Hi all, I haven't posted here for a long time now, but a question has come up and because of all you smart people, this is the first place I thought to come for an answer. I've recently turned my 70-year-old father onto HIIT. He's a scientist, so he gets it. He's also a mad scientist, so he's really getting into it and asking me lots of questions every day about it. He and my mother both go to the gym and work out a lot, doing swimming, weights and cardio. He's doing "HIIT" on the treadmill by walking on a 10% incline. This is how he explains what he's doing: "3.5 mph, took 1m 10sec to reach 140 heart rate, then brought speed down to 1 mph (I keep typing mpg) took 3m for my heart rate to come down to 115. Kept up this cycle for 35 minutes - exhilarating." :treadmill: His question is, is it safe for someone his age to run his heart rate up and down so quickly (though it may not sound quick to us)? He says he doesn't trust anything his doctor tells him and feels no serious strain at 140. This looks OK to me - though maybe 35 min is longer than necessary? - but I want to make sure I am not advising him to do anything dangerous. How safe is this? How much further should he go? What should he be careful of at his age? He's a strong man, always has been, with a strong heart and lungs.