Multiple professions (i.e., law enforcement, firefighting, rescue workers, among others) are subjected to not only the psychological stress during their professional duties but also the additive stress of high physical demands. The increased morbidity and mortality rates observed in firefighters and law enforcement officers (3,16,25,34) illustrate the resultant negative health effects that this combination of stressors can cause.
The human stress response to brief periods of mental or physical stress involves hormonal responses from the sympathoadrenal axis, which causes the release of catecholamines (epinephrine and norepinephrine), and from the hypothalamic–pituitary–adrenal (HPA) axis, resulting in the release of cortisol (6). In modern society, psychosocial and professional stress contributes to the chronic activation of the sympathoadrenal and HPA axes, thus resulting in cardiometabolic and psychological disturbances (11,21,35).
Psychologically, cortisol has been suggested as a possible mechanism for negative affective states during stressful (physical or psychological) situations and is regarded as an index of distress, helplessness, and perceived uncontrollability (14,18). Conversely, cortisol stimulates gluconeogenesis as a catabolic hormone, acts as an anti-inflammatory agent, and depresses the immune system (4). The interaction of stress hormones resulting from combined physical and psychological stress has been suggested to increase the risk of cardiovascular, metabolic, and immunological disorders (11,21,35).
An increase in the level of cardiorespiratory (CR) fitness is often recommended as an interventional strategy to reduce these risks, because it has been demonstrated that trained men exhibit significantly lower cortisol responses to a mental challenge compared with untrained men (31,32). These studies would suggest that individuals of higher CR fitness should have a less exacerbated cortisol response to the combination of mental and physical stress compared with lower fit individuals, although it is unknown whether higher fitness levels can reduce the exacerbated cortisol responses seen in concurrent stress situations (38,39).
The limited number of studies investigating concurrent stressors has demonstrated exacerbated measures of state anxiety, perceived workload, and cortisol responses in a dual-challenge condition (DCC) compared with an exercise-alone situation among average fitness individuals (22–24,38,39). However, none of these studies have directly investigated the effect of CR fitness level on cortisol responses to a concurrent challenge.
Therefore, the purpose of this study was to examine state anxiety, effort sense, and cortisol responses between lower-fitness (LF) and higher-fitness (HF) level individuals exercising at the same relative intensities while exposed to a mental challenge. It was hypothesized that LF individuals would perceive the concurrent mental challenge and exercise as more anxiety producing, requiring a greater amount of effort, and this would result in unfavorable cortisol responses when compared with that of HF individuals.
Sixteen apparently healthy males were recruited for participation in this study from the university community. Participant recruitment was targeted to obtain HF and LF participants. HF individuals were recruited from university sports clubs and teams (i.e., triathlon, crew, surf lifesaving) and reported deliberate exercise an average of 6 d·wk−1 for a minimum of 90 min per day. LF participants in the study reported that they did not participate in a formal exercise program and that their lifestyle was largely sedentary. As an incentive for participation, LF participants were provided with a tailored exercise prescription after their participation in the study.
Before the study, the university institutional review board approved the project, and informed written consent was obtained from each participant before initiation of data collection.
Participants in this study were classified as (a) low-risk individuals as categorized by the American College of Sports Medicine, (2) risk stratification, (b) free of CR and metabolic disorders, (c) free of any known blood disorders (i.e., anemia and hemophilia), (d) without hearing or vision problems (including color blindness), (e) free of a history of psychological disorders and/or chronic illnesses, (f) native English speakers, (g) not using any prescription or nonprescription medication or tobacco products, (h) consuming an average of less than 10 alcoholic beverages per week, (i) not having experienced any significant life events within 30 d of participation (i.e., death in family, divorce, or wedding), and (j) not having any significant coursework (articles, presentations, or examinations) within 3 d of session 2 or 3.
Participants completed a total of three testing sessions. The initial session was composed of participant orientation, including the completion of an informed consent, health history questionnaire, and familiarization with the testing protocols. An assessment of the participants’ maximal oxygen consumption (V˙O2max) was also performed using a Parvo Medics TrueOne 2400 integrated metabolic measurement system (Parvo Medics, Sandy, UT). Air volume was measured continuously using a Hans Rudolph 3813 (Hans Rudolph, Inc., Kansas City, MO) pneumotachometer to measure ventilation, and before each experimental session, the O2 and CO2 gas analyzers were calibrated with gases of known composition. The room temperature and humidity were within 1.9°C and 3%, respectively, between all testing sessions.
