Ashtanga yoga, not a cardio exercise
Yesterday, I broke my 6 days a week of Ashtanga practice (excluding, of course, moon days and ladies' holidays), and did some cardio exercise. I know. I'm surprised the Ashtanga police hasn't arrested me.
I followed a cardio workout video that goes through a series of cardio and strength-training movements. I found myself huffing and puffing after 10 minutes. I've had this video for many years now, and I used to finish this video and then go out for another 2-mile run. What ever happened to my cardiovascular endurance?
I really didn't want to admit this, but I think it is because yoga happened. Back in college I maintained a Mysore self-practice 3-4 times per week, and on other days I would do some form of cardio. I could run 3 miles easy. But starting about 2 years ago, I committed to a "regular" Ashtanga practice, and cardio almost dropped out of the radar completely, besides those occasional--and by occasional I mean once every month or every two months--runs, treadmills, stair masters, or workout videos. I could barely make it 2 miles now without needing a break.
So I did what any normal person would do when they have some burning questions. I asked Google. And Google spitted out a research study published back in 2006 on the heart rates in three different styles of yoga asana practice: Ashtanga, Hatha, and Gentle/Restorative yoga.
The study is very, very small-scale, with 16 participants only. Ten of them had previous yoga experience, and 6 did not. Long story short, the Ashtanga session raised the participants' heart beats per minute by an average of 30.77, while the Hatha session increased their heart beats by 15.75, and the Gentle yoga by 11.62. In addition, the increased heart rates were not held constant throughout the 80-minute session. For example, your heart rate might go up as you breathe your way through Warrior II, but quiet down as you come down to the floor for Paschimottanasana.
This means that for me, an Ashtanga practitioner, my heart rate goes up to approximately 90 bpm (assuming that I have a resting heart rate of 60bpm), the same as performing moderate exercises (e.g., brisk walking). This is below the target heart rate for my age at 98-167 bpm during exercise (your target heart rate is 50-85% of your maximum heart rate, which you can find by subtracting your age from 220). According to the American Heart Association, one should strive for 30 minutes of moderate to intense physical activity 5 days a week to prevent chronic diseases.
The study did conclude that "a more vigorous style of yoga is the heart rate equivalent of moderate exercise and may contribute to increases in cardiorespiratory ﬁtness for individuals who are de-conditioned or not physically active." While yoga might not get you the cardiorespiratory capacity of that of a marathon runner or a swimmer, it certainly has its own myriad of benefits (stress-relief, help with insomnia, strength and flexibility, etc).
I love yoga, and I love Ashtanga especially. While yoga trains my muscles for strength, balance, coordination, and flexibility, I also like to give my heart and lungs a good workout every now and then. I love living in my yoga pants, but my sneakers also deserve some love and attention :)