1. Fluctuations Do Not Correspond To Effort Levels
Perhaps the biggest limitation to heart-rate training is that many changes in your heart rate do not correlate to your fitness level. Sleep, stress, and dehydration can all raise or lower heart rate on any given day. As normal people with jobs, families, and otherwise busy lives, these outside influencers are common and can have a drastic affect on your heart rate readings. Let’s take a look at a few of those factors:
Sleep And Heart Rate Variability
Many studies have concluded that a lack of sleep, a reality that many runners are plagued with, will elevate your heart rate 5-10 beats per minute (bpm). While this may not seem like a big change, coupled with the other factors listed below, a lack of sleep could cause you to train at heart-rate levels that are below your optimal training zones.
In addition, you naturally have a lower heart rate in the morning than you do at night. Even further, heart rate can vary by 2-4 bpm from one day to the next without any changes to fitness or fatigue. Therefore, you need to adjust your heart rate to accommodate for the time of day you’ll be attacking the roads and factor in daily variability.
Stress has the same affect on your heart rate as a lack of sleep. One study in particular showed that workplace stress raised heart rates by 4-6 bpm. This is an important statistic for runners who train after work.
Unlike sleep, an exact measurement of your stress level, and therefore the exact increase in your heart rate, is difficult to determine. While running is a great way to reduce the effects of stress, the elevated heart rates you experience while in a stressful state will change the heart rates at which you should be running.
As runners with busy lives, caffeine often becomes the fuel that runs our day — for better or worse. While staying awake at work is ideal, studies have shown that caffeine elevates heart rate for up to 24 hours after ingestion.
Like stress, it is difficult to measure the exact change in heart rate you’ll experience when consuming caffeine because we all react individually to its effects. Runners who are accustomed to caffeine will be less affected than those who only drink the occasional cup of coffee.
Weather also has a dramatic influence on heart rate. During hot days, your heart rate will increase as your body works to cool itself down. In hot and humid conditions, blood is sent to the skin to aid in the cooling process. This means there is less available blood and oxygen for your working muscles. Therefore, your heart has to work harder to maintain the same pace and effort during your run.
Conversely, heart rate will decrease (or more accurately, underestimate the intensity of exercise) in response to training in cold environments. Researchers posit that training in cold temperatures results in an increase in stroke volume and thus a higher V02 max, which will lower the perceived effort and reduce your heart rate.
Finally, dehydration has a profound effect on heart rate. In one study, researchers found that cyclists who exercised in a dehydrated state exhibited heart-rate readings that were 5-7.5% higher than normal. Like the above factors, training in a dehydrated state can drastically influence your heart-rate training zones.
While each of these factors by themselves aren’t cause to throw your heart-rate monitor out the window, when you combine their effects, you can easily be exercising outside your target heart-rate zones on any given day. Likewise, the exact measurement of your stress levels, caffeine intake, and heart rate variability can be difficult to pinpoint, leaving you guessing at your actual heart-rate levels.