[This article originally appeared in the
issue of Northwest Runner
Many people use heart rate monitors to gauge how hard their bodies are working during exercise. This can be a fun and interesting thing to do; however, tracking your heart rate (HR) is of limited strategic value unless you have enough background knowledge to understand the numbers that your monitor spits out. In this article, we'll review some important things to keep in mind when attempting monitor-assisted workouts or races.
The classic 220-minus-your-age formula is now thought to be less
accurate than the others (Londeree & Moeschberger, 1982;
Whaley et al., 1992; Engels
et al., 1998; Tanaka
et al., Journal of the American College of Cardiology 37: 153-6, 2001). For the serious athlete,
however, none of the formulas can be considered adequate, because,
as discussed by Dr. Robertson in last month's Northwest Runner,
max HR varies considerably even among individuals of the same age.
For example, if a group of 40-year-olds has an average max HR of 188,
95% of them will have max HRs somewhere between
170 and 206 -- quite a large range (Engels
et al., 1998).
Therefore, if you are committed to HR monitor training,
you should put down your calculator and determine your max HR empirically.
In the laboratory, scientists consider a runner's max HR to be the highest HR he/she reaches during a VO2max test, i.e., a 10-to-15-minute treadmill test of progressively increasing difficulty. In the absence of a treadmill, however, this test can be simulated at a 400-meter track. After warming up, do the following without stopping: 2 laps at a comfortable, easy pace, then a third lap about 4 seconds faster than the second lap, then a fourth lap about 4 seconds faster than the third lap, and so forth, making each lap faster than the lap preceding it until you are too tired to continue. (In order to achieve an all-out effort, consider soliciting encouragement and/or pacing help from friends. Also, if you don't regularly engage in vigorous exercise, consider soliciting medical supervision.) The highest HR you attain during this workout should be very close to your max HR.
While the max HR of most individuals remains stable from month to month, it often drops by 4-8 beats in people who take up endurance training after a period of inactivity. Conversely, max HR can rise by a similar amount in trained athletes who scale back their training regimen or abandon it altogether (Zavorsky, Sports Medicine 29: 13-26, 2000). Thus modest changes in max HR at the beginning and end of one's competitive season are to be expected.
Once you have established what your max HR is, what percentage of your max should you train at? Most people, scientists included, would agree that a sequence of hard and easy days is preferable to doing the same workout day after day after day. Exactly how hard to run on which days is a complicated question, and the answer depends on which event(s) you're training for and what you're like as an individual. Nonetheless, a few general comments can be made.
A. Do "race pace" workouts in your training. Some people like to do their intervals and tempo runs at predetermined percentages of max HR, and that's fine. Still, if your goal is to race at a particular pace (rather than at a particular HR), you should practice running at that particular pace, regardless of the exact HR associated with it. This follows directly from the principle of training specificity, which we discussed more fully in last December's column.
B. HR is an imperfect indicator of physical stress, but it's as good as anything else. Critics of heart rate monitors claim that HR is an unreliable measure of workout difficulty. As one coach argues, "the heart can be a terrible indicator of actual running intensity, since it is subject to the whims of weather, psychological state, hydration status, sleep patterns, and other factors" (Anderson, Running Research News 15(8): 1-4, 1999). Well, yes and no. HR is a readout of how hard you're pushing yourself, and that is affected by weather, hydration, sleep, terrain, etc., as well as your actual pace (Gilman, Sports Medicine 21: 73-9, 1996; Lambert et al., Journal of Sports Sciences 16: S85-90, 1998; Jeukendrup & Van Diemen, Journal of Sports Sciences 16: S91-9, 1998). Thus in some ways HR is a better gauge of exertion than split times because of these other influences rather than in spite of them. Though it would be unwise to base all your training on HR with no consideration of pace (see point A above), it would also be foolish to train entirely by pace with no consideration of the effort you're putting out.
It has also been argued that workouts should be conducted at target blood lactate levels rather than at target HRs (Foster et al., Medicine and Science in Sports and Exercise 31: 578-82, 1999), the assumption being that one must train with a very specific amount of lactate in the blood, e.g., "0.3 to 0.5 [millimoles per liter] below the [lactate threshold] in most of the session" (http://www.mariusbakken.com/tr_kenyan3.htm). The goal of this "lactate training" is usually to increase one's lactate threshold (LT) -- the exercise intensity at which lactic acid starts to accumulate in the blood. However, it is not clear whether training just below, at, or above the LT leads to the greatest improvements in the LT -- for details and references, see my March 2001 column -- so why spend a lot of time making sure you're 0.4 millmoles/liter below the LT rather than, say, 0.2 millimoles/liter above it? Given the current limits of our knowledge, such precision is unnecessary.
C. Exercise at low HRs to ensure that your easy days are easy -- not to maximize weight loss. Many fitness machines and HR monitors come with colorful charts of the various exercise "zones," and these charts often refer to the easiest level of exercise as the "fat-burning zone." This terminology is logical in the sense that, during mild exercise, the body's main fuel is fat rather than carbohydrate (Romijn et al., Journal of Applied Physiology 88: 1707-14, 2000). However, it should not be interpreted to mean that slow-paced workouts trim body fat more effectively than fast-paced workouts. A recent review article concludes, "For weight loss, maximizing energy expenditure may be superior to a regimen designed to maximize the use of fat during the exercise bout.... If fat loss is the goal and time is limited, persons should ... expend as much energy as possible during the time available" (Hills & Byrne, Proceedings of the Nutrition Society 57: 93-103, 1998; see also Ballor et al., American Journal of Clinical Nutrition 51: 142-6, 1990).
On the other hand, easy workouts do offer the advantage of giving the body a chance to recover from the hard, fast stuff. For this reason, some athletes employ HR monitors to keep themselves below a certain intensity (e.g., 70% of max HR) on their easy days. There isn't much published research on this topic, so the exact HR cutoffs people use are somewhat arbitrary. As a general approach, though, the strategy of keeping oneself below some pre-established HR on easy days seems sensible, at least for people who might otherwise overtrain.
Racing with a heart rate monitor
The benefits of racing with a HR monitor have yet to be unequivocally demonstrated (O'Toole et al., Journal of Sports Medicine and Physical Fitness 38: 181-7, 1998). Of course, paying attention to your HR can help you maintain a steady effort during races with wildly varying terrain or no mile markers. However, several pieces of data suggest that setting a target HR or HR range for a race is no easy task. First of all, a given speed may elicit a higher HR in a racing context than it does in training (Lambert et al., Journal of Sports Sciences 16: S85-90, 1998). In addition, if you settle into a constant pace during a long (>30-minute) race (or workout), your heart rate will gradually rise, a phenomenon known as cardiovascular drift. This occurs even when fluids are consumed during the race to prevent dehydration (Coyle, International Journal of Sports Medicine 19: S121-4, 1998). Conversely, if you keep your HR constant during a long race, your pace will decline (Boulay et al., Medicine and Science in Sports and Exercise 29: 125-32, 1997). Thus it is difficult to say exactly how pace and HR should be adjusted during races so as to achieve optimal performances. In any case, one should keep the above findings in mind when racing with a HR monitor.