You’ve probably seen it on fitness trackers, and on cardio equipment in gyms (treadmills, cross trainers etc), a colourful graph of heart rate training zones. Usually along the lines of:
60-70% MHR sometimes called ‘light’ or ‘fat burning zone’
70-80% MHR sometimes called ‘medium’, ‘cardio’ or ‘aerobic zone’
80-90% MHR sometimes called ‘hard’ or ‘anaerobic zone’
90-100% MHR sometimes called ‘max’ or ‘Vo2 max’
MHR – Maximum Heart Rate. You can estimate your MHR by subtracting your age from 220, although this is obviously an estimation and does not take into consideration your body composition or fitness levels. So as a 36 year old I could estimate my ‘fat burning zone’ as being 110-128 (220-36)*.6 to (220-36)*.7
So the ‘fat burning zone’? Sounds good doesn’t it? Many people would like to ‘burn a little fat’. However I think it is responsible for one of the biggest misconceptions in fitness – that prolonged steady state cardio is the key to weight loss.
Your hard working muscles demand oxygen to keep them working but that can happen with various exercises – weight training, sprinting, HIIT intervals.
The body relies on different fuel sources to fuel muscles at different intensities and indeed at a lower intensity the body may burn 50% of it’s calories from fat. During high intensity efforts it may burn 35% of calories from fat and also the rest from carbohydrates but the OVERALL fat burn will be more. High Intensity training also results in EPOC (energy post oxygen consumption) which means the body will continue burning calories for up to 48 hours after the training – something not seen in steady state cardio.
I find it fascinating that we can actually manipulate our Heart Rate training zones through HIIT. By working with spikes of intensity that take you beyond 90% MHR you can actually increase your anaerobic threshold and improve your cardiovascular (heart and lung) health.
One of the key factors in how readily your body burns calories is your metabolism. Your metabolic rate determines how many calories your body burns each day and a large factor in how readily it does this is your percentage of lean muscle (as muscle is more metabolically active than fat). In fact every pound of muscle is estimated to burn 45 calories a day so putting on 10lbs of lean muscle will mean your body burns an 450 calories per day – at rest!
If you enjoy steady state cardio (jogging, swimming etc) that’s great. You are far more likely to stick at a hobby you enjoy. However it is well worth bearing in mind the ACSM guidelines that everyone strength trains each major muscle group to fatigue 2-3x per week. And challenge yourself to some HIIT – there are many benefits!