21. Maximum Heart Rate

From the adjunctive exercise essays Isabelle Aubert from UCSD asked the following question:

“…if I work out on the stairmaster only 2-3 times a week, is it acceptable to push the upper limit of the optimal heart rate or it is just not doing any good past a certain point?”

The heart responds to overload. Generally we select an overload with is 60-90% of the maximum heart rate as determined by 220-age. I have exercised my heart (on a life-and-death mountain rescue) until it was much faster than even my maximum heart rate. When I finally climbed into the helicopter I had a heart rate of 232 beats per minute. Well, 232 is far greater than even 220 so by our formula I would have had to be minus 12 years old.

The heart is a pump. The amount of blood it pumps in a minute is called the cardiac output. At rest the cardiac output is about 5 liters per minute. The cardiac output has two determinants; 1) heart rate and 2) the amount of blood the heart pumps with each stroke (called stroke volume). So cardiac output = (stroke volume) X (heart rate)

In the case that a person has a normal resting cardiac output of 5 liters/minute and a heart rate of 60 beats per minute this person’s stroke volume is around 83 ml per beat. The stroke volume does not change (except about 5-10%) from before exercise to maximum exercise. This means that the stroke volume on a given day remains the same in spite of the demands placed on the heart to pump more blood.

Only the heart rate can change, not the stroke volume (at least on any given day). The heart rate changes in order to meet the needs of the body for more oxygen and blood. As exercise begins muscles extract more oxygen and give off more carbon dioxide. Receptors in the bifurcation of the carotid arteries, called the carotid body, respond to the high carbon dioxide levels by sending messages to the brain. The brain, in turn, sends messages to the sino-atrial node in the atrium of the heart which increases the heart rate.

When a person with a stoke volume of 83 ml/beat exercises so as to raise the heart rate to 120 beats per minute we can determine the cardiac output using the above formula. The cardiac output is (83 ml/beat) X (120 beats per minute) = 9.96 liters per minute. For a 50 year old individual this is 71% of maximum heart rate. (220-age) = maximum heart rate: (220-50) = 170 beats per minute for the maximum heart rate. The (120 bpm)/(170 bpm) = 71%. This 50 year old is exercising at 71% of maximum and is pumping just about 10 liters of blood per minute.

Assuming now that the 50 year old has a maximum heart rate of 220-50 or 170 beats per minute we can determine this person’s maximum cardiac output. It is (83 ml/beat) X 170 beats per minute = 14.1 liters per minute. This individual can pump a maximum of 14.1 liters per minute during heavy exercise.

As I stated above a person can exercise so severely that the heart rate will increase to more than 220. So why do we call it maximum heart rate when the rate can actually go faster? A more understandable term might be maximum efficient heart rate or something of that nature. Let’s look at heart beat for a minute.

When the heart muscle contracts it expels blood from its chambers (called systole). We have already discussed stroke volume as the amount of blood that is expelled during any one heartbeat. Well, if the heart expels a stroke volume with each contraction it must receive a stroke volume from the venous side in order to prepare itself for the next heartbeat. The venous blood enters the heart when the heart muscle relaxes (called diastole). There must be adequate time between strokes for the heart to fill with sufficient blood. As the heart rate increases there is less time in diastole for the heart to fill. When the heart rate reaches the level we call the maximum heart rate the heart can just barely fill with enough blood before systole causes the blood to be ejected. When the heart rate increases above the level of the maximum heart rate, which it can do, the heart does not have enough time to fill, so, even if the heart rate increases the cardiac output does not increase (the stoke volume decreases as the heart rate increases). So, then, increasing the heart rate doesn’t improve the cardiac output. In fact, the blood supply to the coronary arteries which fill during diastole may decrease and the heart approaches exhaustion.

As the person ages the muscles of the heart get more rigid probably by the deposition of collagen and other connective tissue in the heart muscle. As the heart gets more rigid it fails to fill during diastole as quickly as when it was more distensible. For this reason the maximum heart rate decreases with age. Whereas a 20 year old female can efficiently fill her heart with blood during diastole at a heart rate of 200 (= 220-age) a 40 year old female’s heart is too rigid to be able to fill that fast. The maximum heart rate for the 40 year old is 180 (= 220-age) because the heart takes longer to fill with blood during diastole.

