Medlife Crisis (a YouTube channel that deserves a much larger following) has a good video related to this topic:<p><a href="https://youtu.be/hT8GZlBBv5k" rel="nofollow">https://youtu.be/hT8GZlBBv5k</a>
I've been doing interval training 3 times a week for roughly a decade. This involves a full 400 meter sprint followed by 4 100 meter sprints at full speed spaced by walking the corners of the track. This has resulted in remarkable gains in my endurance. There are many instances where I'm walking up a hill and I don't even realize everyone else is out of breath and half dying just to keep up with me.<p>Additionally I jog 5 miles twice a week, but I think this had relatively little overall effect on my endurance.<p>From 70 bpm my heart rate has lowered to a point where at it's lowest I measure 39 bpm over the past decade.<p>This raised some concern in my primary care doctor who told me to cut down my exercise by 80%. I got a second opinion from a cardiologist who ran ultrasound and did a V02 max test on me and he told me I was fine and my heart was perfect and that I didn't need to cut down my exercise.<p>Now I'm not so sure, I'm thinking of getting a third opinion. If I keep up this level of exercise my heart rate could go down to below 30 bpm. Although the effects of HIIT are remarkable no one knows the long term outcomes.<p>Is there anybody else here who has been doing similar HIIT training for long periods of time (again a decade for me)... what's your experience?
I would recommend reading "The Haywire Heart: How too much exercise can kill you, and what you can do to protect your heart." The author has some athletes story on similar lines.