A combined mechanical and X-ray diffraction study of stretch potentiation in single frog muscle fibres

1. The nature of the force (T) response during and after steady lengthening has been investigated in tetanized single muscle fibres from Rana temporaria (4 degrees C; 2.15 mu m sarcomere length) by determining both the intensity of the third order myosin meridional X-ray reflection (I-M3) and the stiffness (e) of it selected population of sarcomeres within the fiber. 2. With respect to the value at the isometric tetanus plateau (T-0), I-M3 was depressed to 0.67 +/- 0.04 during steady lengthening at similar to 160 nm s(-1) (T approximate to 1.7T(0)) and recovered to 0.86 +/- 0.05 during the 250 ms period of after-stretch potentiation following the rapid decay of force at the end of lengthening (T approximate to 1.3T(0)); under the same conditions stiffness increased to 1.25 +/- 0.02 and to 1.12 +/- 0.03, respectively. 3. After subtraction of the contribution of myofilaments to tilt: half-sarcomere compliance, stiffness measurements indicated that (1) during lengthening the cross-bridge number rises to 1.8 times the original isometric value and the average degree of cross-bridge strain is similar tc, that induced by the force-generating process in isometric conditions (2.3 nm), and (2) after-stretch potentiation is explained by a residual larger cross-bridge number. 4. Structural data are compatible with mechanical data if the axial dispersion of attached heads is doubled during steady lengthening and recovers half-way towards the original isometric value during after-stretch potentiation.