HIIT has been gaining popularity over the last few years and for good reason, but like with anything that gains popularity, it inevitably gets butchered, usually by people trying to make money.
In my classes we usually do 15 or 20 minutes of high intensity work at the end, because that is all you need, but last week I saw a gym advertising a 60 minute HIIT class. To me this just sounded ridiculous, because there is no way you can do high intensity (90%+ of maximum heart rate) for longer than 20 minutes. You simply cannot maintain that level of intensity because your body wont be able to produce energy quickly enough to enable you to keep going without thing getting messy and risking injury.
With my clients and in classes we always end our high intensity work just before we need to. This allows for better recovery and more frequent training. It would be counterproductive to beat somebody into the ground if it meant they took a week to recover. Doing slightly less work, recovering properly and training 2-3 times a week is a lot more effective than 1 body breaking session.
I wanted to prove that you cannot maintain HIIT for longer than 20 minutes without risking injury. Now I could have tested it on myself, but thankfully I have guinea pigs who are willing to do this sort of thing for me. I'd like to introduce you to Vanessa. Vanessa has been training with us for a good while, she is very fit (fitter than me), she's got some determination and likes a challenge, so was the perfect candidate to test my theory. We first needed to work out 90% of her maximum heart rate, which is (220 - her age) x 0.9. I wont tell you what her age, because it's pretty old and I don't think she'd appreciate it. For the exercise to be counted as high intensity her heart rate needed to be between 90 and 100% during the work sets so I rigged her up to a heart rate monitor.
Kettlebell snatches are an excellent exercise for this type of training so we began with those and decided to do 30 seconds work, 30 seconds rest with a 12kg kettlebell.
After a short 3 minute warm up we started the test. Vanessa had to go all out with the snatches for 30 seconds, then put the kettlebell down rest for thirty seconds and repeat for as long as she could. After 16 minutes she had kept her heart rate in the 90-100% range, but her hands started to to get sore. Grip strength is usually the limiting factor with snatches because you are holding onto a lot of weight. Normally this is where the session would finish, but as she was still standing we decided to change the exercise to Hill Climbers and keep the test going. The hill climbers weren't as effective as snatches, but her heart rate was still above 90%, so it was still high intensity. Much to my dismay the 20 minute mark approached and Vanessa was still going strong, which made me wrong and I hate that, but it seemed to please her.
At 22 minutes however, things started to slow down and she struggled to complete the 30 seconds, the next set was the same. The last 7 or 8 seconds of hill climbers became slow and laboured, it was time to end the test at 23.5 minutes.
Although I was wrong by 3.5 minutes, we (the royal we) did show that there is no way you can maintain high intensity intervals for an hour.
HIIT is a great way to train and everybody (except those with a heart condition) should be doing it. Some people think it is only for people who are really fit like Vanessa, but it isn't. It's just that an untrained individuals "high intensity" might be step ups or star jumps rather than snatches and that is fine as long as you find it hard. Aim for 10 - 20 minutes 2-3 times per week, add in some brisk walks throughout the week and that is all the cardio you need.