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Titin and the so-called sliding filament theory

This is an excerpt from Understanding Fascia, Tensegrity, and Myofascial Trigger Points by John Sharkey.

In 1954, H. E. Huxley and J. Hanson proposed the sliding filament theory of muscle contraction (figure 3.18). The structural unit of muscles, the sarcomere, had been understood to consist of two interdigitating filament systems, which slide past each other when a muscle shortens. Special proteins were identified as forming one thin and one thick protein filament. This description is the most widely used in medical and exercise science texts to this day.

Credit must be given to Huxley and Hanson all those years ago when they noted that removing the actin and myosin protein filaments did not lead to a collapse of the sarcomere. The presence of some third protein filament was proposed, but its size and proximity to the other filaments made firm conclusions regarding its disposition and function difficult to reach. Over the years many terms have been used to describe the function of these filaments, including S filaments, gap filaments, T filaments, and core filaments.

In 1977 a new myofibrillar protein was identified, and later in 1999 it was given the name titin. It appears that titin is the third-most-abundant protein in striated muscle, accounting for about 11 percent of its combined protein content.

I mention this because it is important to recognize that we are still learning about the architecture of sarcomeres and how muscles contract. Of course, that is why we still say the sliding filament “theory,” because it is the best theory we have to explain how muscles go about their business.

But there is no sliding in the human body unless there is pathology, and so there cannot be a “sliding filament” at play. Heat production in the human body comes from the production of energy through the splitting of ATP. Sliding would cause friction, and that would not be a valuable component of muscle activity (figure 3.19).

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