Over on Mark Sisson's blog Doug has a guest post on the Body By Science approach to exercise - Setting Yourself Up to Win: A Body By Science Approach
It is a good introduction to the principles of proper exercise, but the reason I am posting this here is jsut to draw attention to his bodyweight training option. I hoep Mark and Doug do not mind me copying it here:
If you cannot go to a commercial gym, you can get started with simple free-hand exercises that I will describe to you now.
Chin up: This can be done with a chin-up bar that mounts in a door jam, on a sturdy tree branch or rafter board or playground equipment. If you are not strong enough to do chins, you can set the bar height so you can assist with your legs. If this will not work, simply do them negative only by jumping or climbing to the ﬁnished position and lowering yourself as slowly as possible.
Pushups: If you are too weak to perform strict marine pushups, do them from your knees. If you are too weak to do them from your knees, then do only the lowering portion, lowering as slowly as possible. If you are strong enough to do classic pushups, do them with a few modiﬁcations. First is slow movement. Start the ﬁrst inch very gradually, taking 3 seconds to move the ﬁrst inch and then keep smooth movement going. Divide the movement in halves. Do the ﬁrst half (bottom position to elbows bent at 90 degrees) until complete fatigue. After you have exhausted the bottom half, do the top half until complete fatigue (elbows from 90 degrees to almost complete extension).
Squat: Start ﬁrst by doing a static wall squat. Place your back against a wall and descend to a seated position where your hip joint and knee joint are both at 90 degrees. Hold this position for as long as possible. You are done when you start to slide down and cannot hold the 90 degree position any longer. Once you are worn out on the static, do a deep knee bend but with the movement divided in half. Do the ﬁrst half until fatigue (from hips and knees at 90 degrees/thighs parallel to ﬂoor, up to the halfway up point where knees are about 45 degrees). Once you canʼt do the bottom half any more, then do the top half of the movement until you canʼt go on. Remember to not straighten your legs completely, but to turn back around when your knees get to about 15 degrees.
Static Lateral Raise: This movement is done using a door frame. Stand with your feet just outside the door frame and bend slightly forward at the waist. Place the backs of your open hands in the opening of the door frame with your elbows slightly bent. You should be positioned like you are going to a lateral raise with dumbbells. With your hands in the door frame, begin to slowly and smoothly press laterally against the frame. Gradually build up to a 50% effort and keep up a 50% effort for 30 seconds. After 30 seconds, gradually ramp your effort up to 75% effort and continue for another 30 seconds. After this 30 seconds is up, gradually ramp your effort to 100% and continue for a ﬁnal 30 seconds. When you ﬁrst start you will think “this is silly”. However, by the end you will realize that this is probably tougher than anything you could do on a weight machine.The timed static contraction for the lateral raise at the end is a killer. Superb stuff.