The effects of power and stretch yoga on affect and salivary cortisol in women

Abstract

Yoga practice is known to improve well-being and decrease stress. However, acute yoga is understudied. This study investigated the effects of two different types of yoga on affect and salivary cortisol levels in college women. Thirty-three women aged 18–30 years each completed 1-hour sessions of power yoga and stretch yoga. Measures of affect and salivary cortisol were assessed before, during, and after each session. Participants perceived power yoga to be more pleasurable and energizing. Salivary cortisol significantly decreased after both yoga sessions. Thus, even one session of yoga may be effective in improving affect and decreasing stress in college women.

Keywords

affect cortisol females physical activity stress well-being

Introduction

Psychological stress and physiological stress are normal parts of daily living, usually arising from the pressure of life events. However, when chronically present, high levels of stress can cause a disturbance in body’s homeostasis and may impact an individual’s overall well-being (Kagias et al., 2012; McEwen and Lasley, 2002). Chronic stress can be detrimental to one’s overall physical and mental health, leading to increased risk of headaches, depression, anxiety, and heart disease (Bulo and Sanchez, 2014). Physical stress and emotional stress are also major stimuli for activating the hypothalamic–pituitary–adrenal (HPA) axis, thus leading to increased secretion of the stress hormone cortisol from the adrenal cortex (Considine, 2013). Elevated levels of cortisol over a prolonged period of time can lead to suppressed immune responses, insomnia, severe mood swings, depression, and severe hypotension (Kandhalu, 2013).

In recent years, reports have shown an increase in stress levels among millennials (Kandhalu, 2013). One population of millennials especially vulnerable to stress is college students, particularly college women. Women tend to experience more stress on a daily basis compared to men and therefore report feeling significantly higher levels of recent or ongoing stress and depressive symptoms (Matud, 2004). In 2015, the American College Health Association (ACHA) found that 0.9 percent of women experienced no stress compared to 4.0 percent of men, and that 46.3 percent of women experienced more than average stress compared to 36.8 percent of men (ACHA, 2016). Some of the most common stressors among college students include greater academic demands, being independent in a new environment, financial responsibilities, exposure to new people, and making decisions on a higher level (Bulo and Sanchez, 2014). Stress tends to be the greatest factor negatively impacting college women’s academic performance (ACHA, 2016). Due to the increased incidence of stress and its detrimental effects, this population is actively searching for ways to help lower their elevated stress levels.

Recently, yoga has gained momentum as a practice that can alleviate the symptoms of stress in many individuals, including college women. Continually practicing yoga has been shown to improve symptoms of depression, anxiety, stress, post-traumatic stress disorder, and quality of life (Woodyard, 2011). Yoga practice also helps individuals by slowing breathing rate, lowering heart rate (HR), and blood pressure, as well as increasing blood flow to vital organs (Woodyard, 2011). Additionally, a significant decrease in diastolic blood pressure and stress perception, as well as improvements in muscular strength and endurance, flexibility, and health perceptions, were observed after a 6-week intervention in participants performing Ashtanga yoga (a rigorous style of yoga that includes focusing on ujjayi breathing while performing postures), whereas only improvements in muscular strength and endurance, and flexibility were observed in participants performing Hatha yoga (a gentle style of yoga that includes basic postures, breathing exercises, and meditation) (Cowen and Adams, 2005). Therefore, these findings suggest that different benefits may be associated with different styles and intensities of yoga.

Hartfiel et al. (2011) examined the effectiveness of a 6-week yoga intervention on enhancing emotional well-being and resilience to stress among university employees. Improvements in feelings of clear-mindedness, energy, and confidence were observed in those who practiced yoga. In addition, increased life purpose, satisfaction, and feelings of greater self-confidence during stressful events were reported. Smith et al. (2011) investigated the impact of a 7-week intervention of exercise-based yoga versus integrated yoga (i.e. yoga that also includes an ethical/spiritual component) on measures of depression, anxiety, stress, and salivary cortisol levels in 81 undergraduate students with mild to moderate stress, depression, or anxiety. Depression and stress were decreased in both yoga groups, but anxiety-related symptoms were decreased only in the integrated yoga group. Similarly, decreased salivary cortisol levels were observed only in the integrated yoga group.

