One reason that I've always loved George Jowett's books is because he would take the time in his writing to explain how the body works. It's surprising how much he knew about human anatomy and physiology. Knowing how the body works is absolutely critical to being effective with your exercising because if you don't know how it works, you won't know for sure if what you're doing in your exercise is even doing you any good.
Now, Jowett didn't know what caused our muscles contract and how they create the contractions (neither did anyone else for that matter). We now know and studying the chemical reactions that cause our muscles to move can teach us a great deal about how we should be exercising. I'll try to keep this as simple and underwhelming as possible.
The molecule that causes the muscles to move is known as as Adenosine Triphosphate (ATP). As the name implies, the molecule contains three phosphorus atoms. When this molecule blows (converting it to Adenosine Diphosphate or ADP) off one phosphorus atom, we get muscular movement. The muscles have to make these ATP molecules and it does it two ways. The first way that the muscles create ATP is by Aerobic Respiration. This method is done in the muscle cell's mitochondria (look it up if you want to know), requires oxygen and either glycogen (a sugar) or fat to make ATP. It takes more time to make ATP like this but it also yields a lot of ATP. The other method is known as Anaerobic Respiration. As the name implies, it doesn't use oxygen to make ATP and it can only use glycogen. It is produced in the muscle cell's cytoplasm and it makes ATP much more quickly. Trouble is, it also makes lactic acid, which causes the muscles to cramp up so this method doesn't last long.
Now, each muscle cell specializes in one or the other forms of ATP production. Aerobic muscle fibers are known as slow twitch fiber and they are used when the muscles need long term physical activity, such as jumping rope or running, where the physical effort is modest. The Anaerobic muscles are known as fast twitch fiber and they are used for quick, powerful, or explosive efforts, such as wrestling, isometrics, or weight lifting. Now, these cells are laid out next to one another. They exist in varying ratios and there are hybrids of the two types of fibers. In some animals, such as chickens, the fibers are strictly divided. The breast and wing muscles (meat) are almost all fast twitch fiber used for quick flights as opposed to the thighs and legs which are used for walking and are slow twitch fibers.
We don't have that kind of separation between fast and slow twitch fibers so what we can learn from this is that our exercise should condition BOTH of these fibers in our muscles. When we see weight lifters and marathon runners, we are looking at two extremes of the athletic field who overdevelop their muscle's ability to do one kind of activity but are almost incapable of doing another. Since we have both types of fiber coexisting together in the same bunch of muscle, it's true strength lies in being able to run 3 miles as easily as we could lift a heavy object with equal ease. We are the animal version of the SUV or all-terrain vehicle. So, when you're doing your routine, make sure that you are doing exercises that work the muscles in both manners. Do some exercises that require modest efforts over a longer period of time as well as exercises that require intense but brief physical contraction.