October 27, 2017
Recently I’ve had a couple of questions from athletes who are struggling to stay in their HR zone when performing easy run or bike workouts. There are few reasons that could account for this and these will be outlined below. However, first it is worth reminding ourselves why we do easy sessions at low HR values.
The aim of these sessions is to build larger aerobic endurance, improving things like muscle strength, mitochondria efficiency, capillarisation and fat oxidisation etc, whilst limiting residual fatigue that may hinder our really hard workouts. Several scientific studies show that the optimum pace for this kind of training is at an exercise intensity that is known as Ventilatory Threshold 1 (VT1). VT1 is simply the point at which breathing rate increases rapidly.
When we train at ‘zone 2’ all we really mean is an effort somewhere near VT1.
In the real world VT 1 is usually the point at which talking or eating becomes difficult and if you stopped you would be breathing heavily for several minutes.
Now that we have recapped the reason for these easy sessions let’s look at why it can be difficult to keep your HR in the recommended zones.
On different days, for many different reasons,( caffeine intake, volume of sleep, fatigue from previous sessions, temperature) VT1 will occur at different HR values between the bottom of z3 and the bottom of z2. You will be able to asses for yourself if any of these things are likely to be applicable to given session. As long as you can talk or eat in these sessions the HR is less important.
An additional factor that will almost certainly affect HR in easy or moderate sessions is Cardiac drift. In longer running or bike sessions Cardiac drift can cause an increase in HR despite no increase in effort or oxygen uptake. This is a natural phenomenon and occurs after about 20mins of exercise, increasing HR by between 10-20BPM. There is nothing you can do about it and it isn’t in any way a problem. But, again, if you can still talk and eat but HR is climbing then it is likely cardiac drift is the culprit.
Finally discrepancies in the way HR zones are calculated can often cause athletes confusion. For example at Racesnake we use the HR reserve method when calculating training zone percentages. This is:HR reserve =HR max – resting HR (Taken as functional resting not absolute, normally assumed at 60bpm)
Divide HR reserve by 10 to get your 10% brackets.Alternatively, another common method is to calculate Hr as a straight percentage of your HRmax.I.E:
It is not always clear what method your HR device, coach or training manual has used to calculate your HR percentages. If you are using different methods then obviously the outcome will be different. Luckily this one is easy to overcome. Simply check the HR calculation you are using is the same as the one your output device uses.
However, an excellent rule of thumb is (you guest it) the talking and eating rule. If you can count to 20 or hold a reasonable conversation then you are probably exercising at a rate that will build aerobic endurance without impairing the ability the conduct your tough workouts and ultimately that is all you really need to worry about!