While the effects of yoga interventions seem to show improvements in psychological well-being and decreases in cortisol, the effect of acute bouts of yoga on psychological well-being and cortisol have not been examined in as much detail. One study has examined the influence of one 90-minute session of Hatha yoga compared to one 90-minute session of African Dance on affect and salivary cortisol levels (West et al., 2004). In this study, it was found that positive affect increased in the African Dance condition, but did not change in the Hatha yoga condition; however, both treatment groups showed decreases in negative affect. Lower salivary cortisol levels were observed for the Hatha yoga condition, whereas increased salivary cortisol levels were observed for the African Dance condition, but cortisol was not related to these changes in affect. The authors attributed this to the differential changes in cortisol that occurred to the two types of activity and the differences in physiological activation resulting from this activity.

To date, little research has been conducted investigating the effects of different types and intensities of yoga on affect and salivary cortisol levels. Therefore, the purpose of this study was to determine the acute effect of a lower intensity stretch yoga (similar to Hatha yoga) compared to higher intensity power yoga (similar to Ashtanga yoga) on affect and salivary cortisol levels in college females. Since acute bouts of aerobic exercise have often shown improvements in positive affect or positively valenced states (Ekkekakis et al., 2008, 2011; Reed, 2013), it follows that acute bouts of yoga may have similar effects. Based on previous research, it was hypothesized that both stretch yoga and power yoga would improve affect and decrease salivary cortisol levels.

Methods

Participants

In total, 33 young women of all yoga experience levels were recruited for participation in this study. All participants were between the ages of 18 and 30 years and were free of any cardiovascular, musculoskeletal, or other limitations that would make exercise participation unsafe. Participants were recruited from the university graduate and undergraduate programs after obtaining approval from the university Institutional Review Board. Participants were recruited via flyers and word of mouth. Participants with and without yoga experience were asked to volunteer for three evenings of yoga sessions. Informed consent was obtained from all individuals included in the study before proceeding with any study procedures.

Psychological measurements

During the baseline visit, participants completed the Perceived Stress Scale (PSS) (Cohen and Williamson, 1988). The PSS is a 10-item questionnaire which asks how unpredictable, uncontrollable, and overloaded respondents find their daily lives to be. Scores can range from 0 to 40, where higher scores suggest higher levels of stress (Cohen and Williamson, 1988).

Affect was measured based on the constructs of the circumplex model of affect (Russell, 1980) using the Feeling Scale (FS) and the Felt Arousal Scale (FAS) (Hardy and Rejeski, 1989; Svebak and Murgatroyd, 1985) and the Activation-Deactivation Adjective Check List (AD ACL; Thayer, 1989). The FS is used to assess affective valence, that is, the pleasantness or unpleasantness that an individual might associate with a particular situation (Hardy and Rejeski, 1989). The FS employs an 11-point, single-item, bipolar measure of pleasure–displeasure that has a scale ranging from −5 (“Very Bad”) to +5 (“Very Good”). The FAS is used to assess how aroused or activated an individual feels (Svebak and Murgatroyd, 1985). The FAS uses a six-point, single-item measure with anchors at 1 (Low Arousal) and 6 (High Arousal). The FS and FAS were administered at three time points for each session: pre-session, midway through the session, and post-session.

The AD ACL was used to measure positive and negative affect at the beginning and end of each testing session (Thayer, 1986, 1989). The AD ACL is a 20-item instrument listing various adjectives, and participants are asked to rate how those adjectives describe them at that moment on a scale of 1–4 (i.e. “not at all” to “very much so”). The AD ACL assesses the bipolar dimensions of Energetic Arousal (EA) and Tense Arousal (TA), where the EA dimension ranges from Energy to Tiredness and the TA dimension ranges from Tension to Calmness.

