Anterior Cruciate Ligament (ACL) injury | Symptoms & Causes

What are the symptoms of ACL injuries?

The typical symptoms for an anterior cruciate ligament (ACL) injury include:

  • hearing a "popping" noise
  • feeling the knee giving out
  • sudden instability
  • pain or discomfort
  • swelling within 24 hours
  • loss of full range of motion
  • joint tenderness

normal acl vs torn acl

What causes an ACL injury? 

The ACL is typically injured during sports participation. Sports that involve frequent cutting and twisting motions, such as football, soccer, basketball, lacrosse and gymnastics, have relatively high rates of ACL injury. The ACL can be torn when a player stops quickly, changes direction rapidly, lands after a jump or collides with other players.

Teens are the most athletically active age group and have the highest risk of ACL injuries, and girls are five to eight times more likely than boys to tear their ACLs for several possible reasons:

  • They tend to put more stress on their ligaments, compared to their muscles.
  • At puberty, boys grow in height and develop their lower leg muscles at the same time. When girls grow, they don’t tend to develop lower leg strength unless they train for it.
  • Girls tend to land with a straighter knee than boys, reducing the effectiveness of their muscles as shock absorbers.
  • Girls often let their knees drop inward during cutting/pivoting/landing maneuvers, which can put larger stresses on the ACL.
  • Girls tend to have more strength in their quadriceps muscles than their hamstring muscles, placing more stress on the ACL.