Skeletal muscles
The primary function of skeletal muscle is to move the bones at their joints. They do this by contracting or lengthening.
No skeletal muscle acts alone. They always act as part of a group under the control of the nervous system.
Muscle may sometimes be attached directly to bone but, where its bulk would interfere with its function, where it would rub over bone, or where its action needs to be concentrated, it is attached by white fibrous tendons.
The words insertion and origin are often used to describe the attachment of the ends of a muscle.
The origin is the attachment of the muscle that remains more or less stationary. The insertion is the more mobile attachment.
For example, the biceps muscle of the upper arm is attached by a tendon to the scapula. As this attachment usually remains stationary, it is referred to as the origin of the muscle. The other end of the biceps muscle is attached to the radius. This is the more mobile attachment, the insertion.