Aim: 

Stretch-induced changes in X-ray interference and stiffness of active muscle have suggested rapid attachment to actin of the detached motor domain of the myosin dimers with a motor domain already attached (Brunello et al., PNAS, 104: 20114-20119, 2007).

Methods: 

Here we determine the speed and the extent of the increase in stiffness of the half-sarcomere (hs) following 100 ms stretches of amplitude (DL) 2-8 nm hs^-1, imposed on the isometric tetanus of single frog muscle fibres at 2.1 mm sarcomere length and 4 °C.

Results and Conclusion: 

We show that: i, at 2 ms following the stretch the stiffness of the array of myosin motors in each half-sarcomere (e_m) increases above the isometric value (e_m0), depending on the stretch size, up to a maximum value of ~2 e_m0 attained for DL > 6 nm hs^-1; the relation between e_m/e_m0 and the stretch-induced distortion (Dz) of the motor domains attached in isometric contraction can be interpolated with a sigmoidal equation with k, the value of Dz for half maximum, equal to 2.52 ± 0.28 nm; ii, at the end of the 100 ms stretch the relation e_m/e_m0 -Dz is not significantly different from that at 2 ms. These results support the idea that active muscle can efficiently resist rapid stretch through very fast attachment of the second motor domain of the myosin dimer to the actin monomer 5.5 nm M-ward with respect to the actomyosin complex already formed. Supported by MIUR (Italy), NIH (USA).