Maintaining and improving personal health is a key public issue. The American Psychological Association refers to the fact that more than 40% of adults have experienced stress-related health problems (Flight Safety Foundation, 2006). Long-term stress can lead to physiological fatigue and emotional exhaustion (Huang, 2010), with middle-aged women at particular risk (Chang, 2010). Women aged 40–60 years have had several decades of life experience and experience visible signs of aging in terms of physical appearance and health (Chang, 2010). They often need to handle the stresses of family responsibilities and personal careers concurrently. Stress can cause mental and physical disorders if not managed properly (Wu & Lin, 2008).
Stress typically produces a negative psychological impact when an individual is challenged or threatened. The individual can regain balance by using appropriate stress management techniques. Stress stimulates the hypothalamus of the central nervous system to activate endocrine and sympathetic nervous systems, which trigger a series of physiological reactions. Increased and stronger heart rates increase cardiac output, increased blood pressure supplies the body with blood more efficiently, increased blood sugar released by the liver nourishes muscles, and a suppressed immune system increases the possibility of infection (Greenberg, 1983/2000; Ross & Thomas, 2010). Stress has been widely discussed in the literature, with results illustrating that extreme stress takes emotional and physical tolls on the individual (Chen, Kao, & Chen, 2009; Ho et al., 2010; Lucini, Di Fede, Parati, & Pagani, 2005).
Psychological stress is associated with the autonomic nervous system (Cheema, Marshall, Chang, Colagiuri, & Machliss, 2011). Heart rate variability (HRV) is a common physiological indicator used to assess the autonomic nervous system (Furutani, Tanaka, & Agari, 2011; Takada, Ebara, & Kamijima, 2010). A physiological phenomenon in which the time interval between heartbeats varies, HRV, is measured by the variation in the beat-to-beat interval. Methods used to detect beats include ECG and the pulse wave signal derived from a photoplethysmograph (PPG). The PPG measures pulse waves; its signal reflects changes in blood flow detected by infrared light passing through microcirculatory blood vessels. An optical sensor detects the amount of light passed through (or reflected from) the blood flow. This waveform can also be processed to derive beat-by-beat interbeat intervals. Research has shown no statistically significant differences and a significantly high correlation between interbeat interval data measured by both ECG and PPG in short-term steady-state recordings (Bolanos, Nazeran, & Haltiwanger, 2006; Gil et al., 2010). HRV continuous variation is mainly affected by the autonomic nervous system, HRV analysis can be grouped under time domain and frequency domain. Advantages of frequency domain spectral analysis are that it is sensitive, accurate, and quantitative, enabling analysis of short-term steady states and measurement of physiological changes of individual sympathetic and parasympathetic nervous systems (Chen, Tsai, Lo, Tsai, & Jeng, 2005).
Many studies have confirmed that practicing yoga enables individuals to cope with and release stress (Chattha, Raghuram, Venkatram, & Hongasandra, 2008; Satyapriya, Nagendra, Nagarathna, & Padmalatha, 2009; West, Otte, Geher, Johnson, & Mohr, 2004). The usual Western methods define yoga as a complementary or alternative treatment, and it is regarded as a beneficial activity enabling individuals to improve their state of health and maintain mental and physical well-being (Shapiro et al., 2007).
Wu and Lin (2008) analyzed nine studies to study the effectiveness of regular yoga practice for stress reduction. They found that, although various forms of yoga were used, most included Hatha yoga. Hatha yoga uses asana practices to control the autonomous nervous system and muscle function and keep the body clean, flexible, and well lubricated. It also focuses on pranayama and meditation trainings, which relieve chronic stress patterns (Lee & Yang, 2011; Satyapriya et al., 2009; Tu, Hsu, & Huang, 2011). Ross and Thomas (2010) analyzed 12 studies and compared the health benefits of yoga practice with other exercises. They found that yogic practice had profound benefits on physical and mental stress reduction. Yoga acts as both a curative and preventive therapy, helping individuals to manage and prevent stress effectively. Furthermore, it also improves overall physical wellness, helps the practitioner to attain mental and emotional peace, and promotes quality of life (Michalsen et al., 2005; Tu et al., 2011).
