Don’t Worry
If your knees cave when you jump, you likely aren’t one jump away from tearing an ACL. However, it’s important to understand the differences between eccentric and concentric knee valgus and what each one means…
Concentric knee valgus
Concentric knee valgus in terms of jumping would be when your knees come together as you load to jump. When your knees come in, it helps create tension in the muscles of the hip and creates an effect similar to loading a slingshot. This knee triangle creates a power position and is a result of the body naturally organizing itself in the most efficient manner to create force quickly. If you watch a basketball game, you will see NBA players doing this a hundred times a night. If you watch a powerlifting meet, you will see some athletes do this as they come up out of their squat. If you watch weightlifting, you will see the same thing as lifters come up after the catch of their clean or snatch.
HOWEVER, every person is different so if you don’t naturally do this, don’t start doing it artificially. Intentionally manipulating your body’s natural mechanics could lead to injury.
Eccentric knee valgus
Eccentric knee valgus is when your knees cave in on the way down (landing of a jump, down portion of a squat, etc). Eccentric action in sports mostly involves deceleration or absorbing force in some way. A common mechanism of injury is when a force is greater than what the tissue can absorb at that moment. This is why knee cave in this scenario could potentially be more concerning, but there is a lot of context needed.
Knee valgus itself isn’t going to cause an injury, it is when the force that has to be absorbed at a specific angle of valgus is greater than what the tissue can handle at that moment. Therefore if your knees valgus occurs as a result of weakness, that could be a problem. This is why it is important to include plyometrics and single leg strength exercises in your training to help the body become accustomed to the demands it will face.
Knee valgus itself isn't a predictor of injury. Dr. Jason Avedesian is currently the director of sports science at Clemson University and is one of the leading researchers on ACL injury. He is a great source for further information on the topic.
Sample Workout
Here is a sample lower body workout intended to prepare the body for landing and changing direction to hopefully mitigate the risk of ACL injury:
Dynamic Warmup 5-10 minutes
Plyometric block: complete 2 sets
Single leg lateral & medial hops 6 each direction on each leg
90 degree bounds 5 each side
Depth drop 3 reps (height depends on size and ability of the athlete. Start small and gradually work up in height)
Strength block 1: complete 3 sets
Split squat (using dumbbells, barbell, or safety bar) 6 reps each leg
Single leg RDL 8-10 reps each leg
Strength block 2: complete 3 sets
Lateral Lunge 6 reps each leg
Single leg step down 8-10 reps each leg
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