indicate that experimental pain responses may be more clinically relevant for females than males. Potential explanations and implications for this pattern of results are discussed.

Effects of gender and acute dental pain on thermal pain responses

OBJECTIVE

Considerable research suggests that females exhibit greater sensitivity to laboratory pain procedures than do males; however, whether the presence of acute clinical pain influences this sex difference in pain sensitivity has not been investigated. The present experiment investigated the effects of sex and acute dental pain on laboratory pain responses.

DESIGN

Thermal pain onset and tolerance were determined in 46 dental patients (15 male, 31 female) experiencing pain due to acute irreversible pulpitis and in 33 healthy controls (13 male, 20 female). In addition, measures of mood and coping were obtained in all participants. All subjects participated in two experimental sessions. The first session took place immediately before the patients underwent endodontic treatment for relief of pulpal pain. The second session took place approximately 1-2 weeks later, when pulpitis patients were pain free after treatment. During each session, thermal pain onset and tolerance were assessed with a 1-cm2 contact thermode applied to the right volar forearm using an ascending method of limits.

RESULTS

During both sessions, thermal pain onset and tolerance were lower in control females than in control males; however, male and female pulpitis patients did not differ in their thermal pain responses during either session. Pulpitis patients also showed greater affective distress than controls.

CONCLUSIONS

These data suggest that the sex difference in thermal pain sensitivity frequently reported in pain-free subjects appears to be absent in patients presenting with acute dental pain. However, this effect cannot be explained solely based on the presence of clinical pain because the effect on pain threshold and tolerance persisted into session 2, when pulpitis patients were pain free. Potential explanations for these results are discussed.

Sex differences in heat pain thresholds as a function of assessment method and rate of rise

Studies of sex differences in the responses to experimentally induced pain demonstrate greater pain sensitivity among females than males. However, studies investigating heat pain responses have produced inconsistent results. Differences in stimulus characteristics and assessment methods probably account for this variability. This study examined sex differences in the heat pain threshold as a function of two different assessment methods and varying rates of rise. Nineteen female and 18 male healthy volunteers underwent heat pain threshold assessment via the method of levels and the method of limits. In addition, both fast (4.0 degrees C/s) and slow (0.5 degrees C/s) rates of rise were used for the method of levels assessments. In order to examine the reliability of threshold values, each subject participated in two sessions, separated by approximately 8 days. Females evinced lower thresholds than males for the method of levels assessments with both slow and fast rates of rise (ps < 0.05), while no sex differences emerged for the threshold assessed via the method of limits. Test-retest reliability coefficients were relatively high. However, thresholds generally increased significantly from session 1 to session 2. Between method correlations were generally low to moderate. These findings indicate that the method of levels may be more sensitive to sex differences than the more commonly used method of limits. Also, thresholds appear to increase from session 1 to session 2, and thresholds assessed via different methods are not strongly correlated. Potential implications of these results for experimental pain assessment are discussed.

Self-reported abuse history and pain complaints among young adults

OBJECTIVE

Considerable evidence suggests that a self-reported history of physical and/or sexual abuse is more frequently reported among chronic pain populations and is associated with poorer adjustment to pain. However, previous research has typically included patients seeking treatment for pain, whereas few population-based studies have explored the association between abuse history and pain. This purpose of this study was to examine the association between self-reported history of sexual or physical abuse and recent pain complaints, health-related variables, and psychological disturbance among a nonclinical sample of young adults.

DESIGN

Subjects were 426 (275 female, 151 male) college students who completed a series of questionnaires assessing abuse history, recent pain, health care utilization, perceived health, and psychological variables.

RESULTS

Females reported a positive history of abuse (PHA) more frequently than males (43.5% vs. 23.8%), and females reported significantly higher rates for all types of abuse except physical abuse during childhood (p < 0.05). PHA subjects reported experiencing pain in more sites and pain of higher severity over the past month compared to subjects with a negative history of abuse (NHA) (p < 0.05). PHA subjects also reported more health care utilization and greater psychological disturbance, including depression, somatization, negative temperament, and higher levels of catastrophizing (p < 0.05). Interestingly, when somatization and depression scale scores were used as covariates, group differences in pain complaints and health care utilization became nonsignificant (p > 0.10).

