When a muscle contract, it will become shorten. To kick a ball, there are many muscles used for moving the leg. Muscle from the thigh will move the lower leg and the power of kick comes from this muscle group. The muscle in the lower leg will control the feet and determine whether kick using the front or side of the feet.
Since the muscle in one side contract, the opposite muscle in the muscle pair will be flexed. The muscle in flex condition will be lengthened.