How muscles contract

The brain sends a message to the muscle, then the muscle contracts the muscle fibers and the muscle pulls on the bone to move. An example would be when you want to pull something up to you, your brain sends a message to your arm muscles to contract pulling on the bone causing it to move up toward your body. 

      What are Flexor and Extensor Muscles?

 Flexor muscles bring your limbs or objects close toward your body.

Extensors bring your limbs or objects away from your body. A good example of these muscles are in your forearm. The belly of the forearm is the flexor muscle which pulls your wrist closer to you. The top side of the forearm is the extensor muscle, It pushes away from the body.

                             Why do muscles get tired?

The muscle uses oxygen to make glucose and once all the oxygen is used up it has to use lactic acid for energy and when lactic acid builds up the muscles hurt.

            How are Muscles Bones and Tendons related?

Tendons are connected to bones and the muscles are connected to the tendons. The muscles pull on the tendon and that causes the tendon to pull on the muscle making movement.

            The All or Nothing muscle