CONCLUSION

These findings suggest that a self-reported history of physical or sexual abuse is associated with increased pain complaints, health care utilization, and psychological disturbance even among young adults from a nonclinical population. Moreover, the association between abuse and pain complaints appears to be moderated at least in part by the higher levels of somatization and depression observed in the PHA group.

Sexual and physical abuse history in subjects with temporomandibular disorders: relationship to clinical variables, pain sensitivity, and psychologic factors

Recent evidence suggests that a past history of physical and/or sexual abuse is more frequently reported among chronic pain populations; however, the prevalence of reported abuse has not been examined in patients with chronic orofacial pain caused by temporomandibular disorders (TMD). This study compares reported physical/sexual abuse among female TMD subjects recruited from the general population with that of age-matched female control subjects. The association of reported abuse with clinical pain, experimental pain responses, and psychologic variables was examined in the TMD group. Results indicated that a slightly but not statistically greater percentage of TMD subjects (44.8%) reported a history of sexual or physical abuse compared to control subjects (33.3%). Reported abuse among TMD subjects was not related to clinical pain or psychologic variables. Regarding experimental pain responses, TMD subjects reporting a history of abuse exhibited longer ischemic pain tolerances compared to those not reporting abuse; however, the groups did not differ on other experimental pain measures. Results indicate that the reported prevalence of physical/sexual abuse is similar among TMD subjects compared to other chronic pain populations; however, the relationship of abuse to clinical and psychosocial variables remains unclear.

Ethnic differences in thermal pain responses

OBJECTIVE

Although numerous studies have reported ethnic differences in the prevalence and severity of clinical pain, little is known about how these differences affect the perception of experimental pain. The present experiment examined the effects of ethnicity (African American vs. white) on thermal pain responses in a healthy undergraduate population.

METHODS

Thirty white subjects (16 women and 14 men) and 18 African Americans (10 women and 8 men) participated in the study. Thermal testing included evaluation of the following: warmth thresholds, thermal pain thresholds, thermal pain tolerances, and magnitude estimates of both the intensity and unpleasantness of thermal pain (at 46 degrees, 47 degrees, 48 degrees, and 49 degrees C).

RESULTS

Although no group differences emerged for warmth thresholds, thermal pain thresholds, or pain intensity ratings, African Americans demonstrated lower thermal pain tolerances than whites. In addition, African Americans had smaller slopes and larger intercepts than whites for ratings of pain unpleasantness. Additional analyses suggested that these findings were a consequence of group differences in thermal pain unpleasantness ratings at the lowest temperatures assessed (46 degrees and 47 degrees C); at these temperatures, African Americans rated the stimuli as more unpleasant than whites. Finally, group differences in thermal pain tolerance and thermal pain unpleasantness ratings seemed to partially account for greater self-reported daily pain symptoms among African Americans.

CONCLUSIONS

Collectively, these findings seem to suggest ethnic differences in the perception of the affective-motivational dimension of thermal pain.

Resting blood pressure and thermal pain responses among females: effects on pain unpleasantness but not pain intensity

In order to investigate the influence of resting blood pressure on thermal pain responses among females, this experiment studied 21 pain-free, normotensive females (mean age 23) classified as having high (HBP) vs. low blood pressure (LBP) based on a median split of resting blood pressure collected prior to thermal testing. All subjects then underwent thermal pain testing, including determination of pain threshold and tolerance followed by verbal descriptor ratings of the intensity and unpleasantness of suprathreshold thermal stimuli, ranging from 45 to 49 degrees C. Thermal stimuli were delivered to the volar forearm and the ipsilateral face using a 1 cm2 contact thermode. Results indicated that the blood pressure groups did not differ in thermal pain threshold or tolerance or on ratings of the intensity of suprathreshold thermal stimuli. However, the HBP group provided significantly lower ratings of thermal pain unpleasantness than the LBP group (P < 0.01). These data indicate that resting blood pressure is inversely associated with pain sensitivity among females, but this relationship may be selective for the affective component of pain.

Generalized vibrotactile allodynia in a patient with temporomandibular disorder

This report presents the findings from a psychophysical study of vibrotactile responses in a patient diagnosed with temporomandibular disorder (TMD). This patient unexpectedly reported pain due to innocuous vibrotactile stimulation, and this allodynia appeared to have a component of temporal summation. The pain response occurred not only in the region of the clinical pain (the face), but also on the volar forearm, where the patient reported no clinical pain. Administration of the N-methyl-D-aspartate (NMDA) receptor antagonist dextromethorphan (DM), but not vehicle, attenuated the vibration-induced pain at both sites.

