Thunder's Dad is 100% correct. Every time a pitcher lands on the stride leg/foot, she places 3 to 3 and 1/2 times her body weight (measured in force) on that leg/knee. If the knee is at 90%, imagine the force that is being absorbed by that knee, in a position that it is not designed to flex in. 45% allows the entire leg to abosrb the force of the landing, and flex like a shock abosrber. The other issue to consider is that anyone landing at 90 is almost certainly completely opening her hips by over-rotating the drive foot/hip. Most pitchers who do this, cannot engage the back side throughout the motion, thus compromising posture at release. Go 2 Go 2, I've never seen your daughter pitch, but it's possible that the 45% landing helped her not over-rotate to open, allowing her to keep the backside active throughout the motion. One of the biggest issues with over-rotating and re-planting is that the pitcher ends up with a foward lean at release, putting a lot of strain and pressure on the lower back and shoulder. Not saying your DD re-plants, but check and see if her posture at release is any different with the 45% landing; ie more upright as opposed to slightly leaning forward.