The descending limb of the sarcomere length-force relation in single muscle fibres of the frog.

Single muscle fibres, isolated from the tibialis anterior muscle of the frog, were used to study intersarcomere dynamics during muscle-isometric (fixed-end) tetani at long sarcomere lengths. Sarcomere length was measured by an online laser diffraction technique. On the descending limb of the length-force relation, the slow rise of force (creep) was always associated with changes in sarcomere length. Sarcomeres at the ends of the fibres shortened, while those of the central 90\% of the fibre length were stretched. Fibres were found to have a range of passive length-force curves, those with high resting forces developed little creep force, while low resting force fibres developed substantial creep, resulting in a fixed-end sarcomere length-force relation which deviated greatly from that expected from crossbridge theory. These differences in creep force can be qualitatively accounted for by differences in sarcomere dynamics. The simultaneous measurement of force and sarcomere length during force development allows the construction of a 'sarcomere-isometric' length-force curve from minima in the sarcomere length record. Force declined linearly from a plateau at 2.2 microns to zero at a sarcomere length close to 3.65 microns. The online, diffraction-derived sarcomere length was used in a feedback loop to clamp sarcomere length in short (100-200 microns) segments of fibres. A length-force curve constructed from sarcomere length-clamped tetani shows a linear decline in force from a plateau at 2.2 microns to zero at a sarcomere length of 3.65 microns.