We all know about actin and myosin acting as sliding filaments in muscle tissue, but what about titin? Titin, the largest protein known to man, is only recently gaining fame. It comprises over 27,000 amino acids (the building blocks of proteins), which is 10 times the size of an average protein.
Because of this great size it has to fold itself up many times over to fit in the sarcomere. One end attaches to the Z disc (the wall that separates sarcomeres) and is very elastic, or ‘springy’. The other end attaches to the free-floating myosin and is relatively stiff. Each titin molecule fills half the sarcomere length, or 1.2 microns. This is huge. Each half of a sarcomere is a mirror image of itself. Titin may be the primary organizer of the regularly arranged components within a sarcomere.
Why am I telling you this? Well, titin is the primary source of passive tension in skeletal muscle. It’s what you feel most when pressing on soft tissue. In the old days we were told that the passive tension was due to the extracellular connective tissue or muscle membranes. But, in a seminal study published in 1985, Magid and Law demonstrated that the origin of passive muscle tension is mostly due to the intracellular protein titin.
Titin is also very sticky. It may actually get too sticky. Kuan Wang of the NIH describes this as a ‘Myosin train crash’ whereby in a sustained contracture the titin folds up on itself and the myosin like carriages in a train crash. Myosin can stay stuck there. We can feel this as increased stiffness or resistance to our pressure, or as a hardened end-feel of joint movement, both of which are also qualities of myofascial trigger points.
We manual therapists, in most clinical situations, should always follow our pressure-based manual therapy with a lengthening component. The pressure itself changes the stiffened or ‘gel’ state of muscle tissue into a more ‘sol’ state and the stretch may encourage the titin train crash carriages to line up again.
This blog is intended to help put into context all the intricacies of the soft tissue when you study with the Manual Trigger Point Therapy (MTT) seminars.
A summary of some of the unique characteristics of titin;
- Largest protein ever discovered, at 1.5 microns long
- Contributes most of the passive tension (resistance to pressure) of muscle
- Plays a central role of molecular organization within the sarcomere
- Is ideally situated to act as a sensor for altered muscle mechanical conditions
- Altered conditions include chronic length and chronic force change
- Altered muscle stiffness as seen in immobilization, stroke, head injury, spinal cord injury or cerebral palsy may due to the amount and form of titin in various muscles