Muscles and Bones: Working Together for Locomotion

Muscles are attached to bones through connective tissue called ligament, and when they contract, they move the bones, producing locomotion. The movement of bones is achieved through the action of two different bones that are attached by muscles. The point of attachment to bones has two ends, namely origin and insertion, where origin is the stationary end while the insertion moves. The thick part of the muscle between the origin and insertion that contracts is called the belly. Muscles are arranged in pairs, called antagonistic pairs, and each muscle has its own origin and insertion.

Flexor and Extensor Muscles: Antagonistic Pair

Flexor and extensor muscles work in pairs to move bones in two different directions. When the flexor muscle contracts, it bends the bone at the joint, while the extensor muscle straightens the bone at the joint. In the human body, most of the 650 muscles occur in such pairs, and the arrangement of these muscles is called the antagonistic arrangement.

Joints in Action: The Elbow Joint

The elbow joint is a synovial joint between the humerus, radius, and the muscles responsible for its movement are the biceps and triceps. The biceps is also called biceps brachii, and it arises by two heads: the long head from the glenoid cavity and the short head from the coracoid process of the scapula. Brachialis lies below the biceps brachii and arises from the anterior surface of the lower three-fifths of the humerus, inserting into the coronoid process of the ulna. Brachioradialis arises from the lateral side of the lower end of the humerus and extends most of the forearm, inserting into the lateral border of the distal shaft of the radius. Radius is lifted by the contraction of biceps brachii and brachioradialis, while ulna is lifted by the contraction of brachialis.

Triceps brachii is a muscle that causes extension, and it has three heads: long head, lateral head, and medial head. The long head arises from the scapula below the shoulder joint, the lateral head from the lateral surface of the humerus, and the medial head from the medial and lateral surface of the humerus. All three heads fuse at their lower end and insert through a common tendon on the olecranon process of the ulna. When the triceps brachii contracts, it exerts a powerful force on the upper end of the ulna, causing the extension of the forearm.

Muscles in Action