Follow Your Heart: Using HR to Gauge Fatigue
Heart rate is one of the most widely used tools to help measure training intensity. The heart rate monitors of today are more accurate than ever, come with many functions, and provide a reliable source of feedback. Understanding how to use the feedback from your heart is the key. When it comes to training, heart rate is most often used to help measure intensity and maintain a consistent pace. However, heart rate can also provide key information about your current training status and level of fatigue.
Heart Rate and Perceived Effort
To lay the foundation for how heart rate can inform us on our training status, it is first important to understand the consistency of heart rate training and the close relationship between heart rates and perceived effort. A 2004 study called Variation in Heart Rate during Submaximal Exercise1 examined the variation of heart rate data over a week of training from 44 athletic men and women. The study compared heart rate averages from two minute intervals in the 90% of maximal heart rate range, repeated four times daily over five consecutive days.
The results showed a variation in heart rate of around five to seven beats per minute, per day. This represented a 3-5% variation for most, and a small amount of variation when you consider how the test was performed. The subjects in this study only used perceived effort to guide them, and did not view heart rate information until after the test. This study shows a very close relationship between heart rate and perceived effort.
It also demonstrates the consistency of observed heart rate over a one week training period. The study focused on five days of short duration workouts but most of the subjects continued with their regular training schedule outside of the test, naturally providing additional training fatigue. However their heart rates deviated minimally, supporting the consistency of heart rate trends over a short-term training period.
Greater variations in heart rate occur at times due to factors such as training fatigue, heat and humidity, and dehydration. Understanding how these factors affect heart rate will help you understand how to use heart rate data in a more productive way. For example, you may have a targeted heart rate range as a goal for the day but working within that heart rate range may not be possible if you are experiencing a greater amount of training fatigue.
Now let’s discuss how your heart rate trends over a training period.
Heart Rate Trends Over a Training Block
When starting into the first week of a new training period after a week of reduced hours or a longer period of rest, it is a common trend to see heart rates elevate into the 4-5a zones quickly, accompanied by a low to moderate perceived exertion while targeting efforts in this range (see this piece on heart rate zones by Joe Friel). This is a sign of being well-rested, and gives the green light to press on through the week and training period. The study cited above supports that absent of factors such as fatigue, it is a common trend to see heart rates respond into the targeted zones with little variation through the first week and most of the second week of training.
As you work through your second week of training and into your third week, you may experience slightly more variation in heart rate and an increase in perceived exertion in all heart rate ranges. It will most likely become harder for you to elevate your heart rate into the 4-5a plus zones as you build training fatigue - but that is of course part of the goal. You want to experience greater amounts of fatigue towards the end of a training block, but you also want to recognize when you are pushing your limits too far to prevent working into an overtrained state.
How HR Can Tell You When to Rest
If you are experiencing a high level of perceived exertion and a hard time elevating your heart rate into the zone 2-3 ranges following a period of rest, that is a sign that additional rest is needed before taking on more training volume and intensity. At this point, making a change to the plan and backing off for a few additional days would be wise. Complete rest or a few days of easier spins in the zone 1-2 ranges may be all it takes to see heart rates respond more easily and while experiencing lower perceived exertions.
Sometimes it takes much longer than anticipated to gain good rest, so if you’re recovering from a hard race or experiencing overtraining symptoms, listen to feedback from your heart, as well as overall feel and mindset to determine when it is best to start back into more intense training.
If you're in the midst of a training block, know that you are beginning to push the limits when you can elevate your heart rate into the prescribed ranges for the prescribed durations, but experience a higher than normal perceived exertion. For example, if you are able to work into the 3-5a plus zones with a responding heart rate, but you experience a higher than normal perceived exertion or experience more leg fatigue than usual, then you are beginning to push the limits.
It is ok to continue to proceed as such over a two to three week period, until you start to experience greater amounts of fatigue. Observe your heart rate many days to get a better picture of your current training status; you should not judge your overall level of fatigue based off of feedback from a single day. You may have trained a little too hard the day prior, had a bad day emotionally, or experienced an off day with diet.
You are starting to push beyond the limits when you see a trend of having trouble elevating your heart rate into the upper end of zone 2 and into zone 3, accompanied by higher than normal levels of perceived exertion. When you notice higher perceived efforts to get to the upper end of zone 2 and into zone 3, three to four days out of five, then you are viewing a trend in fatigue.
This is when you should consider backing off and getting rest. Otherwise, we start to see consequences related to overtraining such as greater losses in power, loss of sleep, irritability and a lack of motivation to train. It is good to push your limits, but pushing them too far will result in less productive training.
Follow Your Heart
In conclusion, your heart rate is a valuable training tool that not only helps you maintain pace, but also provides feedback on your current level of fatigue and overall training status. A decreasing sub maximal heart rate at a set level of intensity is a sign of increasing aerobic fitness gains while an increasing heart rate can be a sign of fatigue, dehydration, or overtraining.
Our hearts can tell us many things and understanding it is a learning process. So follow your heart and listen to what it tells you. It may help you reach athletic goals and more.
In order to observe trends in heart rate like Mike describes, you'll need to log and track your training.TrainingPeaks makes it easy to monitor your workouts and upload data from your devices, and offers cutting-edge analysis tools so that you can use this information to achieve your personal best. Learn more here.