What Is the Role of Titin in Active Muscle?

In Brief Several properties of muscle defy explanation solely based on the sliding filament-swinging cross-bridge theory. Indeed, muscle behaves as though there is a dynamic “spring” within the sarcomeres. We propose a new “winding filament” mechanism for how titin acts, in conjunction with the cross-bridges, as a force-dependent spring. The addition of titin into active sarcomeres resolves many puzzling muscle characteristics. This paper summarizes a novel hypothesis for activation- and force-dependent increases in titin stiffness and implications for understanding muscle contraction.