Just wanted to share this to all, this might be good info to know.
To Live or Die by the Heart Rate Monitor
It's been over 20 years now that heart rate monitors (HRM) have been a common training tool for triathletes. Unfortunately, they are not the most accurate measure of training stress. Your heart is a muscle, and your heart rate (HR) is a measure of how fast your heart is pumping, which can be influenced by a variety of internal and external factors.. Dehydration, air temperature, heart rate lag, and general fatigue are just some of the mechanisms that effect HR. Triathletes of all shapes, sizes, and abilities can train with a heart rate monitor and still make gains, but with the awareness that heart rate is not always a direct measure of your training stress.
Your heart is a muscle, and just like other muscles it can be come fatigued. When you finish a long ride or run, you normally feel some general fatigue in your legs. Your heart will become fatigued as well. Although you may not feel it, it can be reflected when you monitor your HR. When the heart becomes fatigued, it sometimes is difficult for the athlete to reach a higher HR. Should you stop training? This depends a lot on how fatigued you are and its relation to over training. This is something to be analyzed by your coach or yourself. It is important to remember basic exercise physiology, and that in order to make gains athletes must "over reach" and combat a bit of fatigue at times.
Do you stop training every time your quadriceps are somewhat fatigued or sore? Of course not, you would never make any true gains. Your heart reacts the same way. When the heart is somewhat fatigued, it may be more difficult to raise your HR to reach specific HRM Zones , however you define that. For example if you were supposed to ride your bike at Zone 3 HR 150-160 4 times a week, for 6 weeks, you may find that in the later weeks, you have to work harder at times to reach Zone 3. Now if you were to train with power meter you would notice that regardless of how fatigued you were, maintaining 140 watts, takes the same amount of pressure on the pedals whether you're fatigued or not. Power meters reflect your total work, which is measure in watts. Power meters allow you to keep a steady state of work, regardless of fatigue.
Fatigue is not the only variable that can affect your HR. The more training stress you put on your body, the higher HR becomes. But is this an accurate reflection of the amount of work you are doing? If your endurance run pace is 9min/mile and you go out for a 15mile run, in the later miles your HR tends to continue to creep up. This is known as cardiac drift. You feel fine, but your HR reflects differently. So you decide to run slower, and you know are running at a 10min/mile pace in order to keep your HR under control. You just changed your work load to become a lot easier, therefore you're not placing the amount of stress needed on the body required to make proper gains.
Now this is one of the few times I would suggest that wearing a HRM is not a terrible idea for longer duration workouts. As long as you don't adhere to your HRM every number you can use it to ensure that you are not over doing it during longer workouts. It never hurts to have a GPS watch with HR, this way you can see the correlation between HR and Velocity in your training.
Another area to look at is the duration of exercise and HR response. Have you ever been rushing out of the house and at the last minute remembered you forgot your wallet or cell phone upstairs. So you sprint up the stairs to your bedroom to retrieve what you forgot. Well when you did this you didn't notice the rapid increase of your heart rate, until you were up stairs in your bedroom. This is because your heart rate lags behind. How does this affect your training? Well for long increments of training over 20min you will not see as much of an affect (except for cardiac drift), but for shorter bouts of interval training (which should play a significant role in any athletes training, whether novice or elite), their will be too much lag time to have accurate readings with your HRM.
Lastly, it is important to consider the many internal and external factors that can affect your heart rate. Slight dehydration, air temperature, core temperature, stress, excitement, over training, or improper fueling are just some of the many variables that can affect your HR. Have you ever been at the start of the race and look down at your HRM? You will notice that it will be higher than when you normally start your workouts. This is a result of excitement and the release of stress hormones. This can occur throughout the whole race! How about those long rides in the heat? Take a peak at your HRM and you will notice that it's higher than normal, this is due to the heat, increase core temperature, and even just a little dehydration. These are all variables that athletes must deal with when trying to train with a HRM.
When training with a HRM an athlete has to be aware that these variables can cause falsified measurements in actual training stress. Heart rate can only be somewhat valuable when the athlete partakes in longer duration exercise, taking cardiac drift into consideration. The heart is a muscle, therefore it can become fatigued, at times making it difficult to elevate HR. In addition, the variables such as dehydration, stress, excitement, air temperature, and core temperature can also fluctuate HR readings.
The take away message here is that HRM training is not the most accurate in measuring ones true training stress. The athlete must analyze and be aware of all the factors to ensure that they are training at proper intensities to ensure advancements in their training.
Coach Jason
www.etacoach.com 