The V˙O2max test was performed on a CompuTrainer Pro Cycle (CompuTrainer, San Clemente, CA) ergometer with the workload beginning at 100 W and increasing by 50 W every 90 s until either the primary criterion of a plateau in oxygen consumption (V˙O2) with an increase in workload was met or two of the three secondary criteria were achieved. The secondary criteria included 1) reaching age-predicted maximal heart rate, 2) achieving a RER of greater than 1.1, and 3) reporting a RPE (15-point Borg Scale) of 19 or 20.
The second and third sessions were randomized and counterbalanced within groups and between conditions. The second session was conducted within 1 wk of the initial session, and a minimum of 7 d and a maximum of 14 d separated sessions 2 and 3. For sessions 2 and 3, participants reported to the laboratory at 0700 after an overnight (8 h) fast. An intravenous catheter (Jelco, 20 G, 25 mm; Smiths Medical, Dublin, OH) was inserted into an antecubital vein in the nondominant arm, and a needle-free extension set (Medex, SM5005; Bayville, NJ) was connected to the catheter, which had a CLC2000 positive displacement connector (ICU Medical, San Clemente, CA) attached to maintain patency of the catheter. All venous catheters were inserted before 0715 for both sessions, and in session 3, the catheters were inserted within a ±15-min window based upon the time of venous catheter insertion from the session 2 insertion time. Participants then acclimated to the laboratory for an additional 70 min to allow HPA axis hormones to return to baseline levels (10).
The DCC and exercise-alone condition (EAC) consisted of moderate-intensity cycling at 60% V˙O2max for 37 min. The workload was calculated using the equations developed by the American College of Sports Medicine (2) and validated via CR measurement using a Parvo Medics TrueMax metabolic cart. The DCC also included participation in a mental challenge consisting of a modified computer-based Stroop Color–Word (SCW) task and mental arithmetic (MA) task (1,39) while cycling from minute 12 until 32 min (Fig. 1).
The SCW task is a conflicting color–word task, in which the participant is presented with a color word (RED, GREEN, BLUE, or YELLOW) that appears on a computer screen directly in front of the participant. The participant is asked to identify the font color in which the word is presented, while an auditory conflict (the color words spoken aloud by the computer) is also presented in conjunction with the visual appearance of the color words on the computer screen. Each color word is presented for 500 ms, and the participant has an additional 500 ms in which to respond before a new color word appeared.
The MA task consists of the participants subtracting 3, 7, 8, or 13 from a three-digit number. After typing their response on a computer keyboard located at hand level on their dominant side, the participant received feedback as to the accuracy of his response. In the case of a correct answer, the MA program continued and the next problem was presented in a serial fashion. In the case of an incorrect answer, the word “WRONG” was presented in a 40-point red font in the center of the screen along with disquieting auditory feedback (beep or horn). The participants were then presented with the next calculation to be performed. The participants were provided with a maximum of 10 s per problem to provide their response; failure to do so resulted in a “WRONG” response and auditory feedback.
The mental challenge consisted of five cycles of the SCW and MA (2-min SCW followed by 2-min MA) for a total of 20 min to promote psychological stress. In addition, during the mental challenges, a research confederate provided critical, evaluative feedback to the participant at systematic intervals suggesting that the participant had answered incorrectly, was committing too many errors, or was taking too long to respond. The provision of positive feedback was avoided at all times so as to further enhance the stress load of the DCC.
During all testing sessions, a Parvo Medics TrueOne 2400 integrated metabolic measurement system was used to assess measures of V˙O2 and RER during exercise at 5, 10, 20, 32, and 37 min in both the DCC and EAC to ensure equivalent physiological workload between the two conditions. All exercise was performed on a CompuTrainer Pro Cycle ergometer with workload controlled by a CompuTrainer Coaching Software (Version 1.1; CompuTrainer, Seattle, WA) program specifically written for each participant. An Authorware program (Macromedia, 1999) was specifically written to maintain consistency in data collection timing for the experimental protocols.
To assess perceptions of stress, measures of state anxiety (State Anxiety Inventory (SAI)) (12) were taken before exercise (−75, −30, and 0 min), during exercise (10, 32, and 37 min), and after exercise (recovery 15 (R15) and R45 min). In addition, RPE (8) were taken at 5, 10, 32, and 37 min during exercise. Finally, within 5 min of completion of each condition, participants were asked to complete the NASA Task Load Index (NTLX) (19) to assess the overall perceived workload (Fig. 1).
