It is important to be aware of variables that can affect heart rate numbers. Since you and your clients will be using the numbers represented to make conclusions about the quality of training, it is essential that you have a solid understanding of external and internal variables that can impact heart rate numbers.
The following is a quick overview of what and how different conditions can impact the numbers you see:
During exercise your heart rate is likely to rise over time, especially if you are less fit. You may begin a training session like a bike ride, for example, and your heart rate might start at 135n beats per minute (BPM) and 90 minutes later, it may be in the range of 150 BPM without changing your pace or effort. This is known as cardiac drift. Several factors play into cardiac drift. Core body temperature is most likely the bigger culprit in raising the heart rate without changing the intensity. As internal temperature rises, the body needs to dissipate heat so there is a signal to send more blood to the surface of the skin to help cool the body via evaporation.
As we exercise and perspire there is a certain amount of dehydration that occurs. As we lose water volume, our blood thickens and the heart has to work harder to pump the same amount of blood. One study found a 7 (BPM) increase for every 1% of body weight lost due to dehydration. It is not uncommon to lose 3 to 4%of body weight during exercise in hot conditions.
As muscles fatigue during exercise, the body may shunt some of the workload to other, fresher muscles that may not be prime movers for the exercise and less efficient. This happens so the heart will have to work harder to supply blood, oxygen and nutrients to the muscles. Fatigue can also affect resting heart rate — when fatigued, it is possible to observe both abnormally high and low resting heart rate numbers. This anomaly usually will correct as one recovers. It is a good indicator of overtraining.
Stress and strong emotions will affect heart rate numbers. Any thought or strong emotion, especially ones perceived as challenging or stressful, will raise the heart rate. When we perceive danger, our bodies move into fight, flight or freeze by dumping a chemical cattail that helps the body prepare for a survival episode. Adrenaline is produced and spikes the heart rate.
The younger we are, the higher the heart rate, both resting and exercise. As we age, the heart rate tends to decrease, as does the maximum heart rate.
Women tend to have slightly higher heart rates than men because of total body mass and physical size. Women’s hearts, in general, are smaller than men’s hearts and pump a little faster to get blood and oxygen where it is needed during exercise.