Hiit can be done running or cycling, but most people use an exercise bike. You pedal normally and warm up, then sprint flat out for 30-60 seconds (or as long as you can). Return to normal cycling for a couple of minutes while you recover, then sprint again for 30-60 seconds. Do the sprint 3 or 4 times per session, and do the sessions 3 or more times a week and your good to go.
If you have heart problems or haven't done strenuous exercise for a while, start off slowly and talk to a doctor first. But most people can do it safely as long as you don't overdo it at the start.
If you have heart problems or haven't done strenuous exercise for a while, start off slowly and talk to a doctor first. But most people can do it safely as long as you don't overdo it at the start.