If you have been around the game of soccer for very long, chances are you know someone who has torn their anterior cruciate ligament (ACL).

A torn ACL has become the dreaded injury in many sports -- soccer included. ACL injuries are common, with more than 200,000 ACL injuries occurring each year.

Costly surgery and a long rehabilitation process make this injury daunting. The cost of surgery and rehabilitation alone is estimated to be between $17,000 and $25,000. Non-monetary costs including loss of play time, possible loss of a scholarship, potential depression, and a decline in academic performance are steep as well.

Those who suffer an ACL tear are also 10 times more likely to develop osteoarthritis later in life.

Other debilitating injuries that soccer players frequently experience include muscle strains, other knee injuries, and ankle sprains. In fact, ankle sprains are the most common injury in soccer.

A number of factors can put an athlete at higher risk for an injury. Understanding these risk factors can help you avoid injury and continue competing.

External Risk Factors

External risk factors that contribute to athletic injuries include footwear and weather conditions.

Although improved traction from footwear helps performance, it also can increase the risk of injury by keeping your foot fixed to the ground in one direction while your body continues to move in another direction.

Similarly, playing on a synthetic surface versus a natural surface can increase your risk due to the increased traction.

Weather conditions are another external risk factor. In a study that looked at Australian football, ACL injuries were more common in conditions of low rainfall and high evaporation.

Internal Risk Factors

Internal risk factors deal with anatomical and hormonal changes that increase the risk of injury. Female soccer players are twice as likely to tear their ACL as their male counterparts -- and this probably has to do with anatomical and hormonal differences.

The adolescent years are particularly dangerous for females as evidenced by the fact that most ACL reconstructions in females occur during the 15-19 year age range.

Females have a smaller intercondylar notch  -- the groove in the bone through which the ACL runs -- which is thought to place females at a higher risk of injury than males and possibly play a role in the mechanism of tearing the ACL.

Females also have a wider pelvis, which changes the forces on the knee and may also contribute to the increased likelihood of ACL tears.

Hormones are also known to play a significant role in the biology and physiology of ligaments, muscle, and bone. Changes in hormone levels during the menstrual cycle may affect the strength of the ACL and predispose women to higher injury rates.

Neuromuscular Risk Factors

Neuromuscular risk factors have to do with strength and also with the body's ability to use strength effectively through proper mechanics of movement. Balance and body awareness are also related to neuromuscular risk factors.

During puberty, girls have a rapid increase in size and weight without increased neuromuscular power and control, which increases the likelihood of injury. Boys, on the other hand, have an increase in strength and power along with their increase in size and weight during puberty.

Straight Knees

After puberty, women tend to jump and land with straighter knees, allowing their knees to cave in as they land. This knee position increases your risk for lower extremity injury because the knees don't bend to absorb the weight of the body. Bending the knees while keeping them aligned will minimize knee injury risk.

Quadriceps Balance

Another difference in neuromuscular function is that girls have more quadriceps-dominance than boys. Girls have a greater imbalance between their quads and hamstrings than boys, and they also tend to activate their quadriceps earlier than their male counterparts.

When the quadriceps contract without a simultaneous hamstring contraction, it pulls the tibia (lower leg) forward which can shear the ACL. Therefore hamstring strength and earlier hamstring contraction is crucial in decreasing the risk for injury.

Contact Injuries

An additional concept that is important to consider is the mechanism of injury for ACL tears. Although many tears occur due to direct contact -- such as a direct blow to the knee -- 70 percent of all ACL tears happen without any contact at all.

Non-contact injuries typically occur during landing, deceleration, cutting, or hyperextension.

Injuries can also be caused by pseudo non-contact. Pseudo non-contact injuries are when there is an absence of a direct blow to the knee; however, contact from another player alters the player’s response and results in a “non-contact” injury as described above.

Knowing your risk factors for a sports-related injury can help you understand how to avoid serious injury and highlights the importance of developing a strength-training program to protect yourself from injury.