How hard?

The amount of exercise, or physical activity, you take can be assessed by the three parameters – how long, how often and how hard. How long and how often are easy to measure – but what about how hard?

Heart rate response

Pulse rate or Heart Rate (HR) is one way. The usual recommendation for increasing physical fitness is that you should exercise to between 70% and 85% of maximal heart rate (MHR) for optimal benefit. Two problems:

1. How do you know your MHR? The most accurate way is to exercise to exhaustion – usually on a treadmill – and measure the resulting peak HR.
   For most people, however, MHR is linked to age and can be calculated by taking their age from 220. For a fifty year old, MHR would be 170, 70% MHR about 119 and 80% about 144. For an eighty year old the figures would be 140, 98 and 119. These are “rule of thumb” assessments and may vary from individual to individual but they are accurate enough for practical purposes.
2. Some drugs lower HR – examples include beta blockers like atenolol or bisoprolol, some calcium blockers like diltiazem, and ivabradine. People taking any of these have no other way of calculating MHR than the maximal exercise test.

How hard does it feel?

There are scales of how hard an exercise feels and the best known is the Rate of Perceived Exertion (RPE) also know as the Borg score after its inventor. This week’s illustration shows the two scales for this measurement – from 1 to 10 and from 6 to 20 – with an explanations for the amount of effort which might give such scores. I prefer the 1-10 scale and for most people a score of 5 to 7 would equate to 70 to 85% MHR. That is the amount of exercise to make you a bit short of breathless up to a level which is getting uncomfortable.

The 6-20 scale represents about one tenth of the HR which a young person should reach for that degree of exercise intensity.

The evidence for RPE

As an example, a comparative study of heart rate versus RPE was conducted in 2560 men and women who underwent either treadmill or cycle ergometer tests. Rating of perceived exertion was strongly correlated with heart rate.

The authors comment “Borg’s RPE seems to be an affordable, practical and valid tool for monitoring and prescribing exercise intensity, independent of gender, age, exercise modality, physical activity level and CAD status. Exercising at an RPE of 11–13 (“low”) is recommended for less trained individuals, and an RPE of 13–15 may be recommended when more intense but still aerobic training is desired.”

The usefulness of RPE

Using heart rate measurement as an indication of exercise intensity works well if you have a wearable device which does it for you. If not, you have to stop exercising and count your pulse rate – which creates a problem. By the time you can do this your HR has already begun to decrease. The strength of RPE is that you assess it while you are still on the move and it is equally applicable whether or not you are taking heart rate slowing medication.