The ‘metabolic rate’ is often cited as an important factor when it comes to energy expenditure; furthermore High Intensity Interval Training (HIIT) is reported to increase metabolic rate above that of other forms of training, such as steady state or longer, slower cardiovascular training.
One interesting perspective is to consider what metabolic rate actually is. Metabolic rate is indeed the ability for most of our 100 trillion cells in the body to produce an energy molecule called ATP. Without this ability, metabolic rate and therefore energy expenditure will be lower. Many, many micro-nutrients are required for this complex energy making process to occur perfectly (for example, vitamins B1, B5 and CoQ10!) Nutrient dense eating is therefore our first step to a metabolic rate which ‘fires on all cylinders’!
Next to this, structured exercise becomes an important contributor to daily energy expenditure, via metabolism, which produces energy in the form of ATP. All exercise should of course confer health benefits to the performer, but training for improved health and training for improved body composition (a change in the ratio of muscle and fat to look leaner) can require different plans of attack. Furthermore, international exercise guidelines for health from the American College of Sports Medicine (ACSM), require us to exercise for a weekly total of 4.3 hours, which may be prohibitive for some with respect to the free time that can be given over to exercise.
Very recently Kozaris (2013) highlighted the pro’s and con’s for HIIT in supporting an individual’s fat loss endeavours and made these comments:
“For the personal ﬁtness trainer whose client’s only issue is an overly pressed agenda on certain days, a high intensity sprint protocol may save time”.
So how effective is HIIT for fat loss? The recent research review from Kozaris provides some insight, but first, let’s address one or two important considerations.
As most of us understand, and assuming a given individual is healthy and well nourished, net ATP/energy production and usage across an extended period of time (several weeks) needs to be greater than that which is taken in through food and drink. More commonly, this is known as creating a negative energy balance with exercise being one way to achieve a negative energy balance. Broadly speaking, exercise could be categorised as:
- Cardiovascular training / long slow
- Typically: 60 min +, steady pace, sustained low-moderate intensity
- Cardiovascular training / intermittent high intensity
- Typically: ≤ 45 min, intermittent periods of work and rest, high or supra-maximal intensity
- Resistance training / low intensity endurance
- Typically: 15 reps +, light-moderate loads, short rest periods
- Resistance training / high intensity strength and power
- Typically: ≤ 8 reps, heavy loads or explosive movements, longer rest periods
(There are of course many ‘in-betweens’)
An exercise session which a) burns the most Kcals, b) creates the greatest ‘after-burn’ and c) helps to maintain a higher daily metabolic rate would have the most favourable effects on fat burning and body composition. This could be achieved within a single session (e.g. tough circuit based workout which combines cardio and weight training exercises. Cross Fit or Fitness FX style workouts would be effective here, assuming sound technique) or across a weekly training plan where cardio and weight training sessions are alternated (again, many arrangements would work; it is the principle that matters).
If an exerciser has ‘all the time in world’, then plenty of steady cardio work would be useful to help burn fat- just look at many endurance athletes, there isn’t an ounce of fat on then. These workouts are high in exercise volume and therefore create a large Kcal deficit, however, from an aesthetic perspective this does not always represent the ‘look’ many of us wish to achieve.
HIIT using running, cycling and alike may not allow the same magnitude of energy deficit to be created vs. a longer ‘endurance’ session, but it does allow for the ‘greater activation’ of muscles (fast twitch muscle fibre recruitment through more strength/power orientated tasks) which may help to preserve more muscle tissue in the presence of adequate protein in the diet. Furthermore, this type of training session will generate a greater EPOC affect (excess post exercise oxygen consumption- and remember, oxygen is required to make ATP, which boosts metabolic rate). The extra Kcals used up after a workout has finished are required to replenish energy stores (such as creatine phoshpshte), convert lactate back into an energy form, re-oxygenate tissues, clear stress hormones produced during exercise and generate new proteins for muscle repair. HIIT therefore represents an effective means of either getting similar results in the same period of time or superior results when planned within the context of a weekly periodised (progressively planned) schedule.
In concluding, and with respect to the comparisons between traditional steady state cardio and HIIT cardio Kozaris (2013) in his summary of the recent research states this:
“The most recent study, though, provides evidence that if the repeated effort is an all-out sprint, it can offer an EPOC that is significant. Whereas, during the session, the continuous protocol cost approximately 440 kcal the sprint session cost only 175 kcal. During the remainder of the day, though, the sprint protocol cost approximately 250 Kcal, bringing the 2 types of session to a 24-hour energy cost tie and adding further credence to the viability of sprit interval training for fat loss”.
Try this HIIT session at the gym next time you work out:
- 5 min joint mobilisation/dynamic stretch
- Rowing machine:
- 5 min steady pace warm-up +
- 10 x 100m max sprint (60 sec rest i.e. approx. 1:3)
- Spin cycle:
- 5 x 60 sec max speed (120 sec recovery i.e. approx. 1:2)
- 4 x 400m max speed (match with equal recovery time i.e. 1:1)
- Light cardio cool-down and stretch