Performed properly exercise improves physical condition and health. Done poorly is can cause injury and harm health.

Acute (Sudden) Injuries:

These are injuries that you know about right away because they hurt or harm you right away. A good example is an ankle sprain. A list of common acute injuries is given below:

1.     Muscle soreness: In order to build-up muscle, to increase the amount of muscle we have, we have to injure it slightly through exercise. This leads to muscle soreness and especially to delayed muscle soreness (it hurts more the day AFTER than the day OF the exercise).

a.     Muscle soreness and delayed muscle soreness result from resistance training.

b.    The best way to minimize these problems is to start your resistance training program with light weights and then build-up the weight slowly over many weeks and months

2.     Blisters: Hardly a medical emergency, blisters are painful and commonly result from skin irritation caused by actions like gripping a bicycle handlebar on a long ride or walking (or running) too far in new or poorly-fitted shoes or boots.

3.     Ligament strain: Perhaps the most common exercise-caused injury, ligament strains occur when a tendon becomes overstretched but does not tear. This sort of injury commonly occurs in the ankles, sometimes in the wrists or arms but ultimately can occur anywhere a tendon is overstretched.

4.     Ligament Sprain: This is a step worse than strain. In a sprain, a ligament becomes partially or completely torn.

5.     Tendon Rupture: Worse still is the sudden rupture of tendon. This is where a tendon is torn completely so that there is no longer any connection between a muscle and a bone.

6.     Bone Fracture: A bone will fracture if it is stressed too much. This rarely happens in exercise except when people have accidents like skiing into a tree or dropping a weight on a foot. However, people with osteoporosis (bone thinning and loss) can sometimes fracture a bone performing ordinary tasks.

a.     Ligament strain, sprain, tendon rupture and bone fracture all generally result from exercise related accidents or from trying to perform feats far inexcess of one's physical condition.

b.    The best way to avoid these injuries is to exercise carefully and quit when you are tired. Most exercise accidents occur when people are exhausted.

7.     Heat Injury: Exercising outdoors in hot weather carries special risks. Sometimes we can heat-up faster than the body can manage. This is called "heat-stroke" and it can be deadly. Signs of heat stroke are dizziness, confusion, trouble walking. This is an emergency that requires immediate cooling by getting out of the sun, into shade or a cool building, lying down and splashing cold water on the body.

a.     Heat injury can occur with any exercise if performed in hot weather and especially when people are not "heat-acclimatized".

b.    Summertime exercise should be performed early in the morning or near sunset. Warm-up slowly and carefully monitor your own condition being especially observant of mental-status changes like dizziness, confusion, cloudy or muddled thinking or trouble walking. Stop exercising immediately and seek shade and help if you begin to show these symptoms.

8.     Dehydration: Like heat injury, dehydration tends to happen more often in hot weather, but unlike heat injury, it can also happen in very cold weather. Dehydration is the condition of having too little water in the body and occurs when we lose more water than we replace. In hot weather, we lose water by sweating but we also lose water through breathing and, of course, urinating. In any event, when we lose more than a few pounds (less than half of a gallon) of water, we become mentally and physically impaired. The way to prevent dehydration is to drink ample fluids before, during and after exercise. Sometimes drinking water is fine, other times, weight issues be damned, Gatorade or a similar sports drink is a wiser choice.

a.     Dehydration is a potential risk from any exercise any season, hot or cold.

b.    Drink plenty of fluid before, during and after exercise. Warning signs of dehydration include absent urination or urine that is highly concentrated and dark in color, dizziness, headache, nausea and vomiting. The only treatment for dehydration is rehydration with water or sports drinks like Gatorade.

9.     Hyponatremia (Low Sodium): If people drink too much water, they can dangerously lower their blood-sodium level. This can happen when marathon runners load up on water and keep drinking only water during a hot run. It can happen even absent exercise if people drink huge amounts of water. You may recall that a few years ago a woman died from drinking too much water in order to win a  radio station-sponsored contest in Sacramento, California.

a.     Hyponatremia happens when people over-consume water without salt. This happens when people exercise for a long time and only drink water.

b.    When performing any exercise for longer than about one hour you should consume electrolyte-replacement drinks like gatorade or balanced salt solution and not just pure water.

c.     You can make a pretty good and very cheap electrolyte replacement drink using one teaspoon salt, and two tablespoons sugar mixed into a quart of water. This will be faintly sweet and distinctly salty but it really works.

10.  Acute Asthma (Asthma Attack): Intense aerobic exercise causes rapid and deep breathing in order to keep-up with oxygen demand. This can irritate the airways of the lungs and, in some people, can trigger an "attack" of asthma. Patients who have a history of exercise-induced asthma may need to use an "inhaler" immediately before exercise

11.  Cardiovascular Accidents: In the long-term, exercise helps prevent the sort of heart disease that causes "myocardial infarction" or heart attack, but, in the short term, and in the case of people with existing heart disease, sudden intense exercise can work a sick or damaged heart so hard that

12.  Other injuries: Unless you exercise in a rubber room, there is always a chance of becoming seriously injured from traffic, crime, insects and many other "environmental" factors

Slow (Chronic) Injuries:

These are injuries that gradually get worse each time one exercises. An example is inflammation of the Achilles tendon caused (sometimes) by running. At first there is only vague pain. After a week or two the pain is worse and unless the sufferer stops running, a few weeks later, the pain is unbearable.

1.     Tendonitis

2.     Plantar fasciitis

3.     Sun damage

4.     Bone Spurs

Heart Safety:

Exercise works the heart muscle. Over time, this causes the heart muscle to become stronger and healthier, but in the beginning, especially in the case of someone with existing heart disease, sudden intense exercise can harm the heart by depriving the muscle of needed oxygen or by causing an irregular heart beat (arrhythmia).

Anaerobic Exercise Intensity: Time to Pain

The best way to judge the intensity of weight training is by how long it takes, by seconds or by "reps" to reach unbearable pain.