Wu and Lin (2008) synthesized the literature and recognized that most recent studies have focused on the 12-week, long-term practice of yoga and supported its benefits on reducing stress. However, such a regimen is not easy to maintain in the midst of a busy modern lifestyle. The efficacy of a single class of Hatha yoga in reducing stress is unknown, and previous research on the effectiveness of such has yielded inconclusive results (Wei & Tai, 2008; West et al., 2004). This study thus investigated whether there are significant differences in stress reduction between a single class and regular long-term yogic practice. We hoped to provide stress reduction advice for those unable to practice yoga on a regular basis.
Study Design and Participants
This research used a quasiexperimental design. It was approved by the Mackey Memorial Hospital Institutional Review Board in Taiwan (No. 11MMHIS111). Healthy female residents aged 40–60 years of a community in New Taipei City were recruited. Exclusion criteria included cardiovascular disease, smoking, and currently taking medications. A convenience sample of participants was randomly divided into either the experimental or control group. The experimental group received an 8-week course (one 90-minute class per week) of Hatha yoga, whereas the control group received no intervention.
The freely available software for power calculations, G*Power 3, estimated the sample size needed for the estimated effect size using a pre/post by group interaction. We planned an effect size of 0.3, a significance level of .05, and a test power of 0.80 (Cheema et al., 2011). The calculated sample size was 56 subjects in total. Considering the possibility of dropouts, we recruited 63 participants, with 30 in the experimental group and 33 in the control group. All received prestudy baseline Perceived Stress Scale (PSS) and HRV measurements. In the experimental group, the first postintervention measurement was taken after the first 90-minute Hatha yoga class, and the second postintervention measurement was done after the full 8-week course. Participants in the experimental group were excluded from the second measurement if they missed two or more classes.
Design of the Hatha yoga class was based on reports in the literature (Chattha et al., 2008; Lee & Yang, 2011; Satyapriya et al., 2009) and professional yoga books (Farhi, 2000/2011). Classes were led by certified yoga teachers, with class contents as follows: First, hold one’s breath and adjust inhalation and exhalation for about 15–20 minutes. Next, switch from diaphragmatic breathing to abdominal breathing and chant “om.” Gradually extend inhalation and exhalation times to facilitate mental and physical peace and then progress to the next stage. Total asana practice time was around 50–60 minutes. It started from a sun salutation pose to warm-up, followed by different asanas, including the cobra pose, downward facing dog, warrior pose, triangle pose, boat pose, cow face pose, hero pose 1, hero pose 2, twisted triangle pose, spinal twist pose, seated angle pose, child’s pose, fish pose, wheel pose, locust pose, camel pose, shoulder stand, plow pose, and ending with the corpse pose. Students used 15–20 minutes for meditation training and adjusting breathing. Finally, the entire training came to a conclusion by chanting “om” three times to slow breathing rates down, followed by steady heartbeat and the moderating role to achieve mind serenity. Classes were held once a week with 90 minutes per class. The total intervention lasted 8 weeks.
Outcome measures included PSS and HRV. The PSS is the most widely used psychological instrument for measuring stress perception in clinical trials and studies (Mimura & Griffiths, 2008). Cohen, Kamarck, and Mermelstein (1983) developed a widely used psychological instrument for measuring stress perception. It has been translated into Chinese by Chu and Kao (2005) and validated with good reliability (Cronbach’s alpha = .84–.86) and validity (correlation with symptomatology = .52–.76). The study questionnaire consisted of 14 questions about stress experienced during the previous month and associated coping strategies. It used a 5-point scoring system, from 0 to 4, with the seven positive items reverse scored. The final score was the sum of all 14-item scores, with a higher score indicating higher perceived respondent stress. The HRV instrument used in this research was the SA-3000P Heart Rate Variability Analyzer (Medicore Co., Ltd., Seoul, South Korea). Participants were requested to abstain from caffeinated food and beverages on the day of their assessments. After 5–10 minutes of rest with a regular and calm breathing pattern, participants were seated and a continuous 5-minute ECG recording was collected using a PPG device affixed to the subject’s index finger. During the monitoring period, participants kept the height of the PPG and index fingers at the same level as the heart. HRV analysis was derived from continuous pulse rate recordings at a sampling rate of 1,024 Hz. Interbeat intervals were computed, and the HRV power spectrum was obtained via a fast Fourier transformation algorithm using an appropriate software program. The energy in the specific frequency bands of HRV were expressed as normalized units for low-frequency (LF; range, 0.05–0.15 Hz) and high-frequency (HF) bands (range, 0.15–0.4 Hz), as recommended by the Task Force for Pacing and Electrophysiology. HF norm, LF norm, and LF/HF ratio were derived using spectral analysis of successive interbeat intervals. This analysis technique separates the heart rate spectrum into its frequency components and provides quantitative estimates of sympathetic and vagal (parasympathetic) neural influences on the heart. (Cheema et al., 2011; Hynynen, Konttinen, Kinnunen, Kyröläinen, & Rusko, 2011; Satyapriya et al., 2009).
