Part II of III of exercise types. We did Isometric, now let's tackle Isotonic.
Isotonic exercises are exercises in which you move the body through a range of motion around one or more joints.
There are two kinds of contractions that take place during an isotonic exercise, the concentric and eccentric contractions.
Concentric contractions shorten the length of a muscle. The easiest example is a biceps curl - bring your hand to shoulder. Your biceps muscle fibers shortens against the resistance of your hand (which isn't significant unless you're holding a dumbbell).
Eccentric portion lengthens the muscle. The muscle fibers lengthen, slowing down the weight as it the joint straightens out.
The important thing to realize here is that for your arm, the biceps work when you pull in (bending the elbow), the triceps when you push out (straighten the elbow). So as the biceps contract, the triceps lengthen, and so on.