Why Do ACL Tears Occur?

ACL Tears: Why Do They Occur?

The anterior cruciate ligament (ACL) is one of four main ligaments in the knee and provides a significant amount of stability during agility and sports specific movements.

Unfortunately, over 200,000 athletes are affected by ACL tears each year, and even more importantly the ratio of females to males is 9:1.

The most common age for ACL tears to occur ranges from 15-45 y/o with the primary focus being the youth sports population due to the higher demand and intensity of youth sports. (Griffin, 2000 & Chapman, 2001)

70% of all ACL tears are found to be non-contact in nature. This means that 70% of all ACL tears result from a biomechanical abnormality or insufficiency.

They can also result from an athlete performing a sports specific movement incorrectly causing too much stress on the ACL than it can withstand, resulting in a tear.

The other 30% of all ACL tears occur due to a contact force during a practice or game. For example, a contact ACL injury is when a football player tears their ACL by having a linemen tackle them on the side of the knee.

Both non-contact and contact ACL injuries most commonly occur in basketball, soccer, skiing, and football. (Griffin, 2000 & Chapman, 2001)

One of the Biggest Questions Is…Why do ACL Tears Cccur??

Early Specialization Among Youth Sports

Overall, youth sports in America today are completely flawed, secondary to youth athletes specializing in one or two sports too soon.

When early specialization occurs there is a lack of skill development with basic human movements leading to kids in America becoming undeveloped, under-prepared and over-exposed.

Athletes who focus too soon on one sport during the developmental process can cause their bodies to be exposed to inappropriate developmental stressors, therefore leading to injury and poor performance.

Pro Tip: It is highly recommended to encourage all youth athletes to participate in a variety of sports and activities to allow for proper development and well-rounded athletes. “You need to crawl before you walk.”

Knee Valgus Angles

A knee valgus angle is defined by the angle formed at the knee joint when it goes inward utilizing the tibia and femur as the axes “knocked knees”. During all functional and sports specific movements it is ideal to see 5 degrees or less of knee valgus.

When athletes are not trained correctly and/or do not have the proper body awareness during functional and sports specific movements, increased knee valgus angles at the knee are common causing significant amounts of stress on the ACL. (Hewett et al, 2006)

Poor Form and Muscular Activation/Strength with Functional Movements

It is important for all athletes to understand proper form with all functional and sports specific movements and to develop a solid baseline level of strength in all musculature groups important for their given sport.

It is common to see several athletes on a field performing sports specific movements with poor form, naturally increasing stress on the knee.

The lack of cross training for a given sport to develop the proper amount of muscle activation and strength in muscles is very common.

Pro tip: Due to the higher demands and intensity of sports, it is crucial for all athletes to participate in a well-rounded cross training program including balance, strength, coordination, education with verbal and visual feedback etc. This is to make sure each individual athlete understands proper form with all sports specific movements.

Balance and Proprioception Deficits

Balance is defined by the ability for an athlete to maintain stability and control during sports specific and functional movements. Proprioception is the ability for an athlete to understand where the body is in space during a given time both on and off the field.

Both variables are important for every athlete to develop early on to allow for participation in an open environment and demonstrate control and stability with sports specific movements.

If an athlete does not participate in a training program with balance and proprioception as components of the program, they are at greater risk for ACL injuries during maturation. (AAOS, 2007)

Poor Neuromuscular Education and Control

The development of accurate and efficient neurological pathways to muscles during the maturation of a youth athlete is crucial.

The ability of the brain and important muscle groups to communicate allows an athlete to develop proper muscle activation, timing, control, and stability with functional and sports specific movements.

If the proper neuromuscular pathways are not developed and an athlete does not participate in the proper amount of neuromuscular education within their training, they are more at risk for ACL injuries. (AAOS, 20007)

Gender Specific Anatomy

The primary reason for the 9:1 ratio of females to males being more at risk for ACL injuries is due to gender specific anatomy.

The first area to focus on is the q angle.

It is defined by the angle developed when you draw a line from the ASIS of the pelvis (bony landmarks on front of hips) to the mid-line of the patella “kneecap” and a second line from the tibial tuberosity (bony landmark below your kneecap) through the midline of the patella.

Women by nature have an average Q angle of 11-15 degrees which is 4-5 degrees greater than males. A larger average q angle for women compared to men is related to having a wider pelvis meant for childbirth.

A wider pelvis will naturally cause larger knee valgus angles leading to increased risk of ACL tears. (Griffin, 2000 & Chapman, 2001)

Secondly, common trends among women compared to men tend to lead to significant quad dominance, weakness of hip extensors “glutes” and over pronation of feet “flat feet” during functional movement causing increased stress on the knee.

Lastly, female hormones during their monthly menstrual cycle have been shown to lead to ligamental laxity putting women more at risk for ACL injuries. (Griffin, 2000, Chapman, 2001, & Hewett, 2006)


Encourage all youth athletes to participate in well-rounded training programs and ACL injury prevention programs to ideally allow an athlete to develop and mature properly, prevent injury, and maximize sports performance.

Brittany Lillie PT, DPT, CSCS
Brittany Lillie


  1. Chapman MW. Chapmans Orthopaedic Surgery. 3rd Edition, Volume 3, 2001; 2348-2388
  2. Grifffin LY. Noncontact Anterior Cruciate Ligament Injuries: Risk Factors and Prevention Strategies. Journal of the American Academy of Orthopaedic Surgeons. 2000; 8: 141-150.
  1. Hewett TE, Myer GD, Ford KR. Anterior cruciate ligament injuries in female athletes: Part 1, mechanisms and risk factors. American Journal of Sports Medicine. 2006 Feb; 34(2):299-311. Hardaker W
  1. American Academy of Orthopaedic Surgeons, July 2007, Anterior Cruciate Ligament Injury: Surgical Considerations, http://orthoinfo.aaos.org/topic.cfm?topic=A00297#A00297_R4_anchor (July 11, 2008).

Develop Well-Rounded Athletes by Preparing Them To Perform

Dr. Lillie references the importance of developing well-rounded athletes, which is why it is so important that your athletes are Prepared to Perform!

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