A psychophysical study of discomfort produced by repeated filling of the urinary bladder

Psychophysical studies were performed in 10 healthy, female volunteers using urinary bladder distension (UBD) as a visceral stimulus. Stimulus methodology was similar to that used clinically for obtaining cystometrograms with a fixed-rate (100 cc/min normal saline) filling of the urinary bladder, occasional pauses and simultaneous measure of bladder pressure using a catheter-transducer assembly. During bladder filling, subjects were asked to report sensations by verbal report and by using an electronic, hand-held, visual-analog-scale device. Sensations evoked by UBD were generally localized to the suprapubic region. UBD produced cardiovascular responses which increased with repeated trials. Sensation intensity increased with repeated UBDs as indicated by global pain ratings. Intravesical pressure and volume correlated with sensation intensity. The volume of distending fluid needed to produce a report of discomfort was highly variable from trial to trial and did not change significantly with repeated UBDs. The intravesical pressure which produced a report of discomfort was less variable and significantly decreased with repeated UBDs. The change in intravesical pressure and volume needed to produce discomfort was inversely correlated with initial intravesical pressure measures. Similar to findings in other organ systems, these findings demonstrate that repeated presentations of a visceral stimulus may lead to an increase in physiological and perceptual responses to pain.

Sex differences in temporal summation but not sensory-discriminative processing of thermal pain

Gender differences in experimental pain sensitivity have been widely investigated, and the results generally indicate that females exhibit greater sensitivity to noxious stimuli than males. However, results using thermal pain procedures have been inconsistent, with some studies reporting greater responses among females and other studies reporting no gender differences. The present study investigated gender differences in thermal pain perception using several different psychophysical procedures. Twenty-seven females and 22 males underwent thermal testing, including: determination of thermal pain threshold and tolerance, a thermal discrimination procedure, real-time magnitude estimates of heat pulses, and temporal summation of thermal pain. The results indicated lower thermal pain threshold and tolerance and greater temporal summation of thermal pain among females, but no gender differences in thermal discrimination or real-time magnitude estimates of discrete heat pulses. These findings suggest that gender differences in thermal pain perception may be more robust for sustained, temporally dynamic thermal stimuli with a strong C-fiber component.

Sex differences in the perception of noxious experimental stimuli: a meta-analysis

Fillingim and Maixner (Fillingim, R.B. and Maixner, W., Pain Forum, 4(4) (1995) 209-221) recently reviewed the body of literature examining possible sex differences in responses to experimentally induced noxious stimulation. Using a 'box score' methodology, they concluded the literature supports sex differences in response to noxious stimuli, with females displaying greater sensitivity. However, Berkley (Berkley, K.J., Pain Forum, 4(4) (1995) 225-227) suggested the failure of a number of studies to reach statistical significance suggests the effect may be small and of little practical significance. This study used meta-analytic methodology to provide quantitative evidence to address the question of the magnitude of these sex differences in response to experimentally induced pain. We found the effect size to range from large to moderate, depending on whether threshold or tolerance were measured and which method of stimulus administration was used. The values for pressure pain and electrical stimulation, for both threshold and tolerance measures, were the largest. For studies employing a threshold measure, the effect for thermal pain was smaller and more variable. The failures to reject the null hypothesis in a number of these studies appear to have been a function of lack of power from an insufficient number of subjects. Given the estimated effect size of 0.55 threshold or 0.57 for tolerance, 41 subjects per group are necessary to provide adequate power (0.70) to test for this difference. Of the 34 studies reviewed by Fillingim and Maixner, only seven were conducted with groups of this magnitude. The results of this study compels to caution authors to obtain adequate sample sizes and hope that this meta-analytic review can aid in the determination of sample size for future studies.

