You hurt less and the swelling subsides. Aspirin or aspirin substitutes, such as ibuprofen or acetaminophen, inhibit the production of the chemicals. This, in turn, relieves pain and swelling. Sometimes too much rest can be hazardous. If you've strained your back, more than a day or two in bed will only weaken your muscles and make you more liable to injury once you've arisen.
Injured joints are particularly vulnerable to ill effects of long periods of rest.
Strength and flexibility are inevitably lost during the acute stage of an injury, when you hurt too much to move. Weakness and rigidity most likely contribute to getting injured in the first place and will most likely do the same thing again. So full recovery requires specific exercises to strengthen your supporting muscles and to improve your range of motion. Aerobic exercise may be suggested to improve circulation, decrease pain, relax tension and generally cheer you up. Poor posture may also be an issue, especially with neck or back pain.