During the pre- and postexercise measures, the participants were instructed to read each item and indicate which response coincided with their feelings at that moment in time. For the exercise portion of the protocol, a member of the research team showed the participant each scale item and read the statement aloud, directing the participant to point at the response that corresponded with their feelings at that time. The researcher confirmed each participant response by repeating the participant’s choice aloud to ensure accuracy.
Serial blood samples for cortisol analysis were collected before exercise (−70, −50, −30, and 0 min), during exercise (10, 20, 32, and 37 min), and after the conclusion of exercise during recovery (R15, R30, R45, and R60 min). After each blood draw, an equivalent volume of physiological saline was replaced to equal the blood withdrawn. During each blood draw, the first 1 mL of blood (with saline from the catheter lock) was collected into a discard tube preceding the sample draw and 1 mL of blood was added to a tube containing sodium heparin for cortisol analyses. All blood samples were centrifuged for 15 min at 2500 rpm at 4°C, and the plasma samples were stored at −80°C for subsequent analyses. Hematocrit (microcapillary method) and hemoglobin values (cyanmethemoglobin end point colorimetric method; Pointe Scientific, Canton, MI) were used to determine plasma volume shifts (15).
Cortisol levels were determined using a radioimmunoassay assay technique (Coat-A-Count-Cort kit; Diagnostic Products Corporation, Los Angeles, CA). Interassay coefficient of variation for the cortisol analysis was 4.77%, and the intraassay coefficient of variation was 1.89%. The standard curve for the range of 0 to 50 μg·mL−1 had a correlation coefficient of 0.988.
Statistical analysis was conducted using SPSS version 18.0 (SPSS, Chicago, IL), and all data are reported as mean ± SEM. Average cadence and distance ridden during exercise were analyzed using 2 × 2 (condition × group) ANOVA to compare workload in the two conditions and between fitness groups. Measures of V˙O2 and RER were examined using a 2 × 2 × 5 (condition × fitness × time) repeated-measures ANOVA (RMANOVA) to compare the physical stimulus in both conditions.
NTLX measures between the two conditions were compared using 2 × 2 ANOVA to assess participants’ perceptions of overall workload in each condition between the two conditions. A 2 × 2 × 8 RMANOVA was conducted for SAI measures, whereas a 2 × 2 × 4 RMANOVA was conducted on reported RPE values. Cortisol values were analyzed using a 2 × 2 × 12 RMANOVA. Significant interactions were further analyzed using one-way ANOVA and paired t-tests with Bonferroni corrections, with the α level set at P ≤ 0.05.
Cortisol analysis was further conducted using trapezoidal integrated area-under-the-curve (AUC) calculations (30) from 0 min to R60 to measure the overall (AUCoverall) release of cortisol during the two conditions and between fitness levels. A second trapezoidal AUC was conducted to compare cortisol responses during the exercise-alone or dual-challenge portion (AUCchallenge) of the protocol (0 min to 37 min). Finally, a third AUC (AUCrecovery) was calculated to evaluate the recovery over time (29), to assess the cortisol responses from the end of the physical challenge (37 min) to the final recovery time point (R60), corrected by the last measurement. A 2 (group) × 2 (condition) ANOVA was conducted for each AUC (AUCoverall, AUCchallenge, and AUCrecovery) to assess differences between HF and LF participants in the two conditions.
Eight HF individuals (V˙O2max = 51.18 ± 2.09 mL·kg−1·min−1) and eight LF individuals (V˙O2max = 36.58 ± 3.36 mL·kg−1·min−1) completed the initial familiarization session and two experimental sessions. A significant difference between HF and LF participants for V˙O2max (t 14 = 0.24, P < 0.001) was confirmed, with no other differences seen in the demographic characteristics (Table 1). Participants were classified HF or LF based upon the percentile values for maximal aerobic power (2), with HF participants categorized in the 80th percentile or greater for their age and LF participants in the 15th percentile or lower for their age, which corresponds with “excellent” and “poor” ratings for CR fitness.
No significant differences were revealed for average cadence or distance ridden for HF or LF participants within their respective conditions or between groups. In addition, HF and LF participants did not differ in their performance on the mental challenges (Table 2).
