ToolBox: Understanding Heart Rate Variability
With the increased availability of usable power measuring devices, heart rate (HR) as a tool for measuring intensity may seem demoted to “option #2.” However, heart rate remains the financial and practical mainstay for most cyclists today, so it’s important to understand its strengths and limitations. One thing to keep in mind is how to deal with the potential variability in day-to-day heart rate response…
There is no doubt that for bike racers and triathletes alike, the use of power (when understood) is a training tool that can have substantial benefits to the athlete and the coach-athlete relationship. Of course, the reality remains that power devices are rather expensive and not readily affordable for many athletes. On the other hand, HR monitors, which are inexpensive, have been available and improving since the 1970’s and remain the choice of most athletes to measure training intensity levels. Let’s look at some key components of HR variability that will assist you when using HR to track your intensity levels.
We have all experienced the problem of HR variability on any given training day. For example, you have determined your optimal training zones through performance testing. You plan a training program based on those zones and you go out one day and they are dead on, but the next day they are 5% off the original values (plus/minus.) What are some of the reasons for this variability?
• Lower (or higher) HRs due to training stress and overload. For example, you go out on the second or third consecutive day of hard efforts or a stage race, you feel great, but your HR seems extremely low. Consecutive days of harder intensity increase the blood plasma volume and thus the overall blood volume. The increased plasma volume can have a direct correlation to a lower HR (1), as your heart doesn’t have to work as hard to supply oxygen to the working muscles. Also, consecutive days can lead to lower glycogen stores which affect HR.
• Altitude – Initially, when an athlete gets to altitude, their HR can increase due to the heart needing to supply more O2 in a hypoxic (diminished O2 supply) state. At the same time, maximal HR can decrease, so your training zones can take on new “relative” values; higher in the lower threshold zones, but lower on the maximal efforts.
• Medications – Certain common medications like antihistamines (used for allergies) or albuterol (asthma) can elevate your HR while medications like beta-blockers (hypertensives) can do the opposite and lower the HR. It’s important to check with your doctor when on a medication to see what effects it may have on your training.
• Rested athlete – It seems that early in the early season and after rest periods during the season, HRs are generally on the higher side. You go out to ride and your legs feel super fresh and your HR is responding quickly and possibly higher than you expect.
• Fatigue versus “not being fresh” – We all have been fatigued and know that feeling. Being chronically or really tired from a couple days of training will affect your HR. It will be difficult to hit your target zones and sustain them. In this case, you are best advised to ride home, re-evaluate your efforts, and try again the next day. This is distinguished from “not being fresh”, which occurs after extended periods of training. During these times it may take you longer than normal to reach your target zone, but once you are there, you can sustain the efforts. ”Not being fresh” doesn’t mean you are out of shape, it just means your HR is slow to respond, which could be a result of a dominant parasympathetic branch of your nervous system.
• Caffeine – Although it seems that most athletes who ingest caffeine do it on a regular basis and develop a drug tolerance, high doses can increase an athlete’s HR.
How to Handle Variability
First and foremost, just knowing that you will have variability in your zones for different reasons can be of great assistance. Remember that you have determined “zones” and not a specific HR for each training intensity. Zones have a built-in buffering system to allow for limited variations on a day to day basis. Also remember that for the majority of most active/fit athletes, once defined correctly, perhaps through repeated performance or field testing, HR zones don’t change that much. The changes that occur are the power outputs (or speed when running) within those zones. This is one reason why power, in addition to HR monitoring, can be such a valuable tool.
It’s important to always use two of the three intensity indicators when training. These are defined as: HR (internal), power (external) and rate of perceived exertion (internal). For example, know that threshold efforts are a 7 or 8 (out of 10) on the RPE scale. If getting to that zone and sustaining the HR feels like a 9+, you can compensate and drop the zone by about 5 beats or operate at the low end of the zone (assuming you are not fatigued.) I would argue that RPE is the most important of the indicators, as it represents what you know about your body and what it is capable of accomplishing on a given day.
I think one of the keys to being able to use HR successfully, is watching your HR over many days of training, and thus learning how it reacts to different training stimuli and environmental factors. Being able to correlate HR with RPE will teach you to be aware of what your body is doing. If you monitor yourself enough, over time, you will know exactly what to expect when you are out training. Remember, despite the inherent issues with using HR, many champions have been made using only HR.
(1) Mounier, Plaloux, Mischier, Coudert, Fellmann; Int J Sports Med. 2003 Oct; 24(7):523-9
Bruce Hendler created AthletiCamps to provide cycling specific coaching and training to athletes and cyclists of all levels. Find out more at www.athleticamps.com