There are many muscles in the human body and there are several different kinds. The extensor muscle belongs in the skeletal system and when the muscle contracts, it forces the limb that it is attached to extend or stretch. One example of this type of muscle is the elbow. When the muscle contracts near the elbow, the arm straightens out. Another muscle is called the cardiac muscle.
The cardiac muscle is related to the heart. This muscle works involuntarily. The flexor muscle is similar to the extensor muscle because they both make movements. As the extensor muscle extends, the flexor muscle lessens the angle. The muscle that pulls the arm up at the elbow would be a flexor muscle. The muscle that works to close a joint would be a flexor muscle.