For a long time, heart rate measurements have defined athlete's training programs. The basic premise is fairly sound - the harder your muscles work, the more oxygen muscles need, the faster the heart must beat to supply that oxygen. Athletes divide their heart rate into specific "zones" based on how close they are to a tested maximum heart rate. Maintaining your heart rate at specific zones increases aerobic capacity and fitness. It's a very well established and researched method that has helped countless athletes for many years. However, the longer you spend training with heart rate measurements, the more frustrating it can be.
The strength of heart rate training is its simplicity. A quality heart rate monitor is an inexpensive investment, usually in the range of $30-$100. The system is easy to understand. Once you find your maximum heart rate via a real-life test, it's easy to set up zones at various percentages of your heart rate. Moving between these zones or maintaining long periods of time in a given zone helps an athlete improve his or her endurance. There are volumes of established workouts to allow athletes to gain endurance or power based on heart rate zones.
The biggest problem centers around the variability of heart rate. From day to day and hour to hour physiological and environmental factors can produce drastic changes in heart rate. Anyone who's felt their heart beat race after quickly downing a cup of coffee can attest to this. A few months ago, an article in the New York Times examined the heart rate differences between morning and afternoon workouts. To compound this issue, these changes in heart rate may or may not correspond to changes in effort levels.
I've experienced this first hand myself. On Monday and Tuesday I went on two separate training rides. Both rides were 16 miles, along identical routes, with similar efforts, same time of day, same weather conditions. There were marked differences in my heart rate. On Monday, my heart rate averaged 171 bpm with a max of 195 - much higher than my usual heart rate. On Tuesday, my average was 166 bpm with a max of 175, much closer to my usual.
Equivalent conditions and efforts, but vasty different heart rates...
The only difference I can recall between the two rides is nutrition. On Monday breakfast consisted of two Nutri-Grain bars, definitely not the best cycling fuel. Tuesday, I was able to eat my usual meal of a bagel, yogurt, and plenty of water. That simple difference is the most likely explanation for the difference in heart rate I experienced.
So what's a cyclist to do? First of all, remain flexible with the data you're receiving. Understand that heart rate can and does fluctuate and be willing to compensate. If your heart rate is above normal, but your perceived effort level feels ok, understand that what your heart rate monitor is telling you may not be the entire story. Secondly, keep data on your rides. Careful notes about weather conditions, diet, sleep patterns, and perceived exertion can help explain anomalies in heart rate. If you're committed to training via heart rate, some notes about each ride are well worth your time and can help you understand patterns in your heart rate.
The best alternative, and the one I've committed myself to is power measurement. Rather than relying on a heart rate measurement to estimate effort levels, a power meter uses a strain gauge to measure real-time effort exerted at the crank arm or rear wheel. The data takes into account gradient, wind, and other factors which can increase effort but are often hidden in heart rate measurements. It's worth noting that heart rate also plays a role in power measurement, but isn't affected by the variability as much as stand alone heart rate training.
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