Physiological measurements

HR was monitored continuously throughout each session (Polar Team², Lake Success, NY, USA) After each session was completed, each participant’s average HR for the entire session was extracted using Polar Team² software. Additionally, approximately 1 mL of saliva was collected before and after each session in order to analyze the salivary cortisol levels. Samples were stored in microcentrifuge vials at −20°C until time of analysis. Cortisol concentrations were measured using enzyme-linked immunosorbent assay (ELISA) kits (Abnova, Tapei City, Taiwan). The cortisol assay had an analytical sensitivity of 0.024 ng/mL, intra-assay coefficients of variation of 3.8–5.8 percent and inter-assay coefficients of variation of 6.2–6.4 percent. All samples were run in duplicate.

Experimental procedures

Each participant completed four study visits: one baseline visit, one stretch yoga session, one power yoga session, and one control session. During the baseline visit, participants received an explanation of the study protocol, information regarding the risks and benefits of participation, and were asked to sign the informed consent document. Height and body mass were measured using a SECA mechanical scale and stadiometer (SECA 700, SECA, Chion, CA, USA). Percent body fat was measured using a hand-held bioelectrical impedance analysis device (BF 306, OMRON, Bannockburn, IL, USA). Resting HR was measured by palpating the radial pulse for 1 minute. Additional demographics including age, race/ethnicity, and education level were also recorded. Participants completed the Current Health Status Questionnaire and the Physical Activity Readiness Questionnaire in order to obtain information regarding participants’ current state of physical and mental health and ability to participate in physical exercise (American College of Sports Medicine, 2009; Canadian Society for Exercise Physiology, 2002).

The testing period consisted of the stretch yoga session, the power yoga session, and the control session. All three sessions occurred on consecutive days within 1 week and participants were randomly assigned to the order of the three sessions, and participants with the same session order assignment were tested together in groups. Each session occurred between 5:00 and 7:00 pm and all participants completed the three sessions between the 8th and 16th days of their menstrual cycle in order to control for diurnal and hormonal fluctuations that could affect study variables. The same room was also used for all three sessions in order to control for noise, lighting, and surrounding environment. In order to control for the effect of food and beverage consumption on cortisol levels, participants were given a commercially available granola bar and a 500 mL bottle of water prior to each session, which they were to consume 2 hours before the start of the session. No other food or beverage was allowed to be consumed until the end of each session. Additionally, participants were instructed to refrain from consuming alcohol, tobacco, caffeine, and to avoid vigorous exercise for at least 12 hours prior to each session (Evans et al., 2016).

The power yoga session was composed of opening meditation (including child’s pose, focused breathing, set intention), warm-up (including cat/cow, down-down, yoga push-ups), standing sequence (including sun salutation), seated poses (including pigeon, camel, happy baby), and Savasana (including laying on your back and relaxing your body and mind) with closing meditation. The stretch yoga session consisted of opening meditation (including child’s pose, focused breathing, set intention), yin poses (including butterfly, straddle fold, sphinx/seal, frog), and Savasana (the corpse pose) with a closing meditation. Power yoga was designed to be of higher intensity and faster pace than the stretch yoga session. The control session consisted of watching an educational movie called “Earth” which was produced by Disney (Director Fothergill, Director Linfield, and Writer Megahey, 2009). Showing the movie was chosen to standardize the control session. All yoga sessions were led by the same instructor, and the two yoga sessions and the control session were each 60 minutes in duration.

Statistical analysis

All statistical analyses were conducted using IBM SPSS, Version 22.0. Significance was set a priori at an alpha level of ≤0.05. Repeated Measures General Linear Model (RM GLM) were conducted to determine whether there were differences in affect across time and between conditions as measured by the AD ACL and the FS and FAS. Changes in salivary cortisol concentration before and after each condition were compared using repeated-measures analyses of variance (ANOVAs). Fisher’s least significant difference (LSD) test was used when main effects were found. All data are expressed as mean ± standard deviation (SD). Power analysis suggested that with the sample size (n = 33), a large effect size would be able to be detected with an alpha level at 0.05 and power of 0.8, based on the results and sample size of West et al. (2004).

Results

Physical characteristics and perceived stress

Physical characteristics and PSS scores for the 33 study participants are presented in Table 1. Participant PSS scores indicated that psychological stress levels upon enrollment were within normal limits and perceived high levels of previous stress should not influence the results of this study (Cohen and Williamson, 1988).

Table 1.

Participant characteristics (N = 33).

