Let's explain 'high intensity' before we find out more. The simple explanation of high intensity is physical activity at a performance level that invokes high heart rates, hard breathing, feelings of discomfort, and high levels of oxygen consumption.
Exercise trainers and physiologists sometimes use a 'rate of perceived exertion' scale like the Borg Scale, which rates an individuals feelings of working hard on a scale of 1-10 (and sometimes 1-20). Eight or nine out of ten would be very high intensity, five or six moderate intensity and three or four, low intensity. Whether you can talk comfortably or not while exercising is also used as measure of intensity.
Even so, high-intensity training is not just a measure of short bursts of training at high heart rates, it can be interpreted to incorporate elements of time as well. The FITT acronym for workouts stands for Frequency, Intensity, Time, Type. Training at >90% of maximum heart rate for 20 minutes of intermittent (interval) training is high intensity training, but running for 45 minutes, steady-state cardio at >80% of maximum heart rate is also intense training of another type. Interval or intermittent training is not the only type of 'high-intensity' training. You will see that this has implications for fat burning and post-exercise energy expenditure as well.
As a general rule, HIT stands for high-intensity training as a weight training methodology that evolved in the 1970s and 80s. HIIT stands for high-intensity interval or intermittent training, an acronym that represents a practise that is as old as athletics, but has become popularized as something new for amateur trainers in recent times. Track workouts are generally HIIT.
High-Intensity Weight Training (HIT)
The general principles of HIT, as it applies to weight training, are as follows:
- Repetitions and sets with a load that will lead to muscle failure at the end of each set. This means choosing an appropriately heavy weights for the standard 8 to 12 repetitions of each exercise. Your muscles should feel heavy with fatigue and lactate at the end of your set.
- Because of the high intensity advocated, HIT enthusiasts say that fewer sets and less time is required in the gym to do full-body workouts and to achieve results superior to more traditional workouts involving less intensity and more sets.
Popular exercise regimens like CrossFit emulate this type of metabolic training to some extent. But CrossFit is a general fitness regimen; Don't make the mistake of believing that you can be a class distance runner or Olympic lifter with CrossFit and similar programs. High-intensity training is not magic!
High-Intensity Interval Training (HIIT)
An example of a HIIT workout could be 15 sprints of 70 metres at greater than 90% perceived effort with 60 seconds rest in between. You could also be more calculating and use a heart rate monitor to work at percentages of maximum heart rate.
You might also run for 40 minutes at greater than 80% maximum; or include fartlek intervals in your running. Fartlek is short bursts of higher speeds during longer cardio workouts. For example, every alternate half mile you might increase the pace by 20% for one minute. High-intensity training is not exclusively about short intervals. You could also do repetition 800 metres on the track as your HIIT training.
Fat Burning and the Afterburn
In recent years, a variety of studies have appeared purporting to show that HIIT training, Tabata style, burns more fat than steady-state cardio, mostly, they say, because of increased post-exercise fat burning -- the afterburn. On the whole, these studies are not well controlled, that is, they don't compare high-intensity training with aerobic training of similar depth. Ultimately, work done is still the main factor, and aerobic running at around 75-80% maximum heart rate for 30 minutes or more, is very effective at fat burning.
Summarizing HIT and HIIT
High-intensity training allows you to do more work in a shorter time. This can be useful for busy people looking for general fitness. The down side is that more injuries occur at higher intensities and volumes of training. And it's not magic, you still need specificity for events like triathlon and marathon -- which means lots of miles.
Superior fitness is achieved when you do the triple: weights, cardio, and intervals. For weight loss, combining weights with aerobic exercise helps you maintain muscle, which has metabolic advantages. Interval training provides anaerobic fitness. Do all three.