Let’s review the series on what makes movement possible. So far we learned bones form the hard struts and girders of the body which are used as levers to let us move. Ligaments keep the levers in constant relationship with each other and define how we move at each joint. Muscles provide power to move and that power is only in one direction – muscles only pull.
Joining the muscles to the bones is the last mechanical link, which is the job of tendons.
Tendons have to transfer large amounts of force so they need to be very strong. Structurally, tendons are made of the same material as ligaments. Unlike ligaments, tendons have to allow changes in bone relationships. Often a muscle will have a very short, broad tendon attaching the one end of the muscle onto a bone over a relatively large area. The other end of the muscle will be attached to a narrower, cable like tendon. The result is that when the muscle contracts it stays in place on bone with the broad connection and pull towards the muscle whatever the cable is attached to.
For example – When you bend you elbow your bicep stays where it is in your upper arm, the mass of it does not slide down from the bone due to the broad tendon connections to the humerus. The other end of the bicep, however, is much more cable like and attached to the radius in the forearm. When the bicep contracts the cable end and is drawn up toward the biceps center causing the elbow to bend.
To be clear, other muscles do a lot of the work of bending the elbow, but most of us know what and where the biceps is.
The cable has to move past a lot of other tissues where it could get hung up, so tendons often are wrapped in sheaths in which the tendon can glide and slide like the brake cable on your bicycle. The sheath keeps things running in the same path and the cable runs up and down it to apply mechanical force.
One advantage of this set up is that the cables can be run some distance away from the actual red muscle tissue that generates force. This way we can put muscles that take up space in the forearm and just run cables to the many bones in the wrist and fingers. If we had the muscles in the wrist and fingers the sheer volume of the muscles would limit how dexterous we could be with our hands.
Problems with tendons are probably the most common musculoskeletal injuries.
Tendons get torn, irritated, and inflamed; the sheaths get inflamed, scarred, or damaged, and the connections at both the bones and the muscles can get damaged. Each of these injuries has a different name to indicate the specific problem like tendonitis, tenosynovitis, tendinosis, etc. but in the end, they are all tendon problems.
Unfortunately tendons, like ligaments, are slow to heal and for the same reason. They don’t have a great blood supply to get nutrients in and waste out. And it is really hard to not use them.
Being in a cast or brace will stop a joint from moving and thus prevent straining a ligament – not so for tendons. Even in a cast you will contract the muscles in the cast and pull on the tendons.
That covers the main hardware we use to move with, next up is how we pull it all together to use it. That is the realm of the nerves.
– By John Macy, PT