Answer
Usually a muscle is attached to two bones one at each end. Consequently, either the muscle itself or its tendon crosses at least one joint. Therefore when a skeletal muscle contracts, it moves the two ones closer to each other: Normally, one bone does most of the moving, and the other is relatively stable. On this basis, points of origin and insertion are designated. The attachment site on the bone that does more of the movement is called the insertion. The attachment site of the more stable bone is designated the origin of the muscle. The middle part of the muscle is called the belly-this is usually the thickest part of the muscle.
Work Step by Step
Examples:
The brachialis muscle is the prime mover of flexion of the elbow joint
Origin: (proximal attachment) lower half of humerus and lateral and medial intermuscular septa
Insertion ( distal attachment): Coronoid process and tuberosity of the ulna.
This terminology can be confusing as a muscle may "reverse' its functional origin and insertion
dependent on the action being considered:
Examples . The quadriceps femoris is an extensor of the knee joint. It is usually considered to have its origin (proximal end) on the femur and its insertion (distal end) on the tibia. When one kicks a ball, the tibia moves more, and the femur is relatively stable. In such a case, the tibial attachment is by definition the insertion. However, in the action of sitting down this relationship is reversed--the tibia remains relatively stationary while the femur moves-- the quadriceps braking the action so the sitting is not so abrupt that it hurts. In this case the femur ( by usual definition ) is the insertion and the tibia functions as the origin.
Attempts have been made to obviate this potential confusion by simply speaking of the proximal and distal ends of muscles or of their superior and inferior attachments . These designations are based on anatomy and not on function.
