I have been an athlete my whole life and was doing HIIT type [High Intensity Interval Training] workouts 2-4 days a week in the off season as part of daily training. The did not have a fancy name or label like HIIT but were generally Hard day vs. Easier/Moderate day. As I am now 54 these workouts take names to emphasize the intensity level.
I am dealing with heart failure and Atrial Fibrillation so my weight loss and fitness efforts are specifically aimed at building and rebuilding cardiac muscle. The aerobic workouts get the heart pumping and calories and fat burning. Ultimately aiming for a target bodyweight as recommended by my doctors and a regular schedule of exercise to keep the heart strong and supple.
The following is one type of exercise I have been doing at the gym. I also use a Heart Rate Monitor ( HRM) to track my heart beat as I exercise and the recovery before the next Intense session.
The first time I did this I was only able to do 3 periods of all out effort, but recently i have gotten to 7, and plan to complete 10-12 intense efforts soon.
I will post some additional HIIT workouts I have done or am planning to try in the future.