Friday, 25 May 2012

Art of the Heart: Part I

Art of the Heart: Part I

GPS watches with inbuilt heart rate monitors are excellent tools for tracking fitness. But, I am always a bit concerned about whether athletes and coaches know how to interpret the data. Once you start to get interested in heart rates and begin studying them in detail, things can look quite confusing. For instance two runners - one older and one younger might have very similar performance times, but run at very different heart rates: and not as you might expect. The older runner might be working at heart rates above an 'age-predicted' maximum which the other younger runner might have a lower heart rate at the same speed 20 beats per minute slower. This seems to make no sense. Then you might find that one day your average heart rate rises or falls by 10% at the same speed and distance. Surely all of this means that heart rates are variable and useless? Indeed, that is the stated opinion of some of our high level coaches.

However, there are good reasons for these variations - and once you understand why heart rates can vary they can become a useful tool. Since the heart is a pump, the rate at which it beats tells you something about the metabolic work being done. But, the complication lies with the fact that each beat can pump a different amount of blood. The amount of blood ejected from the heart with each beat is known as the stroke volume. That volume is a proportion of the blood that fills the heart whilst it is relaxing (diastole). If you multiple the stroke volume by the heart rate you can calculate the amount of blood pumped: the cardiac output. The first thing to realize is that the heart has an internal, intrinsic control (known as Starling's Law of the Heart). That control process results in the heart pumping the blood that returns to it through the venous supply, i.e. if you give the heart blood to pump, it will work as hard as necessary to get rid of the blood.

Starling's Law of the Heart

Venous blood, returning to the heart through the vena cava (great vein), flows through the right atrium and into the right ventricle. Almost all of the blood flowing into the right ventricle does so as a result of the pressure gradient caused by the elastic recoil of the ventricle as it relaxes and the venous pressure. The contraction of the atrium adds a bit more blood - and it isn't terribly much at rest, although during exercise it is more important. The force of contraction of the ventricle is determined by the amount of blood that has entered (i.e. the amount of stretch of the ventricle). So, the more blood returning the greater the contraction and the more blood is pumped. The heart acts to get rid of the blood that flows back to it. This is a very important property of the heart since it makes sure that our pulmonary pressure (blood in the lungs) does not rise too high and cause us to lose fluid into the lungs (and therefore suffocate). People suffering from left ventricular failure - where the left side of the heart cannot generate sufficient pressure to get rid of the blood have exactly this problem - they effectively drown.
So, the stroke volume of the heart is determined by venous return (blood flowing back to the heart). It is this variability in venous return that underlies much of the variability of heart rates. When venous pressure is high the heart fills with a lot of blood and each beat pushes out a lot of blood. The result is a low heart rate. When venous pressure falls the stretch is less and heart rate rises. The same amount of blood is pumped, by your heart rate differs. This change in stroke volume is not reported by GPS/heart rate monitor watches and if you  don't realize what is happening you may well end-up making the wrong conclusions....or worse still training at  a different intensity than you intended.

Maximum heart rates

The next problem is maximum heart rates. The term, as used by most people, is nonsense.  Sure, there is a maximum rate at which a heart can beat before the output decreases - but even that probably depends upon the state of the rest of the circulatory system. When you exercise, you are driving your muscles using motor nerves: your heart gets driven by changes that result from this. The maximum heart rate you can achieve will depend upon the amount of muscle you use, the duration of the exercise, your motivation and the state of your circulatory system (in particular the venous portion of it). If you are fatigued, fluid loaded as a result of previous exercise, aerobically fit and exercising for a prolonged period you will not get anywhere near an age-predicted value for heart rate. If your arteries are a bit stiffer, you have good muscle mass, middling fitness and your venous pressure is lower and you are doing a brief period of exercise you should be able to achieve a much higher heart rate than the age-predicted value.

Resting heart rates

Resting heart rate is the value you might get towards morning before you wake-up. Its value is dependent on both long and short term exercise as well as emotional state. It may be of some value - I know some Elite level coaches use it to determine levels of fatigue. I am not a great fan of it and rarely either measure of use it.

Changes in heart rate during an exercise session

Now this is the most useful measure. When you exercise you will notice that your heart rate first rises rapidly and then after a minute or two it either continues to rise or it wobbles around some value. These numbers are the ones that mean something! When you exercise there is an initial drive to the heart to increase the force and rate of contraction. There is also a rise in venous return and a change in arterial resistance. All of these will set stroke volume and heart rate at some value. Then as you exercise muscles will begin to metabolize, use stored energy and release waste products. It is the change in these metabolites and waste products that open up blood vessels and increase the blood flow to the exercising muscles. The result is a general (but small) decline in arterial blood pressure as the peripheral resistance decreases. The baroreceptor reflex (the process that maintains arterial blood pressure) maintains the arterial blood pressure by altering both heart rate, venous return and contraction force. The change in heart rate is what the GPS watch notices.
Once the rapid rise in heart rate is over, your body is in steady-state (almost) with sufficient blood flowing through your lungs and muscles to sustain the exercise intensity. Or, at least if the exercise intensity is low enough that is what happens. If you exercise more intensely then the metabolites/waste products continue to feedback causing ever greater amounts of blood flow and your heart rate keeps rising. The point at which it stops rising depends upon whether your muscles can continue to produce the force and whether your brain can keep driving them.
The beauty of measuring heart rate during a session is that your heart rate, within that aerobic zone, is nearly proportional to the amount of work you are doing. Of course, there are factors that will cause heart rate to drift upwards at constant effort. Thermoregulation places an extra-load on the circulatory system requiring the heart to pump more blood to the skin, and a progressive dehydration will also reduce stroke volume causing heart rate to rise to maintain cardiac output. Also, fatiguing muscles will also require more blood. But, the great thing relationship here is that the number of heart beats required to cover a km remains roughly constant and independent of speed. It is this relationship that I want to consider in my next post.

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