Percentage-wise, the greatest gains in fitness come in the first few weeks and months of any exercise program—and the less fit you are, the greater the relative improvement will be. People who have undergone operations, for instance, often improve their fitness by up to 300 percent in the first month or two of a rehabilitation program. (A trained athlete, on the other hand, would be happy with an improvement of just a few percent.)
2. Being very tired after exercise is a sign that you've had a "really good workout."
In fact, it's a sign that you're pushing yourself too hard. Except for the first couple of weeks of a new exercise program (when the unaccustomed stress may leave you more fatigued than usual), your workouts should leave you feeling invigorated, not worn out. One good rule of thumb: You should finish each workout with the feeling that you could have easily gone a bit longer. An especially bad sign is if you're still tired a day or two after a workout. In that case you're probably over-training, and need to reduce the length and intensity of your exercise sessions.
3. When your muscles are sore, it means they're getting stronger.
What it really means is that you've damaged and torn the microscopic connective tissue that surrounds your muscle fiber. There is also evidence that this soreness may be due to damage to the muscle fibers themselves. "Delayed muscle soreness" peaks between 24 to 48 hours after an offending workout, and then generally disappears on its own. (Apply ice to the sore areas for 20 minutes, several times a day.)
Muscle soreness is usually caused by movements that require a muscle to contract as it's also lengthening. Examples: Lowering a heavy weight to the ground, or using your leg muscle to break your progress during a game of basketball or tennis. If you plan to participate in an activity that requires these sorts of "eccentric contractions," build up slowly over a period of several weeks to give your muscles and connective tissue time to adjust.
4. Heavy people have a harder time burning calories when they're exercising.
In fact, the more you weigh, the more calories per minute you will burn when you're doing weight bearing exercises. It's a simple law of physics — you're doing more work, because you're propelling a greater mass (yourself). For example, someone who weighs 110 pounds will burn about 5 calories per minute during a leisurely 10 mph bike ride; the 220-pounder riding alongside is burning twice that amount — an impressive 10 calories per minute!
5. People who are out of shape sweat more than really fit people.
The truth is exactly the reverse: One of the natural changes that occur as you begin a new exercise program is that your perspiration rate goes up. That's because you're producing heat as you exercise. Sweat (a combination of water, salt and trace amounts of iron) helps cool your body and prevent you from becoming overheated. When your body temperature rises, small blood vessels in your skin widen and draw heated blood to the surface. You also begin to sweat from pores in your skin. As the sweat evaporates, it draws heat away from the superficial blood vessels. The cooler blood then recirculates throughout the body lowering internal body temperature. Faced with the challenge of dissipating this new heat on a daily basis, your body quickly becomes more efficient at cooling itself by increasing the output of your sweat glands.