Methodology
The subject was a 34 year old Asian male who had been practicing ashtanga yoga regularly for more than 5 years. The subject has normal BMI (22.0), resting heart rate of 60 bpm, and an estimated VO2max of 37mL/kg/min. The subject reported no other moderate-to-vigorous physical activities except for ashtanga which he performs 60-120 minutes of ashtanga yoga 3-4 times a week. Recommended age-predicted training heart rate for the subject is 112bpm-167bpm. Based on heart rate reserve, his recommended training heart rate is 123bpm-167bpm.
Instrumentation. I used my Polar RS300x to monitor my heart rate during the asana practice. The heart rate (HR) monitor has a chest electrode transmitter and a wrist receiver which records the exercise data such as total energy expenditure, exercise time, lap time, maximum heart rate, and average heart rate. It can record up to 99 laps. It is actually designed for running so it doesn't record energy expenditure per lap. The HR recording was started at the beginning of the first surya namaskar and stopped after the last vinyasa prior to savasana, the final relaxation pose.
Procedure. After doing my morning rituals, the idea came into my mind. I had a good night rest and I felt that the coffee could wait until I finish practice so it was a good time to observe the heart rate. Caffeine affects the heart rate and blood pressure during exercise so subjects are asked not to ingest caffeine or drink coffee prior to exercise testing according to standard protocols. The "lap" button was pressed at specific points during the practice. Maximum HR, average HR, and duration of the "lap" was recorded every time the button was pressed. A total of 47 laps were recorded. This will be explained in details in the next entry.
Results and Discussion
The American College of Sports Medicine recommends that the intensity of any sustained physical activity has to be between 60%-90% of an individual's age-predicted maximum HR or 50%-85% of his/her heart rate reserve in order to have some cardiovascular benefits. It also has to be sustained for at least 10 minutes at a time. An mentioned above, the aerobic training recommendation for our subject is 112bpm-167bpm (APMHR) or 123bpm-167bpm (HRR).
The graph below shows that, based on APMHR training recommendation, the subject's HR did not reach the minimum 112bpm except in the 9th lap (i.e., warrior [utkatasana-virabadhrasana 1-virabadharasana 2] sequence). The sequence was also only 2min 32.8sec in duration and thus can be considered anaerobic.
Figure 1. Heart rate in bpm during the basic ashtanga practice.
Figure 2 shows the same data but in percent of age-predicted maximum heart rate, %APMHR.
Based on the HRR training recommendation, it did not reach the minimum at all. Target training zone based on HRR or heart rate reserve is computed using the Karvonen's formula, which is %targetHRzone[(220-age)-restingHR] + restingHR. For example, since my resting heart rate is 60bpm. Using this formula, my recommended training zones is 123bpm to 167bpm as mentioned previously.
Figure 3. Heart rate based on %HRR during the basic ashtanga practice.
Total practice duration was 51min 30sec. Total energy expenditure was recorded at 139kcal. MaxHR was 115bpm (62% of APMHR) and AvgHR was 88bpm (47%). On average, the activity required 2.70 kcal/min or 2.65 METs which is categorized under light intensity activity. Interestingly, this is way below the average 6kcal/min that I observed three years ago (Mitahara, 2008). In the previous study, however, the routine observed was the entire primary series. Also, I believe that the performance of yoga asanas require skills like swimming. The more one practices, the more one becomes proficient in the movement, leading to a lower energy requirement because one has learned how to execute the skill efficiently.
On the psychological level, as one becomes more proficient in the performance of yoga asanas, does the mind become steadier also? Is there a relationship between the state of mind and heart rate in the performance of yoga asanas? It would be interesting to know. I wonder if studies have been done already regarding this. Most of the previous studies had investigated the effects of aerobic exercise on psychological states. I'm just beginning to understand all these things.
For now, it appears that the intensity of ashtanga yoga practice varies. In this study, the basic ashtanga practice is not intense enough to elicit cardiovascular benefits. At least for someone who's been fairly active.
References
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