Menstrual cycle, blood pressure and ischemic pain sensitivity in women: a preliminary investigation

Eleven women were tested twice for ischemic pain sensitivity; once during their follicular phase (Days 4-9) and once during their mid-late luteal phase (5-10 days after ovulation) of a confirmed ovulatory cycle. Additionally, in order to examine blood pressure-related hypoalgesic effects, each had 3-4 clinic blood pressures determined during an initial screening interview and each also completed a daily symptom calendar for one complete menstrual cycle prior to testing in order to investigate relationships between 'real life' symptomatology and laboratory-induced pain sensitivity. Results revealed significantly shorter pain tolerance times and marginally shorter pain threshold times in the luteal vs. follicular phase, while verbal descriptors of pain intensity (sensory) and pain unpleasantness (affective) did not vary with cycle phase. Clinic blood pressures were positively correlated with pain threshold and tolerance times assessed during both cycle phases. Real-life physical symptom ratings were predictive of laboratory pain intensity ratings during the follicular phase and tended to predict unpleasantness ratings during both phases. These results not only confirm recent reports of greater sensitivity to ischemic pain in women during the luteal phase of their cycle, but extend the literature by demonstrating pressure-related hypoalgesic effects in women during both cycle phases.

Ischemic but not thermal pain sensitivity varies across the menstrual cycle

OBJECTIVE AND METHOD

Findings from both animal and human research suggest that pain sensitivity changes across the menstrual cycle; however, among humans the nature of these menstrual cycle effects remains unclear. The present study used a repeated-measures design to evaluate changes in thermal and ischemic pain responses during three phases of the menstrual cycle, midfollicular (postmenstrual), ovulatory, and mid-to-late luteal (premenstrual), in 11 healthy women. The cycle phase during which subjects began their participation was determined randomly. Plasma levels of estrogen, progesterone, luteinizing hormone (LH), testosterone, and beta-endorphin were determined at each experimental session. Participants also completed a daily diary of physical and emotional symptoms for two complete menstrual cycles before the experimental sessions.

RESULTS

The results indicated that women showed less ischemic pain sensitivity during the midfollicular compared with the ovulatory and mid-to-late luteal phases, but thermal pain responses did not vary significantly across menstrual cycle phases. Physical and emotional symptoms were minimal and did not change significantly across the menstrual cycle.

CONCLUSIONS

These findings indicate greater ischemic but not thermal pain sensitivity among women after the midcycle LH surge. The practical relevance and potential mechanisms of these findings are discussed.

Pain sensitivity in patients with temporomandibular disorders: relationship to clinical and psychosocial factors

OBJECTIVE

We have previously reported that patients with temporomandibular disorders (TMD) exhibit enhanced sensitivity to experimentally evoked pain (1); however, the clinical relevance of this increased pain sensitivity remains unclear. The purpose of this study was to investigate the relationship of experimental pain sensitivity to clinical and psychosocial variables among patients with TMD.

DESIGN

Thirty-six TMD patients were studied, half of whom were pain sensitive (PS) and the other half pain tolerant (PT), based on their ability to tolerate an ischemic pain task. Responses to painful thermal and nonpainful visual stimuli as well as clinical/diagnostic symptoms and psychosocial variables were compared for the two groups (i.e., PS vs. PT).

RESULTS

Results indicated that, compared with PT patients, the PS group exhibited greater sensitivity to thermal pain and rated innocuous visual stimuli as more intense. PS patients also reported greater clinical pain, but in general the groups did not differ on diagnostic and psychosocial measures.

CONCLUSIONS

The results suggest that ischemic pain tolerance is a clinically relevant marker of pain sensitivity in TMD patients. These findings are consistent with the hypothesis that impairments in CNS inhibitory pathways may contribute to the pain associated with TMD.

The influence of resting blood pressure and gender on pain responses

Recent research suggests that resting blood pressure is inversely related to pain sensitivity, even among normotensives; however, most of these studies have included only male participants. To determine whether this hypoalgesic effect of blood pressure was also present in females, we investigated thermal and ischemic pain responses in a group of age-matched, normotensive females and males as a function of resting blood pressure. Thermal pain threshold and tolerance were determined, and a cross-modality thermal magnitude matching procedure was conducted, after which ischemic pain threshold and tolerance were determined using the submaximal effort tourniquet procedure. Systolic pressure, diastolic pressure, and heart rate were obtained using an automated blood pressure monitor with a pneumatic cuff positioned around the left ankle. Females provided higher normalized thermal magnitude estimates and a shorter time to ischemic pain tolerance, but no gender differences emerged on other pain measures. Systolic, diastolic, and mean arterial pressures were significantly correlated with thermal and ischemic pain responses among males but not females, with higher blood pressure being associated with lower pain sensitivity. After adjusting for resting blood pressure, the gender difference in normalized magnitude estimates was only marginally significant, and the gender difference in ischemic pain tolerance became nonsignificant. These findings are consistent with previous research indicating an inverse relationship between blood pressure and pain sensitivity. Additionally, the findings also suggest that blood pressure may partially moderate gender differences in pain sensitivity. Potential mechanisms and clinical implications of the current findings are discussed.

