In a collaborative effort with Scott over at Straight to the Bar, we will be writing all about rest for the month of October. You can expect a new article on the matter every week.
Rest between workouts is one of the most important factors in determining whether or not you get injured and how well you perform on any given day. Too much rest can sometimes be just as bad as too little. When you workout, you tear muscle fibers. Rest allows your body to rebuild the muscle fibers. Given enough rest, and the muscle fibers will be stronger than the ones that you tore. Given too much rest, and your body will decide that it does not need the extra muscle mass and will begin to break the muscle fibers down again. Muscle is very metabolically expensive; in other words, your body needs to do a lot more work to keep muscle mass on your body than to keep fat on your body. If you are not using that muscle mass, then it will try to cut corners and get rid of it.
A common workout schedule includes 2 or 3 strenuous workouts per week, 2 or 3 workouts that are somewhat easier, and the rest of the week is rest days. You do not want every single workout to be as strenuous as possible, because taking a day off between workouts will leave you a stiff and will usually result in either over-training or under-training. If you take 1 day of rest between each strenuous workout, then you will tear up more muscle than your body can rebuild and you will get hurt. If you only do strenuous workouts but you take 3 or 4 rest days between each workout, then you will not train your body to run at the level that you want and you will not only be under trained but will be at risk of pulling a muscle by working it too hard.
Easy workouts allow you to remind your body that you want to keep the muscle mass, but do not cause damage that can not be easily recovered from. They increase the blood flow and can loosen your legs up for later workouts and make it easier to keep your muscles loose.
More competitive athletes put their bodies on a slightly different schedule. They will usually workout two or even three times per day. Usually, the morning workout will be very easy and its only purpose is to loosen up the muscles and prepare them for an afternoon workout. The afternoon workout will often (but not always) fluctuate between strenuous workouts and medium workouts that can not be termed easy but are not as hard as the strenuous workouts.
Cross training, or doing a workout in a different sport than the one that you compete in, can allow you to work different muscle groups on your rest days. You get the cardivascular benefits of working out, and you help remind your body that you want to keep the muscle mass you have been trying to grow stronger, but the actual stresses applied to the muscles are to different ones than from the last workout. This can also be accomplished by doing the same types of workouts in different ways.
For example, a runner might cross train by lifting weights, riding a bike, or going for a swim. He might mix up how he works his muscles by doing speed training with intervals one day (fast twitch muscles), and going for a long run the next day (slow twitch muscles).
A weight lifter may work different muscles on different days or workouts. A bench press tends to work muscles in the chest and arm, for example, while a squat will work muscles in the back and legs.
To realize any gains in physical fitness and competitive sports, you have to work at it and practice. Muscle does not grow itself. However, you can sabotage any efforts at gaining that muscle mass by not giving it a chance to heal itself. By not resting enough, you can see some very quick gains but can also see even quicker plateaus and declines in your performance. It can be a real balancing act to try to do the right amount of work and take the right amount of rest to see the largest gains possible, especially when you need to time those gains for a specific race or other event.