SPSS 17.0 for Windows (SPSS, Inc., Chicago, IL, USA) was used for data analysis. Chi-square and Student’s t tests were used to analyze demographic and baseline comparisons between the experimental and control groups. In the experimental group, paired t tests and generalized estimating equations examined the interaction effect of the Hatha yoga intervention over time (after the first 90-minute class and after the 8-week course) to analyze differences in PSS and HRV (LF norm, HF norm, and LF/HF ratio) at the two times.
All 63 respondents completed pretest and Posttest 1 (after a 90-minute class) evaluations. However, five experimental group participants were excluded from Posttest 2 due to prior absence from two classes. One control group participant missed Posttest 2 due to illness. In total, 57 participants completed all assessments, including 25 in the experimental group and 32 in the control group.
Participant Demographic and Baseline Comparisons
Table 1 displays the demographic and baseline comparisons. There were 63 female participants in total, randomly assigned to the experimental group (n = 30) and control group (n = 33). Average age was 46 years in the experimental group and 45.6 years in the control group. There were no significant differences between the two groups with regard to age, marital status, education, income, perceived health, or baseline PSS and HRV values.
Efficacy of the 90-Minute Hatha Yoga Class
Table 2 shows the effectiveness of one 90-minute yoga class. After a single class, participant PSS scores and LF/HF ratio were significantly below those of their control group peers (p = .001). Although experimental group biophysical indicators and HRV (LF norm and HF norm) had also changed, these changes were not at statistically significant levels (p = .059).
Comparison of Stress Reduction Efficacy: One Class and the Full 8-Week Course
Table 3 shows the effectiveness of both the first yoga class and the 8-week yoga course in the experimental group. There were no significant differences between the two assessment points with regards to PSS using paired samples t tests. However, the 8-week course decreased LF norm values (p = .005), increased HF norm values (p = .005), and decreased the LF/HF ratio (p = .027) significantly more than the single 90-minute class We further analyzed the effects ([posttest – pretest] / pretest × 100). LF norm decreased 16.32% after the first 90-minute class of yoga and 23.78% after the 8-week course. HF norm increased 24.68% and 35.48% after the 90-minute class and 8-week course, respectively, and PSS decreased 8.85% and 16.96% after the 90-minute class and 8-week course, respectively.
The Effectiveness of Post-Hatha Yoga Intervention Stress Reduction by Time
Table 4 shows the generalized estimating equations analysis of stress reduction (HRV and PSS) after yoga intervention at the two time points. The process time compared pretest and Posttest 1 (after a 90-minute class of Hatha yoga) and pretest and Posttest 2 (after an 8-week course of Hatha yoga). In addition, we used a first-order autoregressive working correlation matrix to verify the effectiveness of HRV and PSS on the interaction of yogic intervention by time. After the first 90-minute class, the experimental group showed a decrease in LF norm and an increase in HF norm in comparison to pretest values, although the differences were not statistically significant (p = .107). However, the LF/HF ratio and PSS scores of the experimental group had significantly decreased compared to the control group (p = .021, p < .001, respectively). After practicing an 8-week course of Hatha yoga, the experimental group showed a significant decrease in LF norm (B = −15.43, p < .001), an increase in HF norm (B = 15.43, p < .001), a decrease in LF/HF ratio (B = −1.15, p = .004), and a decrease in PSS score (B = −4.05, p < .001) compared with the pretest values.
