As a personal trainer I was always taught you had to get into your "Target Heart Zone" regularly for at least 20 minutes to condition the heart. How, I wondered, can Tai Chi strengthen the heart if it doesn't bring the heart rate into the target zone?
A strong well-conditioned heart beats slower but stronger; pumping more blood per stroke. The resting heart rate (RHR) is the best measure of heart strength. This can most easily be measured before you get out of bed in the morning. A normal RHR is 72 beats per minute. The RHR of a trained athlete is much lower.
I grew up in a family of athletes and have always done regular workouts of cardio and strength training while monitoring my RHR. When I started serious tai chi practice, I realized I was doing much less cardio training and feared my RHR of 55 beats per minute would rise. A year into my new program my RHR settled at 42 BPM! My teacher, Master Wang Jiwu, explained that Tai Chi practice releases tension on the cardiac channel (the pathway that allows qi to flow through the body). When the channel is relaxed, the heart does not have to work as hard and gets more rest in between beats.
Recent medical research is confirming what I discovered in my own practice and what tai chi masters have always known. Read more in this article from Consumer Reports.