A tendon (or sinew) is a tough band of fibrous connective tissue that connects muscle to bone, or muscle to muscle and is designed to withstand tension. Tendons are similar to ligaments except that ligaments join one bone to another. Tendons and muscles work together and can only exert a pulling force. Most of the strength of tendon is due to the vertical, hierarchical arrangement of densely-packed collagen fibrils. Tendon length varies in all major groups and from monkey to person. Tendon length is practically the discerning factor where muscle size and potential muscle size is concerned. For example, a man with a 1 centimetre biceps tendon will have greater potential for muscle mass than a man with a longer tendon. Bodybuilders will generally have short tendons and are said to have 'great genetics', however not all bodybuilders are blessed with short tendons.