The mechanism of the resistance to stretch of isometrically contracting single muscle fibres

Rapid attachment to actin of the detached motor domain of myosin dimers with one motor domain already attached has been hypothesized to explain the stretch-induced changes in X-ray interference and stiffness of active muscle. Here, using half-sarcomere mechanics in single frog muscle fibres (2.15 μm sarcomere length and 4°C), we show that: (1) an increase in stiffness of the half-sarcomere under stretch is specific to isometric contraction and does not occur in rigor, indicating that the mechanism of stiffness increase is an increase in the number of attached motors; (2) 2 ms after 100 μs stretches (amplitude 2–8 nm per half-sarcomere) imposed during an isometric tetanus, the stiffness of the array of myosin motors in each half-sarcomere (em) increases above the isometric value (em0); (3) em has a sigmoidal dependence on the distortion of the motor domains (Δz) attached in isometric contraction, with a maximum ∼2 em0 for a distortion of ∼6 nm; em is influenced by detachment of motors at Δz > 6 nm; (4) at the end of the 100 μs stretch the relation between em/em0 and Δz lies slightly but not significantly above that at 2 ms. These results support the idea that stretch-induced sliding of the actin filament distorts the actin-attached motor domain of the myosin dimers away from the centre of the sarcomere, providing the steric conditions for rapid attachment of the second motor domain. The rate of new motor attachment must be as high as 7.5 × 10^4 s−1 and explains the rapid and efficient increase of the resistance of active muscle to stretch.