Heart rate reserve is simply the difference between your maximum heart rate and your resting heart rate.
There is a relationship between heart rate and oxygen consumption - particularly at intensities ranging from 50-90% VO2 max (1). So traditionally, exercise intensity has been prescribed as a percentage of maximum heart rate (calculated as 220 - age). For example, a 30-year old with a maximum heart rate of 190bpm might train at 75% maximum or 143bpm.
One of the problems with the 220-age equation is that it makes no allowances for individual differences in resting heart rate. By incorporating the heart rate reserve into the equation, in theory a more accurate training zone can be determined.
The Karvonen formula uses the heart rate reserve to calculate training zones based on both maximum AND resting heart rate. Heres the actual formula:
Heres an example for a 50year old with a resting heart rate of 65bpm who wants to train at 70% maximum
Using the Karvonen formula this persons target heart rate works out as 139bpm. To create a zone you might want to subtract i.e. 129 to 139bpm
Using the traditional 220 age formula this same person would have a target heart rate of 119bpm, which is considerably lower (220 50 x 0.7). Its worth noting that the Karvonen formula nearly always calculates a higher target heart rate than 220 age.
Here is a rough guide to different heart rate zones and the adaptations they elicit
Recovery Zone - 60% to 70%
Active recovery training should fall into this zone (ideally to the lower end). Its also useful for very early pre-season and closed season cross training when the body needs to recover and replenish.
Aerobic Zone - 70% to 80%
Exercising in this zone will help to develop your aerobic system and in particular your ability to transport and utilize oxygen. Continuous or long, slow distance endurance training should fall under in this heart rate zone.
Anaerobic Zone 80% to 90%
Training in this zone will help to improve your bodys ability to deal with lactic acid. It may also help to increase your lactate threshold.
It is important to remember that the heart rate reserve method of prescribing exercise intensity is by no means flawless. Firstly, estimating a person's maximal heart has been shown to have inaccuracies compared to laboratory testing (2) - where exercise intensity is increased until a plateau in heart rate is found.
Secondly, the heart rate reserve tells us nothing about a person's lactate or anaerobic threshold. By recording heart rate data along side the point at which lactate threshold is thought to occur, a far more effective training plan can be devised.