Study results showed that LF/HF ratio and PSS significantly decreased in participants after a single 90-minute class of Hatha yoga. This suggests a single yoga intervention can be effective for stress, a finding similar to previous studies such as that of Satyapriya et al. (2009), which reported the effectiveness of a 60-minute comprehensive yoga relaxation therapy on women who were 18–20 weeks pregnant, and West et al. (2004), who reported a reduction in perceived stress among 69 healthy university students participating in a 90-minute Hatha yoga class. However, our results differ from those of Wei and Tai (2008), who studied the effectiveness of a 50-minute yoga session on the HRV of sleep-deprived college students, and Telles, Singh, and Balkrishna (2011), who studied the effectiveness of practicing 35-minute HF yoga breathing (kapalabhati; breath rate, 1.0 Hz). We hypothesize that a single yoga class can significantly decrease perceived stress and physical stress (LF/HF ratio), but that HRV is extremely sensitive and easily affected by many factors (Li, Tzeng, & Wei, 2009; Lin & Lee, 2008).
Comparing data from the single 90-minute class and 8-week course, we found significant differences in physiological indicators (LF norm, HF norm, and LF/HF ratio) in the experimental group. A decrease in PSS values was observed but was not statistically significant. An analysis of the effects showed that the LF norm decreased from 16.32% to 23.78%, the HF norm increased from 24.68% to 35.48%, and PSS decreased from 8.85% to 16.96%. Satyapriya et al. (2009) reported the effectiveness of a 60-minute comprehensive yoga relaxation therapy in women who were 18–20 weeks pregnant and found that HF norm significantly increased by 64% in the 20th week and by 150% in the 36th week but that LF norm and the LF/HF ratio significantly decreased. Furthermore, psychological stress decreased 31.75% in the 36th week. Their results were similar to our findings.
Generalized estimating equations modified the interdependence of repeated measurements. Compared with experimental group baseline values, there were significant decreases of LF/HF ratio and PSS after the single class and significant reductions in stress (PSS and all physiological indicators) after the 8-week course. This suggests that Hatha yoga has an immediate stress reduction effect as well as providing additional cumulative benefits over time.
Chang (2010) reported that middle-aged women who engaged in regular yogic practice had lower degrees of fatigue and stress. Similarly, Wu and Lin (2008) reported the benefits of regular yoga participation on reducing physical and mental stress. Their findings are consistent with the results of our study. In addition, many surveys have confirmed that regularly practicing yoga can release stress tension and improve mood (Chang, 2010; Brisbon & Lowery, 2011; Hartfiel, Havenhand, Khalsa, Clarke, & Krayer, 2011).
The main limitation of this study was its relatively small sample size, short duration, and inclusion of women only. Therefore, findings cannot be extrapolated to the general population. Further research should involve a larger sample size that includes men, longer research duration, and additional biological indicators (e.g., salivary cortisol and alpha-amylase) to improve measurement accuracy and the generalizability of the findings. Also, we used a PPG to collect HRV data in this study. Although research has shown no statistically significant differences and a significantly high correlation between interbeat interval data measured by both ECG and PPG in short-term steady-state recordings, the potential for instrument measurement error remains. Finally, PSS typically measures stress during the previous month. However, this study had only 1 week separating pretest and first posttest, representing a potential study limitation.
Modern medicine focuses on preventive measures. Primary prevention focuses on reducing health risks, preventing disease, and encouraging regular exercise. Regular exercise is not always convenient due to busy lifestyles. The results of our study suggest that a single 90-minute class of Hatha yoga significantly reduced perceived stress and physical stress. Furthermore, our results showed that long-term yoga participation is better than a single yoga class in achieving effective stress reduction. Therefore, we suggest that communities, firms, and government agencies offer yoga-related courses as a way to reduce general stress and improve general health among their employees and the general public. Results suggest that practicing yoga regularly over the long term will benefit individual practitioners as well.
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