Bradycardia: Low HR

author : AMSSM

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Is low resting heart rate concerning? How does it affect heart rate zones?

Question from member Purch1977: 

Hi I am currently training for my first middle distance at the outlaw in May and have just been told I have exercise bradycardia? Which the doc says is normal for someone that trains for endurance. My issue is I have been training to heart rate zones and used the standard 220 minus age., I know this isn't the most accurate method but what difference will it make having a low resting heart rate? Do the zones shift with the lower resting or stay the same? My resting rate is 42 and the max hasn't been recorded recently as I am doing low zone training. Any help would be much appreciated.

Answer from Anna Monroe, M.D.
Member AMSSM

Your triathlon training has resulted in a lower resting heart rate which likely also correlates to a lower heart rate (HR) and while exercising at the same intensity as other less well-trained people. As you pointed out the “220-age” equation for maximal HR is not always accurate, and your question raises several issues for further discussion.

Bradycardia:

I’m glad you have seen your doctor and have concluded your low HR is due to your endurance training instead of other medical problems or medication use. I also assume you do not have any symptoms from your bradycardia such as chest pain or shortness of breath. Anyone with a similarly low heart rate, those who use medications which affect heart rate or those with any symptoms should check in with his or her doctor to make sure it is safe to go ahead with HR training.

Use of HR Training:

HR goes up as exercise becomes more strenuous; therefore, measuring HR during a workout can indicate whether or not an athlete is exercising at a high enough level to increase fitness but not so hard as to result in overtraining. Exercise intensity zones ranging from low to high can then be determined from calculating a percentage of an athlete’s maximal HR (HRmax). HRmax is used because it estimates something termed maximal oxygen uptake or VO2max which gives information about a person’s overall fitness.

Finding HRmax:

In order to determine accurate HR training zones, it is important to correctly determine HRmax. The easiest method is to use “220-age” to predict HRmax. However, this formula is often off by 8-12 beats per minute (bpm). “HRmax= 206.9- (0.67 x age)” might be a better option. For the best estimate, you could undergo exercise testing during which you would exercise up to your maximum while a professional observed your heart rate, blood pressure, and other data. This type of test is sometimes available at a health club, a local university, or a private sports performance clinic.

Keep in mind that even on an individual basis your HR can change from day to day based on the length of your exercise session, how hydrated you are, and the conditions outside (whether it is hot or cold or with altitude). Therefore, any HR monitoring method just represents a best guess.

Other Methods:

HR Reserve (HRR) is considered to be a better method than using HRmax. It involves knowing your resting HR (HRrest) and HRmax, but it is fairly easy to calculate if you know those two numbers. Resting HR is your HR first thing in the morning before getting out of bed and ideally one calculated when you have not been awoken by an alarm! HRrest increases with overtraining too, so watching this summer over time can also aid in training.

The equation for using HRR is as follows:
Target HR = [(HRmax - HRrest) x % intensity desired] + HRrest.

Basing your intensity on how you feel (rating of perceived exertion or RPE) is an alternative and low tech method to gauge intensity. Occasionally tuning into your body regardless of the data you notice on your monitor is never a bad idea!

* In summary, HR monitoring can provide helpful information for training safely and at full potential, but in order to realize these benefits, an athlete should understand the technology and work towards obtaining accurate information. Good luck with your race!

Anna Monroe, MD 

References: Achten, J., & Jeukendrup, A. E. (2003). Heart rate monitoring: applications and limitations. Sports Medicine (Auckland, N.Z.), 33(7), 517–538. Thompson, Walter R., Gordon, Neil F., & Pescatello, Linda S. (2010). ACSM’s Guidelines for Exercise Testing and Prescription (Eighth). Baltimore, MD: Lippincott Williams & Wilkins. Author: Anna Monroe, M.D. Staff Physician Appalachian State University Student Health Services

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date: May 31, 2016