Estimating Exercise Intensity

If every workout has a goal, then the duration and intensity of the workout need to be clearly defined and regulated. The duration is as easy as monitoring a stopwatch, but the intensity is a bit more complex. If we assume for the moment that we're talking about continuous exercise modes such as running and cycling rather than intermittent exercise such as most strength training, then we're talking about working at a given intensity throughout the workout (or at least at varying intensities through various portions of the workout). If you know that you want a particular run of 30 minutes to be at 80% intensity, what does that mean and how do you gauge your intensity?

When it comes to doing work such as exercise everyone has both a resting and a maximal metabolic rate. Metabolic rate is simply the rate at which you can turn over energy in your whole metabolism (mostly your skeletal muscles during exercise). The higher your maximal metabolic rate, the higher your exercise capacity. Typically in exercise physiology metabolic rate is measured as oxygen consumption (VO2) since oxygen is the ultimate electron acceptor in the metabolic processes that occur in the cells. The more energy you can turn over the more oxygen you require in the cells and therefore the more oxygen you consume. Oxygen consumption is typically measured in either litres per minute (l/min) or more usefully in millilitres per minute per kilogram of body mass (ml/kg/min). Resting metabolic rate is typically considered to be about 3.5 ml/kg/min and maximal values range dramatically with genetics, training, gender, body composition, and age, but are typically in the range of 30 - 80 ml/kg/min in healthy young adults. If you are in the 30 range then you are probably not involved in any regular exercise training and you might be out of breath walking up a few flights of stairs. If your peak is over 55 then you likely train very regularly and are relatively fit compared to the average person. If your peak oxygen consumption is over 70 then you are almost certainly a trained endurance athlete. Regardless, each person is capable of working at a given percentage of maximum rate and in theory at least 80% will feel similar regardless of how high your maximum is.

But measuring oxygen consumption during exercise requires specialized and expensive laboratory equipment and is not at all practical for training purposes. Fortunately, heart rate, which is relatively easy to measure during exercise, tends to be more or less linearly related to exercise intensity and oxygen consumption.

 This figure shows the relationship between heart rate and exercise intensity (power output on a bicycle ergometer) during a test of progressively increasing intensity to max in a young, healthy male cyclist. Note the roughly linear relationship between the two variables.

Similarly to metabolic rate, everyone has a resting and maximal heart rate. Finding your resting heart rate is relatively straight forward. Ideally it should be measured first thing in the morning before eating or drinking coffee and while still lying supine. Find your radial or carotid pulse and count for a full minute. Resting heart rates in healthy young adults are typically in the range of 50 - 100 beats per minute (bpm). Over 100 bpm (tachycardia) is unusual and might indicate an abnormality. A low resting heart rate below 60 bpm (bradycardia) may also be symptomatic of an abnormality but in reality many individuals have low resting heart rates, and it is not uncommon for endurance athletes to have a resting heart rate below 40 bpm. Measuring your maximal heart rate requires taking this same measure during maximal exercise. But, the exercise must be progressive in nature to allow time for elevation of heart rate rather than all-out for only a few seconds. The best way to actually measure maximal heart rate is to wear a heart rate monitor and progress on a treadmill through one minute stages of increasing speed and treadmill grade until you are running at a high speed and grade and cannot continue (this requires some practice and skill if you are not familiar with high speed treadmill running and is not recommended for novices). There are several ways of estimating maximal heart rate, but the most commonly used one is to simply subtract your age from 220, such that a 20 year old would have an estimated maximal heart rate of 200 bpm.

So, once you have measured and estimated your resting and maximal heart rates, you know the range with which you have to work. It is now simple arithmetic to estimate the intensity that corresponds to a given heart rate. But, keep in mind that your resting heart rate is not zero so you should not simply multiply your desired intensity by your maximal heart rate. Instead, multiply your desired intensity by the difference between maximal and resting heart rates and then add it to the resting value:

exercise HR = [intensity % x (HRmax - HRrest)] + HR rest

For example, if your resting heart rate is 60 bpm and your maximal is 200 bpm and you want to work at 80%, then your desired workout heart rate is calculated as:

exercise HR = [0.80 x (200 - 60)] + 60 = 172 bpm

In reality the relationship between exercise intensity and heart rate is not perfectly linear nor is it perfectly consistent between workouts or over training periods. Metabolic rate is linearly related to exercise intensity, but the rate of oxygen consumption for a given intensity may vary some with training as the body becomes more efficient. In addition, the amount of blood pumped through the heart, delivering oxygen is called cardiac output and is the product of heart rate and stroke volume (the volume of blood pumped each beat). Since stroke volume varies with a number of factors (intensity, hydration, temperature, etc.) the relationship between heart rate and intensity is not perfect. Heart rate also tends to drift upwards at a given exercise intensity within one workout (cardiac drift). And, further, heart rate is affected by many factors including things like caffeine ingestion. So there are a number of potential errors that might shift the roughly straight line on the figure above up or down in one workout compared to the next, meaning that one should be cautious in putting too much stock in very specific workout heart rates. However, heart rate is a relatively valid tool to estimate exercise intensity.