Understanding Heart Rate and Exercise

If you are reading the MAPP, chances are you already use heart rate as a measure of your exercise intensity. The basics are that 1) heart rate serves as a measure of exercise intensity during steady state activities, and 2) If we estimate maximal heart rate as 220 minus age we can use this value to gauge the intensity of our training. That is not the whole story though. Here are a few details that can be important in your training and racing.

• The "220 minus Age" formula is Only an Estimate. Your actual maximal heart rate in a given activity could be 10 to even 20 beats higher or lower than the estimated value. This has important implications for judging training intensity.
• Your Maximal heart Rate Differs in Different Activities. Cardiac hemodynamics and maximal sympathetic drive are influenced by 1) body position during exercise and 2) muscle mass involvement. So, a triathlete with a max heart rate during running of 180, may only hit 176 on the bike, and 171 during swimming. In this case we call the running heart rate "Maximal Heart Rate" and the highest heart rate observed in cycling and swimming, "Peak" heart rate, for that event. Knowing your peak heart rate for each discipline will help you to more accurately guage the intensity of your training. If the activity is restricted to upper-body muscle mass, peak heart rate will generally be considerably lower than in whole body activities. Examples include kayaking and double poling during cross-country skiing. Highly trained athletes can achieve a higher percentage of true max heart rate when performing small muscle mass activities.
• A Better Method for Gauging Exercise Intensity with Heart Rate For a given exercise mode, heart rate will increase linearly with exercise intensity, and therefore, oxygen consumption. However, the resting heart rate creates an offset between % of HR max and the associated % of "peak" Oxygen consumption for that activity. For example, running at 65% of Heart rate max corresponds to approximately 50% of VO2 max. At 87% of HR max, you are at about 77-83% of VO2 max, depending on your resting heart rate, heart rate and VO2 percentage finally converge at 100%. I prefer to use HEART RATE RESERVE as my training intensity guide. To do this I need to know 1) my resting heart rate, and 2) my peak heart rate for that specific activity. The first one is easy to determine. The second one may sometimes be a slight estimate. My current resting heart rate is about 36 beats/min. My peak heart rate during rowing is about 181. So my heart rate range is 181-36 or 145 beats. Now, if I want to train at 85 % of my peak VO2 for rowing, I will take 85% of my heart rate reserve (0.85 x 145=123) and add it to my resting heart rate (123+36 = 159). PERCENTAGE HEART RATE RESERVE will give a better approximation of % maximal oxygen consumption then just % max heart rate. And, it is more accurate because you can adjust for changes in your resting heart rate.
• Body Position on the Bike will Influence Heart Rate. Let's say I am riding on an indoor bicycle trainer with my upper body parallel to the ground (Hands on the drops) at a heart rate of 145. Raising upright while continuing to cycle at the exact same workload will result in an increase in heart rate of about 5 beats per minute. Trust me I have experimented with this effect on many a winter evening! This is due to decreased venous return in the more upright position. Heart rate increases to compensate for the slightly decreased venous return and stroke volume, keeping cardiac output constant. Whe I return to the drops, the heart rate drops again.
• Temperature Will Greatly Influence Heart Rate. Above about 70 degrees farenheight (21C), Heart rate at a standard submaximal intensity will be increased about 1 beat/min per degree F increase in temperature. Thus, a steady state run at a heart rate of 150 on a 70 degree Spring day, may have you close to maximal heart rate on a scorching 95 degree day in July, if you try to maintain the same speed. I am from Texas, so I remember these days well. The oxygen demand doesn't increase in the heat, but the thermal stress load does. As a result, your cardiovascular system must divert blood flow to the skin to enhance heat dissapation. Since you only have so much cardiac output, this means a lower maximal steady state speed in the heat, or early exhaustion. You choose. My choice is generally to avoid running in 95 degree heat.
• Humidity Hurts Too For the same reasons, a higher relative humidity will increase heart rate at a submaximal workload. Increased humidity decreases the evaportation rate of sweat. This means the body has to resort more to heat removal via increased skin blood flow. Data from Wilmore and Costill "Physiology of Sport and Exercise" shows a 10 beat increase in heart rate from 165 to 175 when running in 90% humidity compared to 50%. This is the difference between a morning and afternoon workout in many parts of the country.
• What about the Time of Day? Our bodies show diurnal (time of day) variations in many physiological responses. Within the normal range of times that you might be training, this can result in a 3-8 bpm difference in heart rate at rest, during moderate exercise, and during recovery. The differences during maximal exercise are probably smaller. Data demonstrating this effect is in the literature. However, I suspect the exact pattern of these changes can be altered by your specific exercise pattern. For example, after several years of rowing before sunrise, I am sure my diurnal response pattern was modified. I have no data to support this assumption, but I do know that I was transformed from an afternoon exerciser, to a morning guy! So, my best guess is that you should not be too surprised by small differences in heart rate response if you do your training at an unusual (for you) time of day.
• What is Cardiovascular Drift? If you begin a 90 minute steady state ride on your bicycle trainer at a controlled intensity, your heart rate may be 145 after 10 minutes. However, as you ride and check your heart rate every 10 minutes, you will notice a slight upward "drift". By 90 minutes, your heart rate may be 160. Why is this happening if intensity is held constant? There are two explanations. As you exercise, you sweat (dah). A portion of this lost fluid volume comes from the plasma volume. This decrease in plasma volume will diminish venous return and stroke volume. Heart rate again increases to compensate and maintain constant cardiac output. Maintaining high fluid consumption before and during the ride will help to minimize this cardiovascular drift, by replacing fluid volume.

There is also a second reason for the drift during an exhaustive exercise session. Your heart rate is controlled in large part by the "Relative" intensity of work by the muscles. So in a long hard ride, some of your motor units fatigue due to glycogen depletion. Your brain compensates by recruiting more motor units to perform the same absolute workload. There is a parallel increase in heart rate. Consequently, a ride that began at heart rate 150, can end up with you exhausted and at a heart rate of 175, 2 hours later, even if speed never changed!