Colorado Springs Group Runs

Friday, January 8, 2016

Heart Rate Monitors and Why I Am Not a Fan of Them

Heart rate monitors (HRMs) are relatively cheap and some have moved away from the, often, uncomfortable chest strap feature.  However, I find them to be more trouble than they are worth except in limited situations.

First, why measure heart rate?  Well, as we ask our bodies to work harder and harder on a run (or during any exercise) we increase the metabolic demands and that means the demand for oxygen increases.  One way we meet that demand is by increasing heart rate.  (The amount of blood pumped per beat -stroke volume--also increases early on).  The increase in oxygen demand is pretty linear and so is the heart rate increase.  Linear means that if I increase my running pace by 5% every few minutes, my heart rate will also increase by a certain amount and that those increases continue until we get near our maximal efforts.  Because heart rate and oxygen demand are related, we use HR as a proxy (stand in) for oxygen uptake.

Okay, so what could be wrong with that?  Well, that relationship is solid in a lab setting where temperature and pace is tightly controlled, but in the outside world effort changes a little and you might be going up some rises and down small hills and you have the wind and the sun to deal with.  So the relationship falls apart somewhat.  So running 5 miles at 166 beats per minute (bpm) on a cool October day could be a very different workout from running 5 miles at 166 bpm on a scorching hot August day.

One of the things that happens to heart rate as we get more fit is that at a given absolute intensity, HR decreases.   So if you take a person who has not exercised in years and measured his/her resting HR and then measured it again after several months of running, the resting HR is likely to be lower.  If that person was walking at 15 min/mile on day 1 and then several months later walked at that same pace, the HR would also be expected to be lower.

Here are some of my issues:
1) To get the most benefit, you need to measure your actual maximal heart rate (HRmax).  The formulae you often see such as 220-age or 205-(age*0.5) can be off by 10 bpm or more for an individual.  To get HRmax you need to push yourself to your MAX!  Now this can be done in a hilly sprint type workout or taking the highest HR seen at the end of a 5K.  Both will get you better data than any formula.  There is an alternative of using lactate threshold heart rate, but that is a little messier in my opinion.
2) Many factors affect your heart rate.

  • Temperature is a huge factor.  Doing a run in July at a given pace and another run in November at that same pace will likely yield very different heart rate readings.  (I will put my money on the November rate being lower).  That makes comparing the July and November runs using HR challenging.
  •  Hydration status plays a role.  If you are hypohydrated (underhydrated), your HR at a given intensity will very likely be higher than when you are euhydrated (normal hydration).  One reason is that the plasma (the watery part) of the blood is decreased and the blood does not flow as smoothly requiring the heart to work hard.
  • Sleep or fatigue can be a factor.  If you are not recovered from your previous workout or just did not get well-rested, you might see an increase in heart rate.
3) The heart rate training zones that one often sees are not as clear cut as the proponents make them out to be.  The difference in 75% of HRmax and 76% HRmax is not much a difference at all in reality.
4) Cardiac drift:  this is the phenomenon where during continuous exercise lasting for longer than an hour (although it can occur quicker) even if you maintain the same intensity, your heart rate will rise or "drift" upwards.  While some of this can be accounted for with dehydration, even when exercisers have maintained fluid balance the rate has risen.  So the issue becomes, especially during a long race, where a person wants to maintain some pre-determined heart rate and would need to slow the pace in order to maintain that rate.  This slowing, even when a person is doing fine, could lead to a decrease in performance.
5) The person relying solely on HR may ignore how he/she is feeling.  Are you struggling to maintain that heart rate?  If so, other information could be telling you to slow (or stop).  If you are going to use a HRM, please do not become a slave to it (same for GPS devices!).  Listen to your body and adjust accordingly.
6) How is the data you are gathering being used?  Data management (and this holds true for GPS as well) is tricky.  I suspect that most people will perform a run at some HR and then never look at the data at all.  If they do look at it, what are they looking for?
7) It is impractical for many interval training sessions.  When you increase the intensity, it takes a bit for your HR to get to where it needs to be.  Let's say you are doing efforts lasting 3 minutes and are trying to reach a HR of 170 bpm.  You start out and after a minute the HR is at 155 bpm.  At minute 2, HR is 165 and just as you finish your HR reaches 170 bpm.  Does this mean that you need to run faster?  NO! Heart rate lags behind demand and can take 2 to 3 minutes to reach the required HR to sustain that demand.  So for intervals lasting less than 3 minutes, the HR data can be deceiving.

Now, I am not an absolutist on this.  I see how a HRM can be a useful tool.
A)  In general I think people go too hard on their easy days and a HRM can help to reel them in.  Let's assume you have a good handle on your HRmax, but you (or an athlete you are working with) is the kind of person who pushed hard all the time.  A HRM might be useful in lowering the intensity.
B) Newer HRM models have the ability to capture heart rate variability (HRV).  Now, this gets far more complex, and to be honest, is not something I have ever worked with directly.  We think of our hearts beating in a nice rhythm.  If you have ever seen an EKG, you see the electrical pattern and it looks consistent.  However, in reality the time delay within each complete cycle varies a little bit (talking tenths of  a second).  The more variable the better!  More variability is associated with better fitness and also reduced risk of cardiac events.
C) TRIMP: Again, this is a bit more complex.  TRIMP is TRaining IMPulse and was developed largely by David Bannister (Roger Bannister was the great miler and later physician and no relation) as a way to use HR to assess how well training was going in athletes.  This is an area that I looked into briefly and even analyzed some data, but my duties at the time did not allow me to pursue it.  The TRIMP idea, as best I can tell, is somewhat incorporated into the software that companies like Polar use to assess fitness.

There you have some of my thoughts on HRMs.  I think it is a tool, but one that comes with limitations.  As with any tool, it is the proper application of the tool, and in this case the data it generates, that matters.

Even with a HRM or GPS, I like to know the rating of perceived exertion

I do not require the athletes that I work with to use a HRM.  If they use it, I will look at the data from time to time (assuming they upload it), but in general I go with rating of perceived exertion (RPE).  I will blog about RPE later.

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