Follow this link for an excellent article comparing the differences in joint forces in the leg (primarily the knee) in open versus closed kinetic-chain exercises. The article also includes a very useful description of the differences between the two types of exercises. When you are done reading you will want to go do some  squats and lunges and you may even consider cutting back on your “air kicks”.
Here is the link:
http://www.ejbjs.org/cgi/reprint/75/5/732.pdf

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