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 temporar ia (4 degrees C; 2.15 mu m sarcomere length) by determining both the intens ity of the third order myosin meridional X-ray reflection (I-M3) and the st iffness (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 w as depressed to 0.67 +/- 0.04 during steady lengthening at similar to 160 n m s(-1) (T approximate to 1.7T(0)) and recovered to 0.86 +/- 0.05 during th e 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, respe ctively. 3. After subtraction of the contribution of myofilaments to tilt: half-sarc omere compliance, stiffness measurements indicated that (1) during lengthen ing 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) a fter-stretch potentiation is explained by a residual larger cross-bridge nu mber. 4. Structural data are compatible with mechanical data if the axial dispers ion of attached heads is doubled during steady lengthening and recovers hal f-way towards the original isometric value during after-stretch potentiatio n.