Characteristics	Mean ± SD
Age (years)	21 ± 2
Height (cm)	165 ± 6
Weight (kg)	64 ± 11
BMI (kg/m²)	23 ± 4
Body fat (%)	23 ± 6
Resting heart rate (bpm)	60 ± 7
PSS score	15 ± 8

bpm: beats per minute; PSS: Perceived Stress Scale; BMI: body mass index.

Affect

The results for FS and FAS are depicted in Figure 1 and Table 2. There was a significant main effect for condition (F(4, 29) = 9.12, p < 0.001), time (F(4, 29) = 23.10, p < 0.001) as well as a significant condition × time interaction effect (F(8, 25) = 13.17, p < 0.001). The univariate tests revealed that the condition effect was present for both the FS (F(2, 64) = 5.16, p = 0.008) and FAS (F(2, 64) = 14.60, p < 0.001). This was attributable to FS being more positive in the stretch yoga and power yoga conditions compared to the control condition (p = 0.005 and 0.012, respectively). For FAS, the power yoga condition elicited greater FAS than both the control and stretch yoga conditions (p < 0.001). The time effect was also due to differences in FS (F(2, 64) = 44.18, p < 0.001) and FAS (F(2, 64) = 5.40, p = 0.007). Both the during-session FS and post-session FS were significantly higher than baseline FS (p < 0.001), and post-session FS was also significantly higher than during-session FS (p < 0.001). For FAS, the only significant difference was that during-session FAS was significantly higher than post-session FAS (p < 0.001).

Figure 1.

Feeling Scale and Felt Arousal Scale responses to three different conditions.

Table 2.

Means (± SD) for affective responses for the three conditions (N = 33).

Variable	Time	Power yoga (mean ± SD)	Control (mean ± SD)	Stretch yoga (mean ± SD)
Feeling Scale	Pre-trial	1.64 ± 1.80	2.06 ± 1.70	1.76 ± 1.79
	During	3.03 ± 1.21	2.42 ± 1.35	3.21 ± 0.99
	Post-trial	4.03 ± 0.73	2.39 ± 1.34	3.73 ± 1.01
Felt Arousal Scale	Pre-trial	2.91 ± 1.26	3.09 ± 1.16	2.70 ± 1.24
	During	3.94 ± 1.17	2.52 ± 0.91	2.58 ± 0.75
	Post-trial	3.00 ± 1.17	2.27 ± 0.88	2.24 ± 1.28
Energetic Arousal	Pre-trial	22.24 ± 6.15	24.12 ± 6.57	22.55 ± 7.50
Energetic Arousal	Post-trial	24.88 ± 5.56	18.33 ± 5.08	19.64 ± 5.25
Tense Arousal	Pre-trial	21.88 ± 5.68	22.27 ± 4.60	21.33 ± 5.35
Tense Arousal	Post-trial	16.30 ± 3.67	17.48 ± 4.25	14.82 ± 3.80

SD: standard deviation.

The condition × time interaction was also due to differences in FS (F(4, 128) = 11.10, p < 0.001) and FAS (F(4, 128) = 7.09, p < 0.001). The interaction for FS came from FS being more positive in both the stretch and power yoga sessions compared to the control session both during and post-session. The interaction for FAS was due to a greater increase in FAS both during and following the power yoga condition compared to the other two conditions, both of which had a decrease in FAS both during and post-session.

The results for EA and TA are shown in Figure 2 as well as in Table 2. There was also a significant main effect for condition (F(4, 29) = 4.34, p < 0.001), time (F(4, 29) = 41.94, p < 0.001) and a significant condition × time interaction effect (F(8, 25) = 7.05, p < 0.001) for EA and TA. Univariate analyses found that condition effect was only significant for EA (F(2, 64) = 4.72, p = 0.012) with TA showing no statistically significant difference (F(2, 64) = 2.73, p = 0.073). This condition effect was attributable to EA being greater in the power yoga condition compared to the stretch yoga and control conditions (p = 0.004 and 0.033, respectively). The time effect was found to be due to differences in EA (F(1, 32) = 5.39, p = 0.027) and TA (F(1, 32) = 85.05, p < 0.001), with both EA and TA decreasing following conditions. The condition × time interaction was also due to differences in EA (F(2, 64) = 16.74, p < 0.001), but not TA (F(2, 64) = 1.08, p = 0.35). The interaction for EA was due to an increase in EA post-session for the power yoga condition, while EA decreased in both the stretch yoga and control conditions.

