A new study shows that the amount of preparatory muscle action in the muscles spanning the knee joint prior to landings is associated with knee positions that are considered at risk for ACL rupture, said Riann Palmieri-Smith, lead author and assistant professor at the University of Michigan Division of Kinesiology.

The ACL is one of the four major ligaments of the knee, and women are 2-8 times more likely to tear this ligament than men are while playing the same sport, said Palmieri-Smith.

The U-M research suggests that training programs which promote balanced activity of the inner (medial) and outer (lateral) thigh muscles might help protect the ACL.

This preparatory muscle action helps to control the relationship of the shank relative to the thigh. When the shin bone is positioned outward compared to the thigh bone, it results in a knock-kneed posture, Palmieri-Smith said. This position is referred to as knee valgus, and increased knee valgus (more knock kneed) has been shown to be linked to ACL injury risk, said Palmieri-Smith, who is also affiliated with U-M's new Sport Injury Prevention Center.

Think of a person who jumps and lands with knock knees (knees turned in) as opposed to a person who lands with minimal or no side-to-side angle at the knee (thigh and shank aligned). The person who lands with their knees turned in too much appears, based on previous research, to be more likely to tear their ACL.

The U-M study showed that when women are preparing to land from a jump they tend to activate the muscles of the outer (lateral) thigh more than the muscles of their inner (medial) thigh, and that this pattern of muscle activity was associated with a larger valgus knee angle, Palmieri-Smith said. The results of the study are important because as scientists establish a relationship between preparatory muscle activity and at risk ACL knee positions, they may be able to develop training regimens to help train muscles to behave differently and better support the knees. "The question is can we train these muscles to be more balanced, to really lower valgus knee angle" We want to restore lateral and medial balance," Palmieri-Smith said.

ACL injuries have long lasting and damaging affects in both the short and long term, Palmieri-Smith said. Evidence suggests that up to 70 percent of successfully treated ACL tears will develop osteoarthritis within five to 14 years.

In the study, researchers fitted 11 women and 10 men with electrodes to monitor muscle activity and markers to monitor knee joint position while they jumped on a force platform that collected ground reaction force data.

Note: If no author is given, the source is cited instead.

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