Heart Rate Training Limitations and Phenomena

With the advances in technology, heart rate monitors have the ability to measure heart rate with ECG (electrocardiogram) accuracy.  To get the most out of heart rate training, its important to be aware of various phenomena and limitations leading to misinterpreted data.

1. 220 - age = HRmax?  Recommended by the American College of Sports Medicine (ACSM) to estimate maximal heart rate, it has a large standard deviation of plus or minus 12-15 bpm (4,5).  I highly recommend using RPE in conjunction with this method to determine if your HR max was under- or overestimated.
2. Cardiac drift & Training in heat.  Cardiac drift is a phenomena that causes changes in HR and SV (strove volume) to occur when exercise exceeds 30 minutes at the same workload.  With the onset of heat stress, SV drops due to vasodilation, plasma loss, and circulatory changes.  Collectively, this occurs to improve heat removal.  To compensate for lower SV, HR increases to maintain cardiac output (2,3,4).  In a study on competitive cyclists, cardiac drift caused HR to increase by 20 bpm from 20-60 minutes of exercise (2).  The takeaway is that after 30 minutes of constant aerobic exercise at the same intensity, heart rate will increase without a change in effort or RPE.  This also means that temperature and humidity can also affect heart rate significantly since it directly affects thermoregulation.
3. Dehydration increases heart rate.  During moderate dehydration, it was estimated that for every 1 percent loss of body weight caused by dehydration, heart rate increased 7 beats per minute (1).  A study which required subjects to exercise at 62-67% VO2max for over 100 minutes with no fluid intake found that heart rate increased by 40 bpm (10).  When the subjects were allowed to hydrate, heart rate only increased by 13 bpm (10, 11).  If the difference between hydration and dehydration wasn't clear before, it should be clear now!
4. Heart rate varies daily.  Under the same workload, heart rate can vary anywhere from 1-6 beats per minute (1,3,6).  This variation may be affected by a combination of things such as the environment, motivation, time of the day, hydration levels, nutrition, sleep and medications (3).  This is another good reason to use RPE to keep workouts honest.
5. At the same workload, heart rate in competition is higher than in training (6).  Studies have found that during competition, there is no relationship between heart rate and running speed (3,4,5).  In a 10 km distance, heart rate was 163 bpm (+/- 13 bpm) in competition and 143 bpm (+/- 22 bpm) in training (9). Because heart rate values in training are typically lower, runners tend to unerestimate their pace on race day.  A study targeted towards cycling found that cyclists consistently reached higher maximal heart rates in competition compared to the laboratory determined maximum heart rates (4).  Motivation and pacing could explain this variation.  Paying attention to your pre-race nerves can help with deciding whether your heart rate zones need to be shifted higher or lower.
6. Medications can increase or decrease heart rate.  Stimulants such as caffeine, amphetamines, ephedrine, psudoephedrine and cocaine can also increase heart rate (8).  Beta blockers or Beta-adrenergic blocking agents can lower heart rate (7).

Despite the limitations of heart rate training, heart rate monitors have the potential to provide extremely useful information that can improve the quality of training, track progress and most importantly, prevent overtraining.  As you learn how nutrition, hydration, psychology affects heart rate, the data will become much more reliable for training and racing.

Resources:

1. Astrand, P.-O. and Saltin, B. (1961). Oxygen uptake during the first minutes of heavy muscular exercise. Journal of Applied Physiology, 16, 971-976.
2. Jeukendrup, Asker, and Adrie Van Diemen. "Heart rate monitoring during training and competition in cyclist." Journal of Sports Sciences 16 (1998): S91-S99. Print.
3. Lambert, M.I., Z.H. Mbambo, and A. St Clair Gibson. "Heart rate during training and competition for long-distance running." Journal of Sports Sciences 16 (1998): S85-S90. Print.
4. Palmer, G. Hawley, J.A., Dennis, S. and Noakes, T.D. (1994). Heart rate response during a 4 day cycle race. Medicine and Science in Sports and Exercise, 26, 1278-1283.
5. Plowman, Sharon A., and Denise L. Smith. Exercise physiology for health, fitness, and performance. 3rd ed. Philadelphia: Wolters Kluwer Health/Lippincott Williams & Wilkins, 2011. Print.
6. Selley, E.A., Kolbe, T., Van Zyl, C.G., Noakes, T.D. and Lambert, M.I. (1995). Running intensity as determined by heart rate is the same in fast and slow runners in both the 10- and 21-k, races. Journal of Sports Sciences, 13, 405-410.
7. Van Camp, S.P. (1998). Pharmacologic factors in exercise and exercise testing. In Resource Manual for Guidelines for Exercise Training and Prescription (edited by S.N. Blair, P. Painter, R.R. Pate, L.K. Smith and C.B. Taylor), pp. 135-152. Philadelphia, PA: Lea and Febiger.
8. Thomas, J.A. (1998). Drugs, Athletes and Physical Performance, pp. 217-234. New York: Plenum Press.
9. Wallace, J.: "Principles of cardiorespiratory endurance programming" In: Kaminsky, A. (ed.), ACSM's Resource Manual for Guidelines for Exercise Testing and Prescription Fifth Edition. Philadelphia, PA: Lippincott Williams & Wilkins, 336-349 (2006).
10. Hamilton, M.T., Gonzales-Alonso, J., Montain, S.J. and Coyle, E.F. (1991). Fluid replacement and glucose infusion during exercise prevent cardiovascular drift. Journal of Applied Physiology, 71, 871-877.
11. Montain, S.J. and Coyle, E.F. (1992). Fluid ingestion during exercise increases skin blood flow independent of increases in blood volume. Journal of Applied Physiology, 73, 903-910.