To exercise beyond the maximum heart rate for a person’s age is dangerous. The heart is less distensible than what is needed to insure adequate blood perfusion. The coronary arteries fill while the heart is resting between beats. If a person does not give adequate time for the filling during diastole the quantity of blood needed to keep the heart muscle perfused is inadequate to support the heart. At first chest pain may occur (it may not be apparent during exercise). This woman is aimed at exhaustion and collapse because she is operating outside the limits of cardiac efficiency and capability. I would caution her not to exceed the maximum heart rate for her age.

The target heart rate is between 60% and 90% of the maximum heart rate for a person’s age. As I explained before 60% is quite low and is best for those beginning an aerobic training program and 90% is only for those who want to stress their bodies to the limit. I recommend between 70% and 80%. If you occasionally stress yourself to 90% it is OK for your heart but the rest of your body may soon show signs of abuse. This abuse comes in different forms depending on the exercise involved. Some examples are shin-splints, knee and ankle injuries, lower back pain, hip pain, muscle cramps, pulled muscles, sprains and strains. Unless a person has a really good reason to stress the body to such intensities of exercise, 85%-95% intensity factors should be avoided. Olympic caliber athletes may risk such intensities but the general public looking to get in good physical condition does not need the high risks associated with high intensities.

So, then what are the aims of aerobic exercise for the heart? The benefit of aerobic exercise for the heart lies in the stroke volume, size of the heart muscle, perfusion of the coronary arteries and changes in the LDL/HDL cholesterol ratio.

Although a person cannot change their stroke volume during any one day, aerobic exercise increases the stroke volume over time. After about six weeks of regular aerobic exercise with the proper intensity, duration and frequency the heart chambers begin to enlarge. This allows a greater flow of blood thru the heart with each stroke. In other words the stroke volume increases. As the stroke volume increases the cardiac output at any given heart rate increases too.

Let’s look again at the 50 year old with a stroke volume of 83 ml/beat. After about six weeks of training the stroke volume will start to increase. When it reaches 85 ml/beat only a little difference will be noted in endurance. Say that the person was running 3 miles in 30 minutes as regular exercise, an exercise intensity which requires about 10,000 ml blood/minute cardiac output. Using the formula cardiac output = stroke volume X heart rate we can approximate what will happen as this person’s heart changes. With 83 ml/beat stroke volume her heart rate was 120 beats per minute to pump the necessary 10,000 ml per minute cardiac output. When her stroke volume increases to 85 ml/beat her heart rate while running the 3 miles in 30 minutes falls to 118. This is hardly enough to recognize. But when this person has been training for a year and the stroke volume has increased to 100 ml/beat the heart rate during the same run decreases to 100 beats per minute. This woman is not even getting sufficient aerobic exercise during the same run as a year before. Her heart has improved by increasing the stroke volume.

The size of the heart muscle increases during regular aerobic training programs. The muscle which was weak becomes stronger. More heart muscle leads to less work on the heart during routine living. The heart does not fatigue as easily and has a greater reserve.

In addition, the coronary arteries proliferate and increase in size. The heart muscle is better perfused with blood, and some people say, there is less chance of a thrombotic coronary occlusion (heart attack, myocardial infarction).

Last, but not least, it appears that regular aerobic exercise leads to a decrease in the LDL fraction of cholesterol (bad cholesterol) while increasing the HDL fraction (good cholesterol). With such a change the blood vessels are cleared of atherosclerosis or soft-plaque thus decreasing the chances of thromboic stroke, heart attack and a host of other vascular diseases.

Now, that is a rather wide spectrum of advantages to the heart and blood vessels from regular aerobic exercise done within the limits of 60-90% of the maximum heart rate. Again I caution those who wish to push the upper limit. They may gain cardiac benefits faster but they are at higher risk for a number of abuse related syndromes including heart attack, heart failure, and musculoskeletal injuries.