One of the great things about the "CrossFit" method is the scalability of the exercises. For example, anyone can perform the push up. Here is the progression of the push up from the wall to the rings.
If the athlete cannot perform a push up off the floor, the upper body should be elevated off a bar (on an adjustable squat rack) or the wall until the push up can be performed properly
While performing push ups, the elbows should remain close to the body (not flared out), fingers pointed straight ahead, hands under the shoulders, abs tight, glutes and quads tight, and back straight. The body should remain completely rigid(like a board) with the head in a neutral position.
If the athlete has a difficult time executing all of these things, the upper body should be elevated until all points can be executed.
To add an evil twist to the conventional push up, use rings, medicine balls, or even kettlebells under the hands to add instability or increase range of motion.
For added weigh, use a weighted vest.
The push up is still one of the most functional and effective full body exercises and can be performed anywhere at any time. With some imagination, the progressions to the push up are endless.