Connective tissue is responsible for holding all the individual muscle fibres together. It surrounds individual muscle fibres and encases the whole muscle, forming tendons at the ends. Tendons attach muscle to bone and transmit the ‘pull’ of the muscle to the bones, causing movement and harnessing the power of muscle contractions. Tendons vary in length and are composed of parallel fibres of collagen. They attach directly onto the periosteum of bones through a tough tissue known as Sharpey’s fibres.
The ends of the muscle are referred to as the origin and the insertion. The origin is the more fixed, stable end, and is attached to a stable bone against which the muscle can pull. This is usually the nearest flat bone. The insertion is the muscle attachment on the bone that the muscle moves.
A useful analogy is to nail an elastic band to a wall. Where it is fixed into place is termed the origin. When holding the other end of the band and pulling; where it is being held is termed the insertion. When a muscle contracts it shortens and the insertion moves closer to the origin, creating a movement at a joint.
The muscle belly is the thick portion of muscle tissue sited between the origin and insertion. A muscle can have more than one origin, while maintaining a common insertion. For example, biceps means two (bi-) heads (-ceps), and triceps means three (tri-) heads (-ceps). The biceps has two origins or heads located on the scapula, which pull on one insertion in the radius, raising the lower arm when contraction occurs.
Click to enlarge