Heart Rate Training and The Limitations to Know

With the advances in technology, heart rate monitors have the ability to measure heart rates with ECG (electrocardiogram) accuracy.  Unfortunately, HR monitoring technology moved so fast that physiologist still have not found a way to interpret the data.  As of now, heart rate training is still an imperfect science and has limitations that are important to know prior to training with a HR monitor.

1. 220 - age = HRmax... maybe?  Although this empirically based equation is recommended by ACSM to estimate maximal heart rate, it has a large standard deviation of plus or minus 12-15 bpm (4,5).  This is not very accurate, so it leaves many people working easier or harder than they need to.  This best way to verify this number is to test it in a lab.
2. Cardiac drift.  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.  As a result of heat stress, SV declines due to vasodilation, plasma loss, and redirecting blood flow to improve heat removal.  As SV decreases, 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 take home point is that after 30 minutes of constant aerobic exercise at the same intensity, heart rate will increase beyond the body's control due to heat stress.  This also means that temperature and humidity can also affect heart rate significantly since it can enhance or make heat removal more difficult to perform.
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).  If you're not 100% machine (even if you think you are :] ), don't expect your heart rate to always be the same!  Some days can be a little high and some can be a little low.  To pinpoint the cause of the change or variance, keep a journal and look for a trend.  You might find that on days where you lacked sleep, HR rates were a lot higher!
5. At the same workload, heart rate in competition is higher than in training (6).  Nothing can simulate a race better than a real race.  Studies have found that during competition, there is no relationship between heart rate and running speeds (3,4,5).  In a 10 km distance, heart rates were 163 (plus/minus 13 bpm) in competition and 143 (+/- 22 bpm) in training (9).  This difference in heart rate is a big problem for runners.  Because heart rate values in training are lower, many runners run below their true optimal 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 be some of the most influential reasons why this occurs.  Word for the wise- On race day, don't depend entirely on a heart rate monitor because in this situation, it might hold you back!
6. Medications can increase or decrease heart rate.  Stimulants such as caffeine, amphetamines, ephedrine, psudoephedrine and cocaine can 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.  To get the most out of a HR monitor during competition, don't forget to also use the body's natural monitor- the brain.  Assess the variables that the HR monitor can't analyze such as the environment, muscle fatigue, pain, speed or cadence, climbs/ descents, mood, hydration levels and energy levels.

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.