Quantifying intensity

How to use heart rate to quantify your fitness training intensity

Articles in PP often detail elite and complex aerobic training methods to boost endurance performance, VO2max and lactate threshold. These articles typically refer to target training intensities and heart rates to achieve, say, a new 10K or marathon best; they recommend high-intensity training, with very high target heart rates, to complement the longer ‘steady state’ sessions at more moderate intensities.

However, using target training intensities and heart rates is also useful for those of us whose aerobic training is aimed at improving general health and fitness, or as general conditioning for a recreational sport. In this more modest form, aerobic training involves an endurance activity, such as cycling, running or rowing, performed continuously for a certain amount of time, usually 20-30 minutes. It is recommended that if this kind of activity is performed three-to-five times a week, then it will bring about optimum benefits. Obviously if you do more you will get fitter, but as a general rule, 3-5 x 20-30 mins a week yields a good fitness reward for the amount of time invested, and so is optimal for general fitness needs.

It is also advisable that with this kind of aerobic training the exercise intensity should be moderately hard. The American College of Sports Medicine (ASCM) officially recommend that the optimal intensity is between 60% and 80% of VO2 max. VO2 max is the maximum amount of oxygen, in millilitres, one can use in one minute per kilogram of bodyweight. It is the standard measure of aerobic fitness. However, it is impossible to maintain maximal oxygen use for longer than about 8-10 minutes. Thus, for general fitness training, one should aim to be at 60-80% of maximum capacity and maintain this level for 20-30 minutes. This intensity is comparable to the training levels elite athletes would use on their ‘steady state’ sessions. When performing some of the more advanced interval sessions recommended in PP, elite athletes will be at intensities greater than 85% VO2 max. At the other extreme, activity at an intensity of 40% VO2 max is likely to improve health but won’t significantly improve aerobic fitness.

Take the case of Joe
It is possible to estimate your exercise intensity as a percentage of VO2 max from your training heart rate. This is very useful, for elite and recreational athlete alike, because by monitoring heart rate you can quantify your training effort and target the correct intensity for maximum benefits. These calculations are possible because of the linear relationship between heart rate (HR) and oxygen use (VO2) with increasing rates of work. For example, if Joe is sitting down doing nothing, his resting HR might be 70 bpm. At this HR, VO2 would be at its baseline level, which is approximately 3.5 ml/kg/min. If Joe starts to walk, his HR may increase to around 100 bpm as the VO2 goes up to cope with the extra energy demand. If Joe now breaks into a jog, his HR will go higher again, up to 140bpm, say, as VO2 increases further. Then, if Joe runs as fast as he can for three minutes, his HR might go up to its maximum of 190 bpm. At this point Joe will have reached his VO2 max. Therefore, at VO2 max, HR is also maximum, and at a percentage of VO2 max, there is a corresponding percentage of HR max. This relationship has been shown to hold true across sex, age and exercise type. The ACSM suggest that 40% VO2 max corresponds to 55% HR max, 60% VO2 max corresponds to 70% HR max, 80% VO2 max corresponds to 85% HR max and 85% VO2 max corresponds to 90% HR max. These values are derived from various stuides which have compared VO2 with HR and determined regression equations for % HR max versus % VO2 max.

Revising the ACSM formula
These target values of % HR max provide a means of quantifying exercise intensity to optimise training results. If the optimal training intensity is 60-80% VO2 max then, according to the ACSM, the corresponding optimal training HR is 70-85% HR max. However, the ACSM made these official recommendations in 1991. Since then, a study by David Swain and his US-based research team has criticised the mathematical methods used to derive the regression equations in previous research. Using more correct statistical procedures, they re-examined the relationship between % VO2 max and % HR max and found that the ACSM formula underestimates HR at the target values of % VO2 max. Their results led to a regression equation of % HR max = 0.64 x % VO2 max + 37. This means that 40% VO2 max corresponds to 63% HR max, 60% VO2 max corresponds to 75% HR max, 80% VO2 max corresponds to 88% HR max, and 85% VO2 max corresponds to 92% HR max. Therefore, using these results, the optimal training HR range for general aerobic fitness is 75-88% HR max, significantly higher than the 70-85% HR max from the ACSM. For Joe, with his HR max at 190 bpm, using Swain et al, his target HR range is 143-168 bpm, as opposed to the ACSM’s recommended range of 133-161 bpm. The improved research from Swain et al thus suggests that training HRs should be pushed up a little to 75-88% HR max to bring about optimum results.

For elite athletes, Swain et al showed that % HR max for the same % VO2 max were slightly higher compared to average. Therefore, for steady-state training, an HR range of 77-89% VO2 max would be appropriate for an elite athlete. For advanced interval training, the intensity must be above 85% VO2 max or above 92% HR max. For example, during a session comprising 6 x 800m runs at 5K pace, the training intensity will be at 90-95% VO2 max. This would correspond to a training HR of 95-97% HR max.

We can see clearly from these examples that knowing accurately what % HR max corresponds to a target % VO2 max is very useful for both the average and the elite athlete. By using the formula derived by Swain et al, we can calculate a target training heart rate for the particular goal of the individual. So, how precisely is HR max calculated?

The easiest and best-known method is to use the formula 220 – age. This is the method recommended in the ACSM guidelines. However, the actual derivation for for this regression equation has never been published. It is used since it is a simple way to get a good estimate of HR max. In an attempt to be more accurate, numerous cross-sectional studies have been done to investigate the relationship between HR max, age and other factors. A paper by Londeree and Moeschberger from the University of Missouri-Columbia collates the data from all these studies in order to bring together the findings.

What they show is that HR max varies mostly with age, but the relationship is not a linear one. Thus the 220 – age formula is slightly inaccurate. For adults under 30, it will overestimate HR max and for adults over 45 it will underestimate HR max. This is especially true for well-trained over-45s whose max HR does not reduce as much as with sedentary individuals of the same age. Londeree and Moeschberger suggest an alternative formula of 206.3 – (0.711 x age). Similarly, Miller et al from Indiana University propose the formula 217 – (0.85 x age) as a suitable HR max calculation. In my experience, it is the Miller formula which gives appropriate estimates when calculating HR max from age alone.

 Swimming heart rates are lower
Londeree and Moeschberger also looked at other variables to see if these had an effect on HR max. They found that neither sex nor race make an difference. However, HR max does vary with activity and fitness level. Studies have shown that HR max on a treadmill is consistently 5-6 beats higher than on a bicycle ergometer and 2-3 beats higher than on a rowing ergometer. Heart rates while swimming are significantly lower still, around 14 bpm, than for treadmill running. Running and Versaclimber show similar HR max. Londeree and Moeschberger also found fitness levels lead to a variation in HR max. Elite endurance athletes and moderately trained individuals will have a HR max three or four beats lower than a sedentary inividual. However, as already stated, this is only true for young athletes – well-trained over-50s are likely to have a higher HR max than that which is average for their age.

This is of utmost relevance to those using the rower or bicycle or those who are very fit, since training HRs will have to be calculated differently. To do this, Londeree and Moeschberger offer us another formula, a slightly more complicated interactive equation to calculate HR max for different ages, activities and fitness levels. However, it is very difficult to use without a calculator and a degree in mathematics! (The details are at the end of this article.)

My own suggestion
Having outlined various methods for calculating HR max, I would recommend the following, which combines the Miller formula with the research from Londeree and Moeschberger. Use the Miller formula of HR max = 217 – 0.85 x age for running and Versaclimber training with average trainees. Subtract three beats for rowing training, subtract five beats for bicycle training. Subtract three beats from these estimates for elite athletes under 30. Add two beats for 50-year-old elite athletes and add four beats for 55+ years.

One question that you may be justified in asking is, who cares? Will all these complicated percentages and formulae actually make a difference, when the old ACSM recommendations are so straightforward? The point is that, if you want to use heart rate monitors, it serves little purpose unless you know ACCURATELY what training intensity the measurement represents. For example, a 45-year-old jogging to get fit should maintain 60% VO2 max for a 20-30 minutes continuous run. Using the old ACSM recommendations, he would be aiming for 70% HR max. HR max would be estimated at 175 bpm, using the 220 – age formula. This gives a target training HR of 123 bpm. However, the jogger’s HR max is more likely to be 179 bpm, and following Swain et al, target training HR should be 75% HR max. These two changes give a revised training HR of 134 bpm, a massive 11 bpm difference in target HR. If our 45-year-old had followed the old recommendations, his or her training would have been below optimal intensity, at 50% VO2 max, and he or she would not have got the most from the invested training time.

These inaccuracies can also disadvantage the elite athlete. For example, a 25-year-old elite cyclist using the 220 – age formula may think his HR max is 195 bpm. However, it is more likely to be only 188 bpm. This could mean he is overestimating target training HR for certain sessions, which can be undesirable if mileage rather than intensity is the aim of the session.
The take-home message of this article is a word of warning if you use traditional calculations to quantify training intensities. If 60% VO2 max is the minimum intensity for aerobic fitness improvements, then 75% and not 70% HR max is the minimum training target HR. However, using a range of 75-88% HR max for training targets is probably best. To calculate HR max, the simple 220 – age formula is not always accurate. The alternative formulas and chart provided will give you more accurate estimates.

For beginners and individuals training for a healthy fitness or for a recreational sport, I recommend that you calculate your HR max for your chosen training activity and then the 75% HR max training target. During your workouts, use a HR monitor or take your pulse and make sure that you put in enough effort to get your HR to the required level for a fitness benefit.

For elite athletes, use the new formulae to accurately calculate your maximum and target heart rates. Remember, tough interval sessions need to be really tough, so make sure your HR reaches around 95% HR max. However, sometimes you need to keep training moderate, so aim for 77-89% HR max for steady-state training.

Summary data
Target intensity for health benefits = 40% VO2 max = 63% HR max
Target intensity for aerobic fitness = 60-80% VO2max = 75-88% HR max
Target intensity for elite training = >85% VO2max = >92% HR max
Swain et al equation: % HR max = 0.64 x % VO2 max + 37
Miller et al formula: HR max = 217 – (0.85 x age)

Londeree & Moeschberger interactive formula:
HR max = 199.1 + 0.119 x AEF4 + 0.112 x AE + 6.28 x EF3 + 3.485 x F2 + 2.468 – 0.0006 x A4 – 0.591 x A
A = age; A4 =(age4)/1000; E = exercise type, if run = 1, if bike = 0; if sedentary F2 = 1, otherwise F2 = 0; if active F3 = 1. otherwise F3 = 0; if endurance trained F4 = 1, otherwise F4 = 0

Raphael Brandon