Exercising without proper safety precautions often leads to injury. Muscle overuse and repetitive strain injuries may be prevented by

• Scheduling strength training workouts 48 hours apart to allow muscles to recover
• Keeping workouts varied in regard to exercise method and equipment choices

People should stop exercising immediately if they feel pain other than the usual mild burning sensation in muscles caused by lactate buildup.

Two types of muscle discomfort may be felt after exercise. The desirable or expected type, delayed-onset muscle soreness, does not start until several hours after exercising intensely. Usually it peaks within 48 hours, goes away within 72 hours, and feels better after the warm-up for the next workout. The undesirable type, in which pain indicates injury, is usually felt soon after injury occurs, may not disappear within 72 hours, and may become much more severe over time and if a person continues to exercise.

Warming up: Warming up is meant literally. Warm muscles are more pliable and less likely to tear than cold muscles, which contract sluggishly. Starting exercise at lower intensity (for example, walking rather than running or using lighter weights) raises the temperature of muscles by increasing blood flow. Therefore, warming up helps prevent injuries. Activities that do not raise muscle temperature do not provide such benefit. Warm-up activities also help prepare the mind for more intense activity, thus increasing confidence and motivation and improving the mind-muscle link for higher quality activity.

Did You Know... During exercise, people should drink about 1/2 to 1 cup (120 to 240 milliliters) every 15 to 20 minutes, even if they are not thirsty.

Cooling down: Slowing down gradually (cooling down) at the end of exercise helps prevent light-headedness. When the leg muscles relax, blood collects (pools) in the veins near them. To return the blood toward the heart, the leg muscles must contract. When exercise is suddenly stopped, blood pools in the legs and not enough blood goes to the brain, causing light-headedness. By preventing blood from pooling, cooling down also helps the bloodstream speed up its removal of lactic acid, a natural chemical product that builds up in the muscles during exercise and that can unduly strain the body if it accumulates.

Hydration: Proper hydration is important, particularly when exertion is prolonged or occurs in a hot environment. People should be well-hydrated before activity, drink fluids regularly during extended exertion, and continue to drink fluids after activity. During exercise, people should drink about 1/2 to 1 cup (120 to 240 milliliters) every 15 to 20 minutes, depending on heat and exertion level. They should drink these amounts regardless of thirst, which may be absent or minimal sometimes even when a person is dehydrated. Another way to estimate the amount of fluids needed is to subtract body weight after exercise from body weight before exercise. Then, about 2 cups of fluid is consumed per pound lost (about 1 liter for each kilogram lost).

However, people should avoid drinking too much water because overhydration can cause the level of salt in the blood to fall too low (a condition called hyponatremia—see Electrolyte Balance: Hyponatremia) and lead to nausea or even seizures. Overhydration and hyponatremia are usually only a problem for people who engage in prolonged outdoor exertion (for example, long-distance runners or sport teams playing long games).

To replace lost fluids, plain water is usually fine unless very large amounts are necessary (several quarts, for example). Such a large fluid requirement is unusual unless a person exercises outdoors in high heat and humidity. In this case, electrolyte-containing sports drinks may be preferred. However, if the sports drink has a lot of sugar (more than 8% glucose), it should be mixed with plain water at a 50:50 ratio so that the fluids are not absorbed too slowly.

Last full review/revision November 2012 by Brian D. Johnston