Freely moving synovial joints consist of cartilage and connective tissue ligaments that contain lubricating synovial fluid. Most voluntary muscles attach to bones, forming lever systems. When a muscle contracts, a bone moves. The muscle end attached to the stationary bone is its origin. The end attached to the movable bone is called the insertion.
Muscles form antagonistic pairs, enabling bones to move in two directions.
Muscles are always partially contracted, generating muscle tone. Muscle spindles control muscle tone by sending impulses to the brain or spinal cord when a muscle is stretched. Regular exercise causes microscopic changes in muscle cells that enable them to use energy more efficiently. The McGraw-Hill Companies. Any use is subject to the Terms of Use and Privacy Policy .