RMANOVA for V˙O2 and RER revealed no significant interaction effects, although there was a significant main effect for time with V˙O2 (F 4,56 = 9.80, P < 0.001) increasing across time and RER (F 4,56 = 38.25, P < 0.001) decreasing across time in both the DCC and EAC.
Analyses performed on the NTLX measure to assess the perception of overall workload in each condition demonstrated that there were no significant differences in NTLX scores between HF and LF individuals in either condition, although both HF and LF participants did report significantly greater NTLX scores in the DCC (HF = 71.50 ± 5.22, LF = 76.00 ± 5.31) when compared with the EAC (HF = 46.88 ± 4.94, LF = 46.75 ± 4.80).
Self-report measures of RPE did not reveal any significant interaction effects and revealed only a main effect for time (F 3,42 = 36.36, P = 0.001) with RPE increasing equally across time in both conditions among both HF and LF participants (Fig. 2).
SAI measures did not demonstrate a significant condition by fitness level by time interaction; however, a significant condition by time interaction effect (F 7,98 = 3.37, P = 0.018) was observed with greater increases across time in the DCC compared with the EAC. SAI changed differently in the DCC from 10 to 32 min with a significant elevation at 32 min (DCC = 20.38 ± 1.12; EAC = 17.31 ± 1.04; t 15 = 2.44, P = 0.027). There was no significant difference in SAI values between conditions for HF and LF individuals.
No differences in baseline (0 min) measures were observed for cortisol (Fig. 3A, B). RMANOVA for cortisol revealed a condition by time interaction (F 8,112 = 15.60, P = 0.04) with greater relative elevations in the DCC compared with the EAC. Post hoc analysis revealed greater elevations in the DCC at 32 min (DCC = 27.32 ± 1.30; EAC = 20.55 ± 1.70; t 15 = 2.82, P < 0.05), 37 min (DCC = 26.94 ± 1.48; EAC = 20.20 ± 1.35; t 15 = 3.32, P < 0.05), and R15 min (DCC = 26.16 ± 1.57; EAC = 20.42 ± 1.69; t 15 = 2.22, P = 0.043). There was also a significant time main effect (F 8,112 = 2.99, P = 0.01), whereas during the DCC, cortisol decreased from −70 min to 0 min, increased from 0 to 37 min, and decreased again from 37 to R60 min, whereas in the EAC, cortisol decreased from the beginning of the study (−70 min) to R60. There was no significant condition by time by fitness, condition by fitness, or fitness by time interaction effects. In addition, there were no fitness differences, so that HF and LF cortisol values followed a similar pattern in the DCC and EAC, respectively.
Analysis of cortisol AUCoverall (Fig. 3C) revealed only a main effect for condition, with significantly higher AUCoverall concentration of cortisol in the DCC compared with the EAC (F 1,14 = 14.00, P = 0.01). A significant interaction effect for AUCchallenge (Fig. 3D) was revealed (F 1,14 = 12.81, P = 0.003), with the LF participants demonstrating a greater AUCchallenge response in the DCC (t 7 = 5.50, P = 0.001) compared with HF participants. Finally, AUCrecovery (Fig. 3E) revealed a main effect difference for condition (F 1,14 = 13.62, P = 0.002), with the DCC resulting in a greater cortisol level compared with the EAC.
This study examined the effects of a combination of acute mental challenge and physical stress on psychological and cortisol responses between LF and HF individuals. The DCC did reveal exacerbations in cortisol levels as the result of the dual challenge, and these exacerbations were even greater among LF individuals, especially during the perturbation portion of the protocol. This is the first study to demonstrate differences in cortisol responses to concurrent mental and physical challenge between aerobically high-fit and low-fit individuals.
A fundamental finding of this study was that the participants perceived their overall workload as significantly greater in the DCC as compared with the EAC. The significant differences in NTLX and SAI scores between the DCC and EAC support the idea that responses in the variables of interest in this study can be attributed to the dual nature of the perturbation in the DCC rather than the result of a greater physical workload. Finally, both HF and LF participants in this study demonstrated equivalent RPE responses in both conditions, thus suggesting that the participants perceived their physical effort to be equal even though the physiological responses were different in the two conditions.
Cortisol increases were revealed in the DCC both across time and overall, thereby demonstrating that the DCC caused a greater activation in the HPA axis because of the addition of a mental challenge during exercise. Furthermore, the AUCchallenge demonstrated that HF participants had an attenuated cortisol response to the dual challenge relative to the LF individuals. This finding suggests that the HPA axis responses to stress may be sensitive to CR fitness level. The AUCchallenge was calculated following the formula described by Pruessner et al. (30).
