Little white lies?
Ever since the first equipment manufacturer built energy expenditure estimates into its cardiovascular equipment some time in the 1980s, it was clear that the figures used in the equipment’s software to calculate how many miles you would need to jog or stairs you would need to climb to burn off that chocolate éclair had been massaged upwards, either by mistake – or as Professor David Bassett (quoted in The Guardian) seems to think – deliberately.
You can see why a few white lies might help here, since your average éclair may well contain 500kcal and to realistically expend this much energy on a treadmill means that you would have to cover a total distance of around five miles (8km): and that’s probably an hour of very brisk walking or slow jogging for almost everyone.
- For the average person, walking or jogging a mile expends approximately 100kcal
- For every litre of oxygen your body consumes as you exercise, you will expend 5kcal
At least these days most cardiovascular (CV) equipment is compatible with a heart-rate chest strap and will have an in-built sensor that picks up the transmitted heart rate for display on the equipment’s monitor, which is accurate.
However, runners who wish to equate heart rate to running speed in their training programme need to be wary. As Professor Bassett also pointed out, just because a treadmill says you are running at a given speed, that may not be the speed at which you are actually travelling.
You will definitely be upset when you do that self-same session outdoors, since running outdoors is indeed that little bit more demanding than running on a treadmill. So much so, that not only will exercise science laboratories routinely calibrate the speed of the treadmill they use for exercise testing, they will usually also run the treadmill at a slight (1%) gradient to account for the advantage gained by running on a treadmill indoors compared with running outside.2
With all the information the heart rate monitor/gym equipment can now ask for (weight, 'fitness index', age, gender, etc.) and even if distance is measured accurately, some big assumptions are having to be made about each individual, the work done and how much energy they expend when they exercise in the energy cost calculation that is carried out.
Unfortunately, the one piece of information that you would really like to have is the amount of oxygen you are consuming for the work (exercise) you are doing, since from this figure you can actually calculate energy expenditure.
However, exercise involving small muscle groups above the level of the heart, or exercise involving bouts of sustained isometric (static) muscle contractions and/or bouts of breath-holding, lead to a disproportionate rise in heart rate relative to oxygen cost.3 In the simplest of terms, the relationship between heart rate and oxygen cost is only a good fit with the two parameters mirroring each other when traditional aerobic activity (i.e., large muscle groups in rhythmical contractions) is performed.
Also, exercising for prolonged periods in hot, humid conditions is when you are most likely to see the phenomenon of cardiovascular drift, in which heart rate steadily climbs even though the same amount of exercise is being performed.4
Cardiovascular drift is especially associated with dehydration, since as fluid is lost through evaporation, fluid is then 'borrowed' from the blood leading to a decrease in total blood volume, a fall in stroke volume (i.e., the amount of blood being ejected by the heart with each beat) and a rise in heart rate, in order to maintain cardiac output to the working muscles and body in general (since cardiac output = stroke volume x heart rate).
As for the 'fat-burning' zone – obviously, there is arguably no such thing. Of course, the higher intensity interval workout involves more work and hence a greater total energy expenditure; and when it comes to weight management, total energy expenditure matters above all.
In the novice exerciser however, higher intensity exercise is associated with a greater risk of injury and may pose an increased cardiac risk too. Thus, the safest weight management exercise programme for new exercisers is one that is of relatively moderate intensity and can be repeated several times a week, but yet is still of sufficient duration to expend an appropriate number of calories: probably around 2,000 per week.5
This feature was first printed in the December/January 2010 issue of Fitpro magazine.
For a list of references, visit www.fitpro.com/references