The sarcomere length-tension relation determined in short segments of intact muscle fibres of the frog.

1. Single fibres isolated from the tibialis anterior muscle of Rana temporaria were stimulated to produce a 1 