Got an email query on the bodyweight exercise stuff, thought it worth answering here.
Muscles -- by which I mean here those that are voluntary, striated, and skeletal, as opposed to those smooth and involunatary ones in the internal organs, like the heart and bladder and blood vessels -- work but one way -- they contract, shorten, and by that action, pull on bones and whatnot.
By means of this and some clever levers, you get flexion, extension, rotation, supination, pronation, abduction, adduction, all like that. The reason a chimp is so strong compared to most people is that his leverage is better.
Large skeletal muscles work in pairs -- for every protagonist, there is a antagonist. And some muscles do more than one thing. Take the one little boys default to when somebody says, "Show me your muscle!" the biceps. This is a two-headed thing on the upper arm. If you flex your forearm with your palm up, you can see how that works. If you do it palm down, when you get to ninety degrees, turn your hand so that it is palm up, and you can see that it is also used to supinate the hand.
Sometimes the attachments are tendons, and obvious; sometimes like the muscles of the lips, their origins and insertions are less apparent. Eventually, something somewhere connects to the skeleton. (The "origin" is the more fixed end, the "insertion," the part that tends to move the most. Thus, the biceps originates on the shoulder and inserts on the forearm just below the elbow. The Latin names are jawbreakers:
Long head:supraglenoid tubercle of scapula. Short head: coracoid process of scapula with coracobrachialis
posterior border of bicipital tuberosity of radius (over bursa) and bicipital aponeurosis to deep fascia and subcutaneous ulna
If you want to know the other Latin names and such, go here.
The triceps, on the back of the humerus, extends the forearm. Generally, flexion is toward the body, and extension away from it. A curl is flexion, a bench-press extension, leastways, of the arm. When the chest comes into it, then you get pectoral flexion, and some shoulder rotation.
All you need to do to work your body is to figure out what motion works a muscle, then the muscle(s) that reverse it.
The easy way is to grab something and move it and see what muscles tighten. Compound exercises, such as squats, work a lot of muscle groups. Isolation exercises, such as sitting in a chair and curling a dumbbell, work fewer groups, though it is almost impossible to completely isolate one muscle, since there are a bunch of them that come into play stabilizing the body. If you stand and do curls with a barbell, the biceps are doing most of the work, but everything from your hands to your shoulders, to the rest of your back, belly, hips, and legs that are keeping up upright also come into play. Sit in a chair, lean back, you remove some of the muscles from the exercise.
One of the reasons the old Six Million Dollar Man was so utterly silly was that Steve Austin had this extremely powerful arm, but it was connected to his not-so-powerful body, and that was the weak link. The arm could pick up a car, but his non-bionic parts couldn't. No writer I knew could watch that show without laughing.
I'm talking about exercise a lot, since my back is still tight enough from my strain and resulting spasm that doing anything really vigorous is still a bad idea ...