Figure 2.

Energetic Arousal (EA) and Tense Arousal (TA) responses to three different conditions.

Salivary cortisol and HR responses

Salivary cortisol concentrations before and after each session are shown in Figure 3. Significant decreases in salivary cortisol concentrations were observed pre- to post-session for power yoga (3.66 ± 2.91 vs. 2.31 ± 1.17 ng/mL, p = 0.002), stretch yoga (3.85 ± 2.10 vs. 2.43 ± 1.36 ng/mL, p < 0.001), as well as for the control condition (3.09 ± 1.65 vs. 2.08 ± 0.98 ng/mL, p < 0.001). The magnitude of change in salivary cortisol levels were similar for all three conditions (power: −20.83% ± 40.76%, stretch: −31.35% ± 25.4%, control: −27.57% ± 28.55; p = 0.29). Average HR was significantly higher for the duration of the power yoga session compared to the stretch yoga session and the control session (107 ± 13 bpm, 81 ± 10 bpm, and 74 ± 14 bpm respectively, p < 0.05).

Figure 3.

Salivary cortisol responses to three different conditions.

Discussion

The purpose of this study was to determine the effects of two different types of yoga on affect and salivary cortisol levels in college females. The results of this study indicate that the power and stretch yoga both showed a significant increase in pleasure as indicated by the FS. These findings are similar to what has typically been found in acute aerobic exercise (Ekkekakis et al., 2008, 2011). In addition, lower salivary cortisol levels were observed from pre- to post-session for both of the yoga conditions and control session. These lower salivary cortisol levels in response to an acute yoga session are consistent with previous research by West et al. (2004). However, unlike the West et al. (2004) study, this study found increases in positive affect as measured by EA and decreases in negative affect as measured by TA.

Previous intervention-based yoga studies found favorable results similar to this study. Noggle et al. (2012) examined the effects of a 10-week Kripalu-based yoga intervention (a yoga style that is characterized by physical postures, breathing exercises, relaxation, and meditation) in high school students. Total mood disturbance and tension anxiety improved in yoga students and worsened in those who participated in the usual physical education program. In addition, positive affect remained unchanged, while negative affect significantly worsened in the physical education group and improved in the yoga group. Meissner et al. (2016) examined the effects of a short-term (2 weeks) traditional yoga practice approach compared to that of a western yoga practice on determinants of well-being. Positive and negative affect scores remained constant for the western yoga group, while traditional yoga practitioners displayed an increase from baseline in positive affect and decrease in negative affect. A strong negative relationship was also observed between positive affect and perceived stress. Additionally, Smith et al. (2011) observed decreased depression and stress following a 7-week yoga intervention. They also found that the participants in the integrated yoga group (an exercise-based yoga accompanied by a meditation) experienced decreased anxiety-related symptoms compared to participants who were performing only an exercise-based yoga. Therefore, the results of this study support existing findings of the intervention-based studies by Noggle et al. (2012), Meissner et al. (2016) and Smith et al. (2011). These findings suggest a positive effect of yoga on psychosocial well-being and reduced perceived stress.

In this study, reduction of similar magnitudes was observed for salivary cortisol levels after both the power yoga and stretch yoga sessions. These findings are consistent with those of West et al. (2004) who observed lower cortisol levels in response to a session of Hatha yoga as well as those of Smith et al. (2011) who observed a significant reduction in salivary cortisol levels in response to a 7-week intervention of integrated yoga. It was expected that salivary cortisol levels would only decrease after the stretch yoga session since the power yoga session was performed at a higher intensity, and continuous exercise of a moderate or high intensity is associated with an increase in cortisol levels (McMurray and Hackney, 2005). Although average HR throughout the power yoga session was significantly higher compared to the stretch yoga session, the relative exercise intensity for both sessions based on these HR responses was still quite low (<40% of HR reserve based on the Karvonen formula) which is less than the intensity that is generally required to elevate blood cortisol concentrations (McMurray and Hackney, 2005). Additionally, the inclusion of the 10-minute cool-down period (Savasana) at the end of both sessions may have been further responsible for the similar decrease in salivary cortisol levels that were observed after both the power yoga and the stretch yoga sessions. As West et al. (2004) postulated for their study’s results, the concentrated breathing and postural positions that were performed during both the stretch and power yoga sessions in this study may have led to the decrease in salivary cortisol levels independent of the changes in affect.

