Friday, July 4, 2008

Heart Rate Monitors and Training

This is something I wrote some time ago, originally to help my daughter who got a heart rate monitor. Periodically I change it slightly and send to someone. I post it here because it may be of more general interest.
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How you train depends on what you want to accomplish. Genetically people tend toward a predominance of one or the other of two muscle fibers: Fast twitch or slow twitch muscles. The ratio of these is not fixed. Slow twitch fibers can be recruited to be fast twitch, but there is a genetic diffference that tends people toward one or the other. Those who have lots of fast twitch fibers tend to be sprinters. Those with lots of slow twitch tend to be better at endurance. Related to the sprinter/endurance dichotomy are several metabolic pathways for producing energy. There are aerobic (with oxygen) and anaerobic pathways. The anaerobic pathways tend to be good for very short duration activities, a few minutes or less. I have never been tested, but I am the anti-sprinter, so I suspect I tend toward the slow twitch. At any rate, my goals tend to be fitness, endurance and speed (strength) over periods of about an hour. I choose an hour because that tends to be the amount of time I can manage for exercise on any given day. This puts me in the aerobic, endurance camp. I have no idea how sprinters (or their cousins, weight lifters) train.
Several things limit athletic performance. One of them is oxygen. A second is Lactate buildup. Another is muscle strength. In a sense, muscle strength is the simplest to train for. Muscles get stronger as they adapt to stress. To make your muscles stronger you stress them to the point of minor damage, then allow them to recover. Both stress and recovery are important. No stress, no need for adaptation. No rest, no chance for the damage to be repaired and the muscles to get stronger. In addition to simple strength there is also technique. Every activity can be done more or less efficiently. The more efficient you are, the less energy is required for a given result. This is good.
The oxygen/lactate systems are more complicated because there is so much going on. Every person has a maximum rate at which oxygen can be absorbed through the lungs into the bloodstream, pumped by the heart to the body and utilized by the muscles and other organs. This can be measured as your VO2 max. If you exceed your VO2 max value, by definition you can’t provide enough oxygen to support the work you are doing and something either has to stop or go anaerobic. The point at which you start going anaerobic is called your anaerobic threshold. The best thing you can do to improve your VO2 max is to choose your parents well. However, even the genetically ungifted can improve their VO2 max through training.
When you produce energy anaerobically, one of the by products is lactic acid. If the lactic acid accumulates in the muscle it changes the PH and the muscle loses the ability to contract. Of course none of this is black and white. Even when you are exercising mainly aerobically, some lactic acid is produced. You should be able to increase performance by improving your capacity to get oxygen to your muscles, improving your ability to remove lactic acid from your muscles when it occurs, and by making your muscles more tolerant of lactic acid when it is present. Lactic acid itself is burned oxidatively by mitochondria to produce energy. That is one of the ways that lactic acid is removed. See http://home.hia.no/~stephens/lacthres.htm.
Two other factors in oxygen transport are your maximum heart rate and your resting heart rate. There is a speed past which your heart will not beat. The maximum differs from person to person and is also dependent on age. You lose roughly one beat per minute per year as you get older. Your resting heart rate serves as an indicator of heart efficiency. Take your resting heart rate while lying down, preferably in the morning before getting up or right before bed. At rest, your body still demands a certain amount of oxygen. The fewer heart beats needed to satisfy this requirement, the more efficient the heart is. As you get more fit, your heart simply delivers more blood per beat.
What does this have to do with training with a heart rate monitor? Your heart rate roughly corresponds with the amount of oxygen being demanded by your body. This, in turn correlates with the energy your body is putting out. The harder you work, the faster your heart beats. In an endurance sense your heart rate is limited on one end by your resting heart rate and on the other end by your anaerobic threshold. The range is expanded by making your heart pump more per beat (lowering the resting heart rate) and raising your anaerobic threshold (increasing oxygen delivery and lactate removal).
As long as you aren’t "overtraining", a heart rate monitor allows you to judge just how hard you are working. Surprisingly this can be more useful on easy days than on hard days. Using the monitor you can get a sense of your real anaerobic threshold (it hurts and you can’t sustain for long). You can gauge effort by finding how close you are to either your anaerobic threshold or to your maximum heart rate. This is usually expressed as a percentage (90% of maximum heart rate). Be aware that these measures are different because your anaerobic threshold is below your maximum heart rate. However, your anaerobic threshold is something you can figure out. Your maximum heart rate is difficult to determine. I knew my predicted one was wrong when I looked down at the monitor one day and saw a number above my "maximum".
Conventional wisdom is that training should be a mix of stress and recovery. Recovery does not mean inactivity. It means activity at a level below that which stresses you. Most of your time should be spent in recovery. Exercise at a recovery level can be performed for a longer duration than exercise at stress levels. This recovery not only allows you rebuild your stressed muscle, it increases endurance and gives you a chance to work on efficiency. Although technique is critically important, efficiency is not synonymous with technique. I believe some of it is a physiologic adaptation.
Stress levels for your heart lung system are right around the anaerobic threshold. To work on raising the anaerobic threshold, lactate recovery, and increasing VO2 max, intervals are the prescribed medicine. To stress the aerobic system rather than just using your sprint abilities, the intervals should be at least a couple of minutes long. These are very painful and it is recommended that you not do this more than about once a week.
A step below intervals is a hard workout. Ignoring my own advice about recovery, these are typically what I do on the bike. For a hard workout I basically work for an hour at just below my anaerobic threshold. For me on the bike this is around 90% of my predicted maximum heart rate. My excuse is that I am not an athlete and don’t have enough time to recover as an athlete would (easy days). I recover through inactivity. There are many days where I simply don’t get any exercise and it is uncommon for me to have the same exercise activity two days in a row.
For athletes and sensible people who actually schedule easy recovery workouts, the heart rate monitor is most useful. The trick is to set a level before you start. It is very easy to have a recovery workout turn into yet another hard workout. The heart rate monitor gives you a number. If that number goes too high, you ease off regardless of how you feel. I find this to be very difficult. Several sources give target recovery workout rates of about 70% of your max heart rate. I find it hard to give advice that I don’t follow, but conventional wisdom would probably have you do as many recovery workouts as you do hard workouts and intervals. Hard days should be followed by recovery days. I do not know much about recovery time. For example, I don’t know how long it takes muscles to heal from the micro-tears induced by a hard workout (but I think it is on the order of one to two days). This is probably what should govern your recovery schedule for any given muscle set.
My goal is general health and fitness, so I don’t tune my training toward any particular event or activity. I’m on maintenance and (I hope) gradual improvement regime. If you are tuning toward an event or season, there are longer cycles (weeks and months) of buildup and relaxation you should use. At that point you should find a coach.
All this assumes that your heart is a pretty good gauge of effort. For the most part it is. After using the heart rate monitor for some time on the bike and blades, I find that it correlates pretty well with perceived effort, that is, how hard it feels that I’m working. This isn’t always true though. It is not true when I’m boosting or dropping my heart rate. For example, increasing exertion to go from a heart rate of 150 to 155 can feel worse than a steady state of 165. Dropping from 170 to 165 can feel like heaven.
A second factor that can change heart rate is over-stressing. When I measure my resting heart rate after a hard workout day, it tends to be high. I think this is part of the normal recovery. If you over-stress for days or weeks, your resting heart rate will remain high and your total capacity to work will be reduced. Resting heart rate is both an indicator of general fitness (low is good) and of "overtraining" (goes up and stays up over a period of days). I think resting heart rate also goes up during illness.

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