Assessment of aerobic power in chronic pain patients before and after a multi-disciplinary treatment program

Physical exercise is widely used in the treatment of chronic pain patients, and direct measurement of physical capabilities is needed to objectively document change. In this study, 46 residential chronic pain patients undergoing treatment at a multidisciplinary rehabilitation center were administered a cycle ergometer graded exercise test, using a Medical Graphics CAD/NET exercise system, to measure aerobic fitness and other physiological parameters before and after the four-week treatment program. Patients evinced highly statistically significant changes in all major indices of cardiopulmonary functioning, including MAXVO2 and METS, and a measure of lower body power (WATTS). Possible mechanisms underlying such dramatic changes in this short time period include improved physical fitness, learning or desensitization to symptoms associated with exertion, and improved effort. Documenting treatment-related changes is important, and metabolic exercise testing provides an objective method for assessing changes in functional capacities. Such changes may have important practical implications for these individuals. The importance of assessing and improving aerobic fitness in chronic pain populations is discussed.

The effects of aerobic exercise on cardiovascular, facial EMG, and self-report responses to emotional imagery

This study examined the effects of exercise on subsequent psychophysiological and self-report responses to emotional imagery, using excitation-transfer theory as a guiding conceptual model. Twenty-four female undergraduates engaged in aerobic exercise (stationary cycling) for 15 minutes, and an equal number of subjects rested quietly for the same time period. All subjects then engaged in anger and sadness imagery trials. Cardiovascular, facial electromyographic, and self-report responses to the imagery were assessed. The results indicated that the subjects in the exercise group showed less peripheral vasoconstriction in response to the imagery than did the quiet rest subjects. Subjects in both groups displayed greater electromyographic activity in the depressor and zygomatic muscle regions during anger than sadness imagery, and subjects in the exercise group tended to show greater corrugator tension during sadness than during anger imagery. Few differences between the groups were found on self-report measures. These findings are discussed with reference to previous research, theoretical implications, and future directions.

Acute emotional and cardiovascular effects of stressful mental work during aerobic exercise

The acute effects of engaging in challenging mental work during a single session of aerobic exercise were examined on measures of subjective mood and cardiovascular function. Fifty-seven female subjects were randomly assigned to participate in either a 10-min aerobic exercise condition or a no-exercise control condition. Half of the subjects in each group performed digits backward problems during this time period, and no mental stressors were presented to the other subjects. The results indicated that the exercise and mental stress conditions had additive effects on subjective anxiety levels and on cardiovascular responses during exercise. Both exercise and mental stress increased heart rate. In addition, exercise had anti-anxiety and vasodilative effects, but both of these influences were attenuated by opposing main effects for mental stress exposure. No effects were found for exercise on measures of cardiovascular reactivity to a later digits backward stressor. The results are consistent with previous research in suggesting that exposure to mental stressors during aerobic exercise provides no acute psychological benefits but attenuates some of the mood improvements and vasodilative effects of the exercise activity.

Life events, fitness, hardiness, and health: a simultaneous analysis of proposed stress-resistance effects

The effects of exercise participation, self-perceived fitness level, and dispositional hardiness for promoting stress resistance were examined in a sample of 373 college students. Self-report measures of stressful life experience and recent physical illness were positively correlated, and fitness and hardiness were negatively correlated with illness as expected. Multiple regression analyses indicated that neither fitness nor hardiness provided a stress-moderator effect because neither was found to significantly interact with stress in the prediction of illness scores. Structural equation analyses suggested that hardiness may affect health indirectly by first influencing either the occurrence or subjective interpretation of stressful life events. No direct effect on health was found for exercise participation, although exercise may reduce illness indirectly by improving fitness. Implications for the multivariate modeling of proposed stress-resistance-enhancing effects are discussed.