This study is one of the only studies to examine acute responses to two different styles of yoga. Additionally, this study is one of the first to examine and compare the effects of different yoga intensities on both psychological and physiological parameters. As with any study, however, there are several limitations to this investigation. Salivary cortisol levels and affect questionnaires were assessed and administered following, rather than before, the 10-minute Savasana at the end of the power and stretch yoga sessions, which may have impacted the results observed. Decreased post-session activation, arousal levels, and salivary cortisol were observed following both the stretch and power yoga sessions, which may have been largely due to the effect of the Savasana. Therefore, it is difficult to determine the true impact of higher intensity power yoga session without having obtained these results both before and after the Savasana. Although previously it has been suggested that there were no differences found between yoga experience levels and stress (Brisbon and Lowery, 2011), participants varied in yoga experience ranging from non-practitioners to experienced, as well as all participants were women. Therefore, the results of this study may be difficult to generalize to other populations. Another limitation may have been time constraint (Brisbon and Lowery, 2011). Data collection took place during spring and fall semesters. Students may have been pressed for time to complete questionnaires and may not have been able to dedicate enough time to finish questionnaires accurately which may have influenced their scores.

Given these limitations, the results of this study do provide meaningful information regarding how college-aged women may respond to one session of both lower intensity and higher intensity yoga. As stated previously, college-aged women are a population that seems to be particularly vulnerable to experiencing negative effects of stress, and may actively look for ways to reduce stress (ACHA, 2016; Matud, 2004). As yoga has gained popularity as a practice that is often used to alleviate the negative symptoms of stress, it is important to continue to objectively investigate the psychosocial and physiological effects of yoga, in order to strengthen the findings in the literature. Additionally, while most studies examine the cumulative psychosocial and physiological effect of yoga interventions that extend over a period of days, weeks, or months, this study addresses how individuals might respond after just one session of yoga. The information gained from this study suggests that even one session of yoga may improve the mental state as well as decrease a physiological marker of stress levels (i.e. cortisol). Therefore, the beneficial effects of yoga may be experienced in both the short term and long term, which may be attractive to individuals who are thinking of engaging in yoga as part of their physical activity.

In conclusion, the results of this study indicate that power and stretch yoga would be effective ways for college-aged females to acutely improve psychological well-being and decrease stress, and that these effects are evident even after one session. Future yoga intervention studies could be used to confirm and expand the results of this study. Since this study only examined the acute effects of power and stretch yoga, future studies may need to focus on the effects of longer exposure of different style of yoga on reducing stress to determine long-term effects on one’s well-being.

Footnotes

Declaration of conflicting interests

The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding

The author(s) disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: The authorreceived financial support from the Office of Undergraduate Research and Faculty Research and Development funds at Elon University.

References

American College Health Association (ACHA) (2016) American College Health Association-National College Health Assessment II: Reference Group Executive Summary Fall 2015. Hanover, MD: ACHA.

American College of Sports Medicine (2009) ACSM’s Guidelines for Exercise Testing and Prescription (8th edn). Baltimore, MD: Lippincott Williams & Wilkins.

Brisbon

Lowery

(2011) Mindfulness and levels of stress: a comparison of beginner and advanced Hatha yoga practitioners. Journal of Religion and Health 50(4): 931-941.

Bulo

Sanchez

(2014) Sources of stress among college students. CVCITC Research Journal 1(1): 16–25.

Canadian Society for Exercise Physiology (2002) Physical Activity Readiness Questionnaire (PAR-Q). Victoria, BC, Canada: British Columbia Ministry of Health.

Cohen

Williamson

(1988) Perceived stress in a probability sample of the United States. In: Spacapan

Oskamp

(eds) The Social Psychology of Health. Newbury Park, CA: sage, pp. 31–68.

