HIIT certainly has a cult following. Sometimes it seems like it’s the only type of workout that I hear mentioned these days. There are excellent reasons for people who are physically able to do HIIT to do so. Namely, it improves cardiorespiratory fitness both during exercise and at rest. You will continue to burn calories long after the workout is over (due to something that is known as “excess post-exercise oxygen consumption”).

However, HIIT was never supposed to be a daily workout. If you are genuinely doing HIIT in the purest sense, the exercises are supposed to completely and utterly exhaust the body. This is why it’s pretty well known that people who can do the workouts on consecutive days are probably not pushing themselves quite hard enough.

If you don’t give yourself rest in between HIIT workouts, you’re likely to suffer injury, as your muscle tissue won’t have enough time to repair and grow. Overdoing it with HIIT risks soft tissue injuries to the joints, such as stretches, sprains, and tears of the ligaments, tendons, or muscles around the joints. In the long-term, overdoing it could cause damage to the joint itself. This is why HIIT should only be done two or three days a week.

Proper HIIT training purposefully creates strategic overload and fatigue so that the system recovers stronger. Recommendations for efficiently working HIIT into your exercise program include:

    • Follow a program that provides for incremental progression along with planned rest and decreases in intensity.
    • Take at least one day completely away from training each week.
    • Make sure that high-intensity days are genuinely high, and those low-intensity days are truly low.
    • Every four to six weeks, insert a training week where intensity is significantly decreased (known as de-load).
    • Designate a few exercises each day for the primary purpose of improving movement/mobility.
    • Don’t merely “go as hard as you can” every day. Develop progressive training intensity goals.

The pendulum always swings, and it’s fun to watch as you get older. The pendulum is beginning to turn away from HIIT as people are starting to crave a gentler and more mindful approach to exercise. I like to say things like, “I remember when this was cool the FIRST three times around!” At any rate, Low-Intensity Steady State training (LISS) is once again vying for position at the gym. It has a new name, but it’s anything but fresh (Jane Fonda or Richard Simmons…anyone else remember them?). It’s only working out at a slower pace for a more extended period; it is the opposite of HIIT. Jogging, swimming, or biking are examples of LISS workouts. LISS is working out at an intensity of 4-6 on a 10-point scale (50-60% of your maximum heart rate) for 45-60 minutes. LISS is undoubtedly less daunting to exercise newbies and is a good fill-in for days that you won’t be doing HIIT training. LISS proponents also point out that it’s often safer on the joints, increases blood flow, and promotes oxygen delivery within cells. LISS is also highly accessible – you can do it in the park, in the backyard, on the stairs to your basement, or at the gym. Some research has shown that people who choose LISS over HIIT tend to stick with the routine for a more extended period, which anyone can attest is half of the battle. A 2013 study published in Nutrition, Metabolism, and Cardiovascular Diseases concluded that “prescribing a higher-intensity exercise decreases adherence and results in the completion of less exercise.” Unlike HIIT, you can do LISS for several days in a row if you want (hopefully, you’ll find a routine that you enjoy enough that you’ll start to crave the movement). Some research has shown that longer durations of low-intensity activity improve insulin sensitivity and blood lipid levels than short stints of vigorous studies, but not all research agrees.

Both HIIT and continuous aerobic exercise programs have many benefits on the body, including increased cardiac muscle mass, increased fat oxidation, and increased disposal of metabolic wastes. Athletic trainers warn that if you’re looking to build muscle and burn fat, LISS alone might not reach your goal. Your body will also adapt to LISS over time, so what was once challenging for you will cease to be after many repeats of the same workout.

On the other hand, people who skip LISS are entirely likely missing out on the stress relief and cardiovascular conditioning that it provides.

In most everything, variety is vital – eating a variety of foods, doing a variety of physical activities. When in doubt, shake up your routine.