This is an excerpt from Heart Rate Training by Roy Benson & Declan Connolly.
One of the most valuable long-term pieces of information you can gather is resting heart rate. When you wake up each morning, take a minute to get an accurate resting heart rate and keep a log. You'll find this an invaluable tool, providing feedback on injury, illness, overtraining, stress, incomplete recovery, and so on. It is also a very simple gauge of improvements in fitness. We know athletes who have gathered resting heart rate data for years and in a day or two can identify a 1 or 2 bpm elevation that precedes an illness or a bonk session. Some newer heart rate monitors have the capacity for 24-hour monitoring.
Several factors affect heart rate at rest and during exercise. In general, the main factors affecting heart rate at rest are fitness and state of recovery. Gender also is suggested to play a role, albeit inconsistently (more about this later). In general, fitter people tend to have lower resting heart rates. Some great athletes of the past have recorded remarkably low resting heart rates. For example, Miguel Indurain, five-time winner of the Tour de France, reported a resting heart rate of only 28 bpm. The reason for this is that, with appropriate training, the heart muscle increases in both size and strength. The stronger heart moves more blood with each beat (this is called stroke volume) and therefore can do the same amount of work with fewer beats. As you get fitter, your resting heart rate should get lower.
The second main factor affecting resting heart rate is state of recovery. After exercise, particularly after a long run or bike ride, several things happen in the body. Fuel sources are depleted, temperature increases, and muscles are damaged. All of these factors must be addressed and corrected. The body has to work harder, and this increased work results in a higher heart rate. Even though you might feel okay at rest, your body is working harder to repair itself, and you'll notice an elevated heart rate. Monitoring your resting heart rate and your exercise heart rate will allow you to make appropriate adjustments such as eating more or taking a day off when your rate is elevated.
These same factors of recovery and injury also affect heart rate during exercise. The factors that elevate resting heart rate also elevate exercise heart rate. If you're not fully recovered from a previous workout, you might notice, for example, at your usual steady-state pace, an exercise heart rate that is 5 to 10 bpm higher than normal. This is usually accompanied by a rapidly increasing heart rate throughout the exercise session.
An extremely important factor affecting exercise heart rate is temperature. Warmer temperatures cause the heart to beat faster and place considerable strain on the body. Simply put, when it is hot, the body must move more blood to the skin to cool it while also maintaining blood flow to the muscles. The only way to do both of these things is to increase overall blood flow, which means that the heart must beat faster. Depending on how fit you are and how hot it is, this might mean a heart rate that is 20 to 40 bpm higher than normal. Fluid intake is very important under these conditions. Sweating changes blood volume, which eventually can cause cardiac problems. The simplest and most effective intervention to address high temperature and heart rate is regular fluid intake. This helps to preserve the blood volume and prevent the heart from beating faster and faster.
Another important factor affecting exercise heart rate is age. In general, MHR will decline by about 1 beat per year starting at around 20 years old. Interestingly, resting heart rate is not affected. This is why the basic prediction equation of 220 - age has an age correction factor. As a side note, this decrease in MHR often is used to explain decreases in .VO2max and endurance performance with increasing age, because the number of times the heart beats in a minute affects how much blood is moved and available to the muscles. We have coached and tested thousands of athletes, and the general trend is that athletes of the same age who produce higher heart rates often have higher fitness scores. However, your MHR is what it is, and you cannot change it. Don't obsess over it.
A final factor is gender. Recent studies have suggested a variation in MHR between males and females. However, the data are inconclusive with the calculations resulting in lower MHRs for males versus females of the same age, while anecdotal reports suggest that the MHRs are actually higher in males. In general, females have smaller hearts and smaller muscles overall than males. Both of these factors would support the conclusion of a higher MHR in females, certainly at the same workload. We have to conclude that the jury is still out on the gender effect.
Read more about Heart Rate Training.