That being said, if you can't do a pull-up, I still would not recommend taking the alternative that Tony Horton shows you and performing pulldowns with a resistance band. One of the big ironies of doing body weight exercises to get in shape if you’re overweight is that, thanks to your higher body weight, you’re actually forced to lift substantially more weight than someone who isn’t overweight – and if you’re also out of shape, you’re forced to lift more weight with less muscle! If you sag, you’re changing the mechanics as well as shifting the weight in ways which may be more likely to cause injury. In case you’re wondering why a straight line is so important, it’s because the goal in a push-up is to lift a static weight – in this case, your body weight – primarily by using the muscles in your chest and shoulders. This will stabilize your “plank” position. To help you maintain that straight line, concentrate on contracting your abdominal muscles by ‘pulling’ your belly button up and in, squeezing your glutes, and making sure to drive your heels backwards rather than leaning forward on your tiptoes.
As a matter of fact, focus on making yourself into a straight line all the way from neck to ankles. Your head position should be neutral, neither drooping towards the floor nor looking up. Do not round your shoulders, but keep them lightly pulled back. We’re talking about spinal position and muscle control here.) Conversely, make sure that your butt doesn’t stick up in the air, either. (If the fat on your belly sags all on its lonesome due to the inexorable downward pull of gravity, that’s okay. This means that your back should be flat with no hip or belly sag towards the floor.