Have you started to dread working out? Not the normal, "I don't feel like it today," but a deeper, "I can't do it" signal coming from your muscles themselves? While most would blame it on laziness, you may be pushing yourself too far; it happens more than one might think. So often, in fact, that it actually has a name: overtraining. "Exercise is like taking two steps forward, and rest is one step back," says C.C. Cunningham, owner of PerformENHANCE Sport and Adventure Athlete Training in Chicago and a spokesperson for the American Council on Exercise. "In order to take the next step forward, you need to take one step back with rest. If you overtrain, you never get past step one and your workouts become hard."

The irony of exercise is that while it's so good for your body it actually causes damage to the muscles. Not damage in the conventional negative sense but a "good" kind of damage that the body adapts to, overcomes, and gets stronger because of. Namely, when weight training, you cause small tears—known as microtears—in the muscle, which stimulate muscles to adapt both in size and chemistry. During aerobic exercise, another kind of damage occurs, because the muscles utilize oxygen to make energy from fuels like glucose and glycogen. When you're exercising extra hard or for a long time, the body goes into anaerobic cell metabolism, where the muscles use only glucose for energy and produce lactic acid. This buildup of lactic acid causes your muscles to feel fatigued. Between workouts, your muscles clear out the lactic acid and rebuild themselves and their glucose and glycogen stores. The effects of overtraining manifest themselves when you don't give your body enough R and R between workouts. The next time you exercise, your muscles' energy levels won't be refurbished and your body won't be ready to handle the "good" damage. The more run-down your body becomes, the less you'll gain from your training activity. Workouts that were once easy become hard, because your body simply lacks the energy to perform.

As for the exact cause of overtraining, some researchers attribute it to a low energy supply to the muscles, which forces the body to discourage more exercise until stores can be replenished. Others blame free radicals—groups of atoms that build up during endurance training and damage muscle cells. When you start accumulating a lot of free radicals, you may start damaging tissue faster than you can repair it—thus your power, speed, and endurance may decline.

So, how much is too much? While there is no specific quantity of exercise that will induce overtraining, "It would be just as normal for a marathoner who usually runs ten miles a day to overtrain on fifteen miles as it would for a sprinter to overtrain after running straight through for an hour," says Cunn...

A lot of people have the idea that running on a treadmill is a lesser form of exercise than running outside. It's not. Running on a treadmill is of equal value in terms of aerobic fitness as running on the roads, trails, or tracks. That's your biggest question answered, isn't it? As long as you keep your heart rate in the training range—70 to 85 percent of your estimated maximum heart rate (MHR equals 220 minus your age)—your heart will work just as hard and you'll burn a similar number of calories. A 30-minute run is a 30-minute run, no matter where you do it.

While running outdoors offers changing scenery and terrain and fresh air, running indoors has its advantages. It sure is more pleasant to run indoors when it's cold and icy, hot and humid, or raining outside. It's also safer: You don't have to worry about being hit by a car or getting attacked on a dark street if you're running alone. Treadmills not only wipe out a lot of excuses for skipping runs but also offer options for getting more out of those runs.

Convenience Treadmills are the most popular cardiovascular machines at many clubs. They make it possible for friends who have different speeds to enjoy a side-by-side run. For home use you can purchase machines that are more than adequate for less than $1,000, or you can get luxury models that cost more than $4,000. I had a $700 ProForm treadmill that folded up, which would be a good choice for starters. Now I have a True high-end model with lots of gadgets, which is great. Try the treadmill out before you buy it; as long as it has adequate cushioning and meets your needs in terms of pace and incline, it will work well for you. If you're a slow fitness runner, you won't need a fast-paced machine with a steep incline. But if you're a running pro, you can buy a treadmill with more incline and greater speed. Don't be tempted by cheap nonmotorized machines—most don't have proper cushioning and strain the legs due to the sliding action. Add a basic set of free weights, and you'll save enough money in club dues to make the investment worthwhile quickly—especially if more than one person will be using it.

Consistency Running on a treadmill is different from outdoor running. You can set a consistent pace per mile on a treadmill—which will keep you from running too fast or too slow—though the speeds per mile indicated on computerized machines are often inaccurate. As long as you're exercising at the same perceived exertion level as you do when you're outside and you keep your heart rate in your training range, your exact pace isn't important.

Variation

You can add hills at the touch of a button, and—just as important—you can make the hills go away just as fast! Adding hills to your run works different muscles; flat running is powered mostly by the hamstrings, while hills require the quadriceps to do m...