Many athletes use a heart rate monitor (HR monitor) when training and racing. Knowing your heart rate under various conditions can teach you much about your performance physiology. In endurance athletics, monitoring heart rate may reveal problems in your training program, errors in your race pace, and warn you of stress caused by environmental factors. Understanding the implications of heart rate can help you avoid training and racing errors. The intention of this article is to caution you about the difficulties and limitations of interpreting exercise heart rate.

CONCEPT VI: The HR monitor

The heart rate monitor is an electronic device which conveniently records heart rate while you are exercising. Heart rate is an approximation of exercise intensity. It is an indirect measurement of physiologic workload (oxygen consumption, energy expenditure, aerobic capacity, or VO2). Therefore, knowing your training heart rate or the perceived effort associated with that heart rate, is important so that you can intentionally train significant elements (VO2max, VO2submax, and lactate threshold) of your physiology.

Physiology: Heart rate is related to VO2 in a linear manner. The approximate relationship (+/- 8% error) is:

This tells you that if you plan to train at 70% of your VO2max, then you must paddle at 80% of your maximal HR (HRmax). However, this straight line relationship between HR and VO2 tends to break down at the low and high values. For example, maximum heart rate is achieved before maximum VO2. This relationship is still practical because the higher the heart rate response, the greater the exercise intensity.

Factors other than oxygen consumption can influence heart rate. These factors include temperature, emotions, food intake, hydration state, body position, muscle groups exercised, continuous vs. stop-and-go exercise, or isometrically or rhythmically contracting muscles. In many arm exercises, heart rates are higher when compared to leg exercises. Consequently, heart rate during upper body work, tends to over predict the actual VO2. Heart rate response to paddling varies for paddlers of different experience, skill, and fitness levels.

More variables: In order to use the heart rate monitor for training and racing, paddlers must know their 1) maximal heart rate (HRmax) and 2) training or racing target heart rate (percentage of HRmax).

1. The usual calculation used to predict HRmax is 220 minus age. However, for at least 1/3 of athletes, 220 minus age will over or under predict HRmax by at least 15 beats per minute, leading to workouts which are too intense or too easy. The only sure way of determining HRmax for the paddler is an actual field measurement. Warm up for 10 minutes, paddle 250m hard, rest for one minute, then paddle 500m and check heart rate at the end.
2. There are two methods commonly used for calculating target HR. The first is the percentage of HRmax (__% X HRmax). The second is the percentage of the difference between HRmax and resting heart rate (__% X (HRmax - HRrest) + HRrest). Be aware that these two methods result in different heart rates!

More problems: Lactate threshold, which is very sensitive to training, is a key physiologic factor for predicting performance. The threshold is the VO2 at which lactic acid begins to accumulate because it is produced by the working muscles faster than it is metabolized. Much training effort is devoted to having this occur at the highest exercise intensity possible so that you can paddle fast and far. HR is commonly used to identify this threshold intensity, but be aware of the following concerns:

1. HR is a deceptive measurement for lactate threshold training. What is threshold HR? It is often suggested to be 82 to 88% of HRmax. Actually, it may be 65 to 94%. Or another source says “threshold intensity is about 80 to 88% of VO2max which is about 85 to 92% of HRmax.” Careful! Threshold occurs at different levels for different people.
2. HR associated with threshold will increase over time with proper training
3. HR varies to environment and mood, but threshold doesn’t follow HR.
4. HR only partially determines the amount of blood being circulated by the heart to the active muscles. Cardiac output = heart rate X stroke volume. Imagine the effects of fatigue and dehydration on HR during a long, hot race! The blood volume drops causing the stroke volume to decrease and the HR to increase in an attempt to compensate.
5. Lactate threshold is a function of how hard the muscles are working, not how fast the heart is beating.

HR may be affected by other things.

1. Cardiac drift is the tendency for the heart rate to rise slowly as you paddle, even at a constant pace. HR can rise by as much as 20 bpm during constant velocity paddling lasting 30 minutes. Hydration may help to stabilize the drift. Drift will cause you to paddle slower even though you have the ability to maintain an even pace.
2. HR is sensitive to environmental conditions and to psychological state. HR is higher when weather is hotter or more humid and when you are tense and irritable.

Should you train by pace or by heart rate? VO2, your oxygen consumption, not heart rate is significant for determining work intensity. The heart is along for the ride and will do what muscles require it to do. The skeletal muscles become fatigued faster than the heart. The heart slows down when the paddling muscles do, not the other way around. Your goal is to develop greater fatigue resistance in the paddling muscles at the desired pace.

Training based on HR makes the heart primary and paddling muscles secondary. Practicing pace rather than HR trains the precise neuromuscular coordination and precise muscle functioning that you need to race. The less you practice pace, the lower the coordination and economy. If you want to race at a goal pace, practice that pace, not a HR.

Conclusions: Use the HR to monitor condition and change, not to control the training program. The HR monitor is a device which collects data and it is not a coach. Information collected by a HR monitor refers specifically to your heart and doesn’t tell you whether your paddling muscles are really ready for your goal racing speed. Only training at race pace can do that.

Remember that your HR is along for the ride. It will do what your exercising muscles tell it to do. If your muscles cannot sustain high intensity work for a long period of time, they will not drive your heart to its limit or its target rate. If you improve the fatigue resistance of your muscles at high speeds by practicing those speeds, then your ability to sustain race pace for longer periods of time will force your heart to work harder to keep up with your muscles.

HR can be a powerful paddling assistant. Get used to how you feel at different work intensities and check the HR monitor occasionally for objective feed back. After some practice, you’ll be able to guess your HR within a few beats. You’ll also be able to feel lactate threshold intensity and sense how hard you can push yourself for the ambient conditions. A high HR may tell you that you are racing too fast, or that you are fatiguing or dehydrating and that you’d better conserve your energy if you want to maintain pace. Or you can learn these warning signs by how you feel. Don’t fall short of your performance goals by letting a misleading heart rate control your training and racing!