This is an excerpt from Cutting-Edge Cycling by Hunter Allen & Stephen Cheung.
Heart Rate Monitors
With improved fitness, the defining change in your body is that your cardiac output, or the volume of blood pumped by your heart each minute, increases. Cardiac output is simply heart rate multiplied by stroke volume; the latter is the volume of blood pumped with each heart beat. In an untrained individual, the maximal cardiac output can reach approximately 25 liters per minute, but an elite aerobic athlete may have a value of 35 liters or higher. Obviously, as more blood is pumped, more oxygen is delivered to the muscles, increasing your aerobic energy production. Physiologically, what are some of the major adaptations in your cardiovascular system to enable this?
- Your maximal heart rate will not change with fitness. This value is largely determined by genetics and age.
- Overall blood volume will increase slightly, and some correlation is found between higher aerobic fitness and higher blood volume (Sawka et al. 1992). This result occurs mostly because of increased plasma volume rather than more red blood cells, such that hematocrit (the fraction of solid to liquid in blood) usually decreases with fitness.
- The heart, like your leg muscles, becomes stronger and able to pump blood with greater force. This increase in contractility helps to increase the stroke volume, or the amount of blood pumped with each heart beat.
- Stroke volume also increases by improvements in your body's ability to return blood from the body to the heart, resulting in greater filling of the heart each time.
- The capillary network in your muscles increases in density, permitting greater blood flow to the muscles themselves.
As can be seen from the preceding description, the cardiovascular system is a complex interplay of many things going on inside the body. Overall, the body adjusts the heart rate to achieve a cardiac output that will deliver adequate blood, oxygen, and nutrients to the muscles during exercise. Heart rate is also affected by the nervous system, as can be seen at the start line of a bicycle race when your heart rate is at 150 beats per minute before the starting gun even goes off! Or you could be tired and pushing yourself harder than ever, yet your heart rate reaches only 160 beats per minute as compared with your normal 175 beats per minute at threshold. In both cases, your nervous system is sending signals to your heart that override your body's physiological demands for cardiac output.
Therefore, although heart rate is an indication of effort, it is an indirect one at best. Similar to a tachometer on a car that tells you how fast the pistons are pumping rather than how many horsepower the engine is producing, heart rate does not tell you the speed or power that you are generating on the bike, but only how fast the heart is pumping. Hunter likes to say that “heart rate tells me the ‘intensity of an athlete's intention,' and that can help me to better understand the athlete in workouts and races.” Also, heart rate can be influenced by many factors related and not related to exercise, such as sleep, caffeine intake, and hydration status. That being said, monitoring your heart rate is beneficial, and you can track fitness changes over time by comparing heart rate with the data channels.
Read more from Cutting-Edge Cycling by Hunter Allen and Stephen Cheung.