With all exercise, the intensity at which you work can range from doing a 1 rep max lift (the largest possible weight your muscles can move), to extreme endurance (like marathons or even longer events). Different types of exercise have different training effects. Pure strength work makes muscles get bigger and stronger. Endurance work makes the heart get bigger and stronger and causes other adaptations to increase muscle energy supply. Between these two ends of the spectrum are all the other types of training. High intensity interval training (HIIT) is somewhere in the middle.
The idea of HIIT training is to work at maximum effort for a specific time interval, then to recover before the next interval. Within this basic format, there are many possible variations. Interval time, recovery time, type of recovery and exercise intensity are all variables.
Aerobic and anaerobic training
Muscles need energy to function. Although our energy comes from the food we eat, the muscles can’t use the food directly. They need to take units of glucose and convert them to something called ATP, which they can then use as energy. The harder a muscle is working, the more ATP it needs. If there is enough oxygen available to a muscle, it can make a large number of ATP molecules, while only producing a small amount of carbon dioxide. If there is insufficient oxygen, it can only produce a small amount of ATP, accompanied by a large amount of carbon dioxide. This will cause the muscle to tire quickly.
Aerobic exercise is the sort of exercise that we can keep up for several minutes, because the energy demands are relatively light and there is enough oxygen going to the muscles. Anaerobic exercise is higher intensity exercise in which the muscles don’t get enough oxygen to produce enough ATP efficiently. The effort in HIIT training needs to be intense enough to keep the intervals anaerobic.
So why work anaerobically?
If aerobic exercise is less effort and it’s good for the heart, then why choose to do high intensity anaerobic intervals? While high intensity intervals aren’t the right choice for everyone, they do have several advantages:
- They are time efficient. As they are very challenging, you can achieve more in a shorter time.
- Working at high intensity can be very effective at improving aerobic fitness, even though it isn’t aerobic exercise in itself.
- As well as improving aerobic fitness, HIIT training improves muscular endurance. Very short, very high intensity intervals will also improve muscular strength and power.
- Although anaerobic work can’t use fat as a fuel, high intensity intervals are effective for weight loss. This appears to be because the body’s metabolic rate remains higher for several hours after the workout.
Drawbacks with high intensity interval training
Due to the intensity of effort, there are some risks. HIIT training is not suitable if you have any medical conditions that would be of concern for other types of exercise – see here. HIIT exercises are usually done at high speed and often involve high impact. This means that there is higher risk of injury. It’s important to have good technique and movement control to minimise the risk. For beginners, it would be better to build up some cardio fitness, core strength and muscle endurance before attempting hit training. See the bottom of the post for some suggested workouts.
Types of interval training
As noted above, there are many possible variations for time and type of intervals. The intervals need to be short enough to allow for high intensity work, or it just becomes aerobic. The rest period can be complete rest or active rest, where you do some light activity. Opinions are divided as to the ideal length of intervals and type of rest, but really it depends on what you’re trying to achieve. In general, the longer the intervals and the more active the rest, the more you will be improving your cardio fitness. Shorter intervals with complete rest, on the other hand, will be tending towards strength and power.
Intervals can range from 20 seconds to about a minute. Longer intervals will be ordinary interval training, rather than high intensity interval training. The rest period should allow you to recover sufficiently to work at your maximum effort for the next interval. How long this takes will depend on the individual, but it should be at least half the time of the high intensity interval.