Human Body 101: Muscle Facts #3
The fibers in your muscles are kind of like rowing a boat. There is the hook, the pull and recovery. The hooks can only hook and pull in one direction. This is the muscle contracting. Just like when you put the oar into the water, it pulls the boat along its it moves through the water.
This is why most joints you have an agonist (prime mover) and an antagonist (opposing) so that the joint can both extend and flex.
For example, your Bicep (agonist) flexes your elbow joint and your Tricep (antagonist) extends it.