Considine

(2013) Adrenal gland. In: Rhoades

Bell

(eds) Medical Physiology: Principles for Clinical Medicine (4th edn). Philadelphia, PA: Lippincott Williams & Wilkins, pp. 633–648.

Cowen

Adams

(2005) Physical and perceptual benefits of yoga asana practice: Results of a pilot study. Journal of Bodywork and Movement Therapies 9(3): 211–219.

Ekkekakis

Hall

Petruzzello

(2008) The relationship between exercise intensity and affective responses demystified: To crack the 40-year-old nut, replace the 40-year-old nutcracker! Annals of Behavioral Medicine 35(2): 136–149.

10.

Ekkekakis

Parfitt

Petruzzello

(2011) The pleasure and displeasure people feel when they exercise at different intensities: Decennial update and progress towards a tripartite rationale for exercise intensity prescription. Sports Medicine 41(8): 641–671.

11.

Evans

Hackney

Pebole

, et al. (2016) Adrenal hormone and metabolic biomarker responses to 30 min of intermittent cycling exercise in breast cancer survivors. International Journal of Sports Medicine 37(12): 921–929.

12.

Hardy

Rejeski

(1989) Not what, but how one feels: The measurement of affect during exercise. Journal of Sport and Exercise Psychology 11(3): 304–317.

13.

Hartfiel

Havenhand

Khalsa

, et al. (2011) The effectiveness of yoga for the improvement of well-being and resilience to stress in the workplace. Scandinavian Journal of Work Environment & Health 37(1): 70–76.

14.

Kagias

Nehammer

Pocock

(2012) Neuronal responses to physiological stress. Frontiers in Genetics 3: 222.

15.

Kandhalu

(2013) Effects of cortisol on physical and psychological aspects of the body and effective ways by which one can reduce stress. Berkeley Scientific Journal 18(1): 14–16.

16.

McEwen

Lasley

(2002) The End of Stress as We Know It. Washington, DC: Joseph Henry Press.

17.

McMurray

Hackney

(2005) Endocrine responses to exercise and training. In: Garrett

Kirdendall

(eds) Exercise and Sport Science. Philadelphia, PA: Lippincott Williams & Wilkins, pp. 135–160.

18.

Matud

(2004) Gender differences in stress and coping styles. Personality and Individual Differences 37(7): 1401–1415.

19.

Meissner

Cantell

Steiner

, et al. (2016) Evaluating emotional well-being after a short-term traditional yoga practice approach in yoga practitioners with an existing western-type yoga practice. Evidence-Based Complementary and Alternative Medicine 2016: 1–9. DOI: 10.1155/2016/7216982.

20.

Noggle

Steiner

Minami

, et al. (2012) Benefits of yoga for psychosocial well-being in a US high school curriculum. Journal of Developmental and Behavioral Pediatrics 33(3): 193–201.

21.

Reed

(2013) Effect of acute and regular aerobic physical activity on positive activated affect. In: Ekkekakis

(ed.) Routledge Handbook of Physical Activity and Mental Health. New York: Routledge, pp. 413–421.

22.

Russell

(1980) A circumplex model of affect. Journal of Personality and Social Psychology 39(6): 1161–1178.

23.

Smith

Greer

Sheets

, et al. (2011) Is there more to yoga than exercise? Alternative Therapies in Health and Medicine 17(3): 22–29.

24.

Svebak

Murgatroyd

(1985) Metamotivational dominance: A multimethod validation of reversal theory constructs. Journal of Personality and Social Psychology 48(1): 107–116.

25.

Thayer

(1986) Activation-deactivation adjective check list: Current overview and structural analysis. Psychological Reports 58: 607–614.

26.

Thayer

(1989) The Biopsychology of Mood and Arousal. New York: Oxford University Press.

27.

West

Otte

Geher

, et al. (2004) Effects of Hatha yoga and African dance on perceived stress, affect, and salivary cortisol. Annals of Behavioral Medicine 28(2): 114–118.

28.

Woodyard

(2011) Exploring the therapeutic effects of yoga and its ability to increase quality of life. International Journal of Yoga 4(2): 49–54.