I also have written more about the two types of knee extensions for knee injury and injuries.
I'm going to explain which products to buy for more agility. The first is a "flexed" knee flexion. This is a good "soft" foam product to purchase because the "softness" is less than the softness of a foam knee extension (KF). I recommend Flex KF as a soft, cheap product. I bought some of these from Amazon. I like that it is foam, which is cheaper than a KF product. It also has more flexibility than the KF. I bought my Flex KF on the 5/17/2015. The second product is a "hard" knee extension. You may be able to use a soft KF knee extension with this product, but this is a hard knee extension and it may not be as flexible as the Soft KF. Also, some people have experienced pain on their knees when they start using this product. So if you have problems with your knees, you may want to look into a Soft KF knee extension instead of the Hard KF. But I don't know if it is the same. I was very impressed with the Flex KF, so I gave it a 5 stars.