An ACL injury often occurs with a popping sensation or feeling of instability in the knee during sports that involve sudden stops, changes in direction, landing from a jump, or collisions. Risk factors include being female, playing sports like soccer or basketball, poor conditioning, faulty movement patterns, poorly fitted equipment, or playing on artificial turf. Symptoms are swelling, instability, and inability to bear weight on the injured knee.

21. Many people hear a pop or feel a "popping" sensation in the
knee when an ACL injury occurs. Your knee may swell, feel
unstable and become too painful to bear weight.

23. ACL injuries often happen during sports and fitness activities
that can put stress on the knee:
•Suddenly slowing down and changing direction (cutting)
•Pivoting with your foot firmly planted
•Landing awkwardly from a jump
•Stopping suddenly
•Receiving a direct blow to the knee or having a collision, such
as a football tackle

25. There are a number of factors that increase your risk of an ACL injury, including:
•Being female — possibly due to differences in anatomy, muscle strength and
hormonal influences
•Participating in certain sports, such as soccer, football, basketball, gymnastics and
downhill skiing
•Poor conditioning
•Using faulty movement patterns, such as moving the knees inward during a squat
•Wearing footwear that doesn't fit properly
•Using poorly maintained sports equipment, such as ski bindings that aren't
adjusted properly
•Playing on artificial turf