A sprain is a stretch or tear of a ligament, the band of connective tissues that joins the end of one bone with another. Sprains are caused by trauma such as a fall or blow to the body that knocks a joint out of position and, in the worst case, ruptures the supporting ligaments. Sprains can range from first degree (minimally stretched ligament) to third degree (a complete tear). Areas of the body most vulnerable to sprains are ankles, knees, and wrists. Signs of a sprain include varying degrees of tenderness or pain; bruising; inflammation; swelling; inability to move a limb or joint; or joint looseness, laxity, or instability.

A strain is a twist, pull, or tear of a muscle or tendon, a cord of tissue connecting muscle to bone. It is an acute, non contact injury that results from overstretching or over contraction. Symptoms of a strain include pain, muscle spasm, and loss of strength. While it's hard to tell the difference between mild and moderate strains, severe strains not treated professionally can cause damage and loss of function.

Because of its complex structure and weight-bearing capacity, the knee is the most commonly injured joint. Each year, more than 5.5 million people visit orthopedic surgeons for knee problems.

Knee injuries can range from mild to severe. Some of the less severe, yet still painful and functionally limiting, knee problems are runner's knee (pain or tenderness close to or under the knee cap at the front or side of the knee), iliotibial band syndrome (pain on the outer side of the knee), and tendinitis, also called tendinosis (marked by degeneration within a tendon, usually where it joins the bone).

More severe injuries include bone bruises or damage to the cartilage or ligaments. There are two types of cartilage in the knee. One is the meniscus, a crescent-shaped disc that absorbs shock between the thigh (femur) and lower leg bones (tibia and fibula). The other is a surface-coating (or articular) cartilage. It covers the ends of the bones where they meet, allowing them to glide against one another. The four major ligaments that support the knee are the anterior cruciate ligament (ACL), the posterior cruciate ligament (PCL), the medial collateral ligament (MCL), and the lateral collateral ligament (LCL). (See diagram.)

Knee injuries can result from a blow to or twist of the knee; from improper landing after a jump; or from running too hard, too much, or without proper warm up.

In many parts of the body, muscles (along with the nerves and blood vessels that run alongside and through them) are enclosed in a "compartment" formed of a tough membrane called fascia. When muscles become swollen, they can fill the compartment to capacity, causing interference with nerves and blood vessels as well as damage to the muscles themselves. The resulting painful condition is referred to as compartment syndrome.

Compartment syndrome may be caused by a one-time traumatic injury (acute compartment syndrome), such as a fractured bone or a hard blow to the thigh, by repeated hard blows (depending upon the sport), or by ongoing overuse (chronic exertional compartment syndrome), which may occur, for example, in long-distance running.

In the United States, about 30 million children and teens participate in some form of organized sports, and more than 3.5 million injuries each year, which cause some loss of time of participation, are experienced by the participants. Almost one-third of all injuries incurred in childhood are sports-related injuries. By far, the most common injuries are sprains and strains.

Obviously, some sports are more dangerous than others. For example, contact sports such as football can be expected to result in a higher number of injuries than a non-contact sport such as swimming. However, all types of sports have a potential for injury, whether from the trauma of contact with other players or from overuse or misuse of a body part.

• More than 3.5 million children ages 14 and under get hurt annually playing sports or participating in recreational activities.
• Although death from a sports injury is rare, the leading cause of death from a sports-related injury is a brain injury.
• Sports and recreational activities contribute to approximately 21 percent of all traumatic brain injuries among American children.
• Almost 50 percent of head injuries sustained in sports or recreational activities occur during bicycling, skateboarding, or skating incidents.
• More than 775,000 children, ages 14 and under, are treated in hospital emergency rooms for sports-related injuries each year. Most of the injuries occurred as a result of falls, being struck by an object, collisions, and overexertion during unorganized or informal sports activities.

• Playground, sports, and bicycle-related injuries occur most often among young children, between the ages of 5 and 9 years old. Bicycle- and sports-related injuries also affect older children, in addition to overexertion.
• The highest rates of injury occur in sports that involve contact and collisions.
• More severe injuries occur during individual sports and recreational activities.
• Most organized sports-related injuries (62 percent) occur during practice.