In studies by Rimmele et al. (31,32), it was demonstrated that trained men exhibited significantly lower cortisol and heart rate responses than their untrained counterparts in responses to a psychosocial stressor alone. These, combined with the lower state anxiety levels of the trained men, indicate an overall reduced reactivity to psychosocial stress. The investigators speculated that trained individuals would generally appraise acute psychosocial stressors as less threatening and more controllable than untrained individuals, which may then alter HPA axis activity, resulting in the observed cortisol responses.
Moya-Albiol et al. (27) also demonstrated that trained individuals had lower cortisol responses to a maximal ergometer test when compared with untrained individuals. One questionable result in this study was that the cortisol levels of the trained individuals actually decreased from the basal measurement to the postergometer measurement, whereas the levels of cortisol increased significantly in the untrained individuals. This finding is notable because cortisol is shown to be sensitive to intensity levels, with cortisol increasing significantly at V˙O2 levels >80% V˙O2max (20,40). However, the finding still suggests that HPA axis may respond to a physiological challenge differently based upon training status.
The findings of these studies and Dienstbier’s model of physiological toughness (14) support the results of the current study suggesting that the HPA axis is sensitive to CR fitness. However, this is the first study to examine a dual-stress challenge.
The current study involved acute exposure to the dual challenges, which resulted in unfavorable and uneconomical cortisol responses in LF participants. The results of the current study suggest that although there is no difference in HF versus LF individuals at a specific time point, the difference in responses shown to occur during the dual-challenge portion of the protocol is greater among LF individuals. Given the catabolic function of cortisol, prolonged cortisol activation has deleterious consequences such as decreasing lean body mass, increasing fat mass, inducing insulin resistance, as well as contributing to cardiovascular disease (9,11,21,35). Inferences may be made to the possible pathophysiological effects of recurrent exposure to dual stressors in occupations where this occurs on a regular basis.
The findings of this study are particularly applicable to populations of individuals typically exposed to concurrent challenges, especially in light of the prevalence of being overweight and obesity among populations of firefighters (5,28,41), law enforcement officers (17,42), military personnel (26,33), and emergency medical technicians (36). Although it must be understood that obesity or being overweight is not necessarily equivalent to a lack of CR fitness, it represents another risk factor for cardiometabolic diseases. It may also be important to inform individuals exposed to combined challenges that CR fitness level is important to coping with occupational stressors, although more research is needed to determine what the minimum CR level might be to obtaining these benefits.
Interpretations of the results of this study are limited by the specific types of stressors used in this study. The physical stress of cycling in the laboratory setting may not be as intense as experienced by those individuals whose occupation necessitates exposure to dual stressors. Likewise, the psychological stress of the mental challenges may be perceived differently by each individual, according to familiarity with MA and other factors such as sensitivity to the social evaluation by the research confederate. The combination of physical and psychological stress during the DCC is non–life threatening and may not concur with the reality of working in an occupation exposed to combined physical and psychological stress. Furthermore, it has been suggested that the HPA axis is not particularly sensitive to the effects of MA or SCW (7) and that tasks that are perceived as uncontrollable with a social-evaluative threat such as public speaking (13) are more effective in stimulating the HPA axis. However, previous studies have demonstrated that the stressor used in this study is effective in stimulating an HPA axis response (22,37,39).
Overall, this study demonstrates that a dual-stress laboratory condition results in exacerbated cortisol responses, which are more greatly exacerbated in LF individuals. It was expected that increases in these responses would be attenuated in individuals with higher fitness levels, which holds true for this study, because the release of cortisol was higher in LF participants during the DCC (mental challenge during cycling) compared with the exercise-only condition, whereas there was no difference between conditions in the HF participants. The cortisol responses described in this study suggest that a high level of fitness may provide another benefit that has not been previously described in the literature. Further investigation in this area is required to develop strategies to prevent stress-induced dysfunction in individuals whose occupation includes psychological and physical stress such as the dual stress condition.
This study had no external funding sources.
The authors wish to thanks the participants of this study for their time and the reviewers for their insightful comments.
The authors report no conflicts of interest.
None of the results of this study constitute endorsement by the American College of Sports Medicine.
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