As a dynamic, high speed game where physical contact occurs both incidentally and deliberately, soccer creates many circumstances where injury may result. Most soccer injuries are relatively minor in terms of the degree of disability created; more serious injuries often result through the execution of a hard sliding tackle or other sudden physical collisions between players. Data from researchers regarding soccer injuries indicates that there are over 150,000 soccer injuries reported annually in the United States, among a playing population of over three million athletes; approximately 45% of these injuries occur in players under the age of 15 years.
As would be expected in a sport that centers on kicking a ball, injuries to the lower legs are the most common injuries in soccer. Ankle sprains are another common occurrence, often created by either an awkward plant of one of the feet while running or changing direction, or by stepping on another player's foot, causing the ankle to twist forcefully. Most soccer players wear a cleat that is low cut to permit greater maneuverability, and this footwear is not naturally supportive of the ankle.
The Achilles tendon is vulnerable to two kinds of injury. Given the explosive movement required of a soccer player, the Achilles must instantly respond to the impulses of musculoskeletal movement. If the Achilles tendon is imbalanced in terms of either its strength relative to the connected muscles of the calf, or if the tendon is not sufficiently flexible, the fibers of the tendon can become overstretched or subjected to micro tears. The second type of injury to the Achilles results from the tendon being kicked from behind by an opposing player. The resulting trauma can significantly damage the tendon fibers.
Soccer players are subjected to numerous varieties of accidental kicks from an opponent in the course of play. Most of these kicks result only in contusions, as the players wear relatively durable shin guards. More serious injuries to the lower leg may occur as a result of a defender's sliding tackle, where the defender slides forcefully along the turf to strip the ball from an opponent. If the tackle is not executed cleanly, the offensive player's leg may be caught and twisted, the mechanics necessary for either a significant ankle sprain or a fracture of the tibia/fibula bones in the lower shin.
The knee can also be injured by a sliding tackle, if the offensive player's leg is planted on impact and the knee joint is forced laterally (sideways); this type of collision prevents any of the force of impact being directed and absorbed anywhere but the knee joint. In such circumstances, the anterior cruciate ligament (ACL), a large connective tissue between the femur and the tibia in the knee joint, is at the greatest risk of injury. Other knee injuries occur in the same fashion as ankle injuries, where the leg is planted forcefully on an uneven surface, and the ultimate stress radiates directly into the knee.
Thigh injuries in soccer are typically one of two types. The first are contusions, as the thigh is exposed to all manner of physical contact in the course of a game. The second type of injuries are those common to all other running sports, muscle strains and pulls caused by repetitive and often explosive acceleration. Soccer players who have an imbalance in the function of the hamstring, which provides flexion to the knee, and that of the quadriceps, which gives the knee its ability to extend, will often experience injuries to these muscle and tendon groups.
Groin injuries are often the bane of the high-level soccer player. The structure of muscles, tendons, and ligaments in the upper thighs and the lower abdomen is complex; these tissues are also vulnerable to injury in soccer due to the almost constant lateral and stop and start movements that place stress on them. The abdominal injury that has attracted attention throughout the sports world that is popularly called a sports hernia is, in fact, a tear of the groin inguinal hernia, first identified among English professional soccer players in 1980. Such injuries require surgical repair.
Other than contusions, injuries to the upper body in soccer are less common. The collisions in the sport will occasionally cause a shoulder separation, which is damage to the acrimoclavical (AC) joint, the connection between the shoulder blade and the collarbone. Soccer goalies are more exposed to shoulder injury as a result of diving across the crease to make saves and striking the goal post.
Head injuries may occasionally arise due to collisions with opponents—concussion and damage to the player's teeth are the greatest risk. Many players wear mouth guards to protect their teeth, which has the additional benefit of reducing the effect of concussions by keeping the tempomandibular joint (TMJ) from being driven upward into the skull. Since the mid-1990s, there has been controversy in the international sports science community as to whether the repeated heading of a soccer ball will cause damage to the brain or to the muscles and structure of the neck. Various studies initiated by soccer nations have not yet resolved this question.