Re: Weighted box jumps and ACL strain
Jumping can put a strain on the ACL especially if you have suffered a previous knee injury and/or your mechanics are wrong. The risk areas are taking off and landing. The risk from taking off is small and can essentially be eliminated by strengthening the leg muscles and practicing sound jumping mechanics. 99% of jumping-related knee injuries are the landing. The landing impact causes a lot of force on all the leg joints; somewhere between x3 to x5 times your body weight.
Now here's why box jumps are so good for the knees. You still gain the benefits of jumping exercises but minimize the risk of injury. If you jump 24 inches up then you land with the force of a 24 inch landing. With a 20 inch box jump (for example), you're only landing with a 4 inch landing and greatly reducing the impact portion of the jump.
Hope that helps.