Sometimes after a not-so-restful night of sleep, my heart rate is slightly elevated when I wake up. I always run in the morning, and on these mornings my heart rate is also higher than normal during my run. I've always been told that the more intense the exercise, the better. So my question is, do two runs of the same time but at different heart rates produce the same changes in my fitness? If one were to consider only heart rate, it would seem that exercising when tired would be better since your heart rate would be higher, and this seems strange. So which should I use to "rate" a workout, heart rate, distance, or a combination of both?

— Siddharth Sawkar, Madison Wisconsin

Athletes have longed used morning pulse to help monitor recovery from training and other stresses, such as a poor night's rest. It's a good habit to acquire. A pulse five to ten beats above normal in the morning could indicate that your body is not quite ready for another high stress day. Rather than run at the same intensity on these days, you would be better off taking it easy that day with a shorter, slower run. Also, keep in mind that heart rate is only one variable (although probably one of the best), and there are many other factors to consider when deciding what you're up for on a given day. Pay attention to how you feel (your perceived exertion during your workout), your attitude, and your general level of life stress.

We can all exercise at a variety of intensities, from putting along to really huffing and puffing, and the body adapts differently to different levels of effort. A nice, easy run at, say, 60-70 percent of your maximum heart rate (MHR), allows you to use fats as a primary source of energy, and helps develop your body's ability to deliver oxygen to muscles by improving circulation. This intensity is important for the endurance athlete as it establishes a strong foundation of fitness. Up the intensity to 80-90 percent of your MHR, and you improve your ability to move quickly for a sustained period of time, you burn more carbohydrates (and calories), and your mechanics of movement change, which develops coordination and power. Exercising at different intensities is the key to developing optimal fitness.

Let's say you are running five days per week. Ideally you'll run two days a week at a lower intensity (60-70 percent of your MHR), two days a week at a relatively high intensity (80-90 percent of your MHR), and one day a week at a moderate intensity (70-80 percent of your MHR). By mixing the intensities, you'll develop a more complete level of fitness and be less likely to get bored with the same old routine. Rate your workouts with a combination of time and intensity, rather than distance. A five mile run can have different meanings depending on the terrain, climate conditions (temperature and humidity), speed, and your attitude. By monitoring these variables, you'll have a good idea of the amount of stress the workout placed on your body. After a high stress workout, you'll want to have an easy day or day off to allow your body to recover, and thereby improve your fitness. By mixing exercise intensities and learning to listen to your body's signals, every workout will be one you'll look forward to and enjoy.