Recently, my son John, who lives in the San Francisco Bay area and is an avid bicycle rider, asked me if I could explain to him something about the technique of heart rate monitoring with exercise. Apparently, many of his friends hold to the theory that a work-out is not worth doing unless one can monitoir his heart rate to be certain the activity is being performed optimally. John told me he was completely baffled about the subject. As a good father, I told him I would do what I could to allay his anxiety about the quality of his bicycle rides.
First- the short answer
I reminded John that I had discussed heart rate monitoring in an earlier entry about using an exercise bicycle. I suggested that for most people it was quite sufficient to first calculate their maximum heart rate (220 minus age in years) and aim for a heart rate of 60-70% of the calculated maximum to assure a ride that would contribute to cardiovascular fitness. John, who is quite serious about his level of cardiovascular fitness, told me he had read the entry but really wanted quite a bit more information.
More information than almost anyone needs about heart rate monitoring
First, I need to say something that may upset many serious exercisers. It is fine to carry out heart rate monitoring during exercise (unless one falls off his bicycle or the treadmill while checking the pulse), but for most people it is not very important. For all but elite athletes and those in cardiac rehabilitation, very little heart rate monitoring is necessary. Now that I’ve offended lots of people, including manufacturers of heart rate monitoring equipment, I need to defend my audacious statement.
Heart rate monitoring is used during exercise as a measure of work rate and oxygen uptake, which are really what we want to know but which can only be assessed properly in a laboratory. So, heart rate (HR) is a surrogate measure of energy expenditure. Its main virtue is that it is easy to measure. HR does increase linearly with work rate and oxygen consumption but the HR response to exercise depends on many factors including age, body position during exercise, level of fitness, environmental factors (e.g., air temperature, relative humidity, altitude), whether or not heart disease is present, medications, blood volume, and on and on.
What happens to the heart rate with training?
At rest, energy expenditure is very low (1-2 kcal/minute) with the skeletal muscles contributing very little, about 20%. With exercise, things change drastically. Energy expenditure can increase 10-30X, mostly for the muscles. For this to happen, the muscles need more oxygen which can only occur if the heart gets more blood to the muscles. The heart has a limited bag of tricks for increasing blood flow to muscles. For one, it can shunt blood from certain parts of the body, i.e., the gut, to the muscle. The heart can also increase its rate, resulting in more blood, hence more oxygen, to the muscles per unit time.
With training, the heart can also enlarge, allowing more blood to be ejected per beat (the amount of blood ejected per beat is called the stroke volume or SV). This is very important since the heart needs more oxygen during exercise just like the rest of the muscles. The blood vessels that supply the heart muscle with oxygen are called the coronary arteries. These arteries fill only during diastole, the resting phase of the heart. The more rapid the heart rate, the shorter and shorter the diastolic phase and the less time for the heart to feed itself. The trained heart with its greater SV can pump as much blood as the untrained heart but at a much lower heart rate, thereby increasing the supply of oxygen to the heart muscle. Pretty nifty, huh? In addition, the trained heart has decreased oxygen demand at rest and during exercise.
Resting heart rate
As you would suspect based on the fact that the conditioned heart is larger and thus ejects more blood per unit time, the resting heart rate is lower in people who are fit. The “normal” heart rate is about 65-75 beats/minute; the highly conditioned athlete can have a heart rate as low as 40 beats/minute and still deliver as much oxygen as the body needs in the resting state. If the resting heart rate is consistently less than 60 beats/minute, that suggests reasonable cardiovascular fitness assuming the low heart rate is not due to some disease of the heart or some medication (my son’s was 48 beats/minute).
With increasing age, the heart changes: the maximal heart rate decreases, the maximal SV decreases, and as you would expect, the maximal cardiac output (measured as the amount of blood pumped/minute) decreases- in a 65 year old the cardiac output is 10-30% less than in a 30 year old. The maximal SV also decreases with age.
We could go on and on and discuss many other aspects of cariovascular fitness. We didn’t even get into “METS” or metabolic equivalents which are a measure of the metabolic cost of activity. Put in another way, a MET is an estimate of the intensity of an activity looking at a ratio of the working metabloic rate to the resting rate.Â One MET is equivalent to a certain amount of oxygen uptake (3.5 mL/kg/min, if you are interested).Â Studies have determined how many METs are used in various activities.Â For example, using a stationary bicycle with very light effort uses 3 METs while vigorous effort uses 10.5 METs.Â walking the dog uses 3 METs and walking at a 4 mile per hour pace (considered a brisk pace) uses 5 METs.Â Maybe we’ll come back to METs sometime?
For those of you who are not aiming for a gold medal at the next olympics, I would relax a bit about quantifying your cardiovascular effort with each exercise session. I would calculate maximum heart rate and try to sustain at least 60-70% of that for at least 20-30 minutes. If at that level of physical activity you barely raise a drop of sweat, maybe you need to rethink your maximal heart rate. Remember, the 220 minus age in years for maximum heart rate is just a rough guideline. Also, it is important to keep track of your resting heart rate- after 2-3 months of fitness training, it should be under 60 beats/minute. One other thing- if after exercising 30-60 minutes at 60-70% of your calculated maximum heart rate your heart rate is still quite a bit over your usual resting heart rate (e.g., greater than 100 beats/minute) 30 minutes out, you may have overdone it or you may be a bit dehydrated.
Well, maybe I was a bit brash in denegrating the use of heart rate monitoring with exercise. Maybe I should have said, heart rate monitoring can be useful but don’t go crazy with it? One other thing- I never did discuss how to monitor heart rate. As I discussed in my exercise bicycle entry, checking the radial pulse is the easiest and cheapest way- the index and third finger of one hand placed on the wrist of the opposite hand thumb side. You’ll find the radial artery pumping away. If you count for 10 seconds and multiply that by 6 you have a heart rate; the longer you count, the more accurate the estimate of the heart rate. I think expensive fancy monitors are just fine but very optional (if these gadgets are fun to use and increase the pleasure in the exercise, they are probably worth the cost even if they do not directly do much to improve cardiovascular fitness).
For those who want to explore the subject of cardiovascular/fitness training in depth, I recommend the American College of Sports Medicine (ASCM) Resource Manual For Guidelines For Exercise Testing And Prescription, Fifth Edition, Lippincott Williams & Wilkins, 2006. This is “the bible” for exercise testing and is both comprehensive and readable (and expensive- $59.00). You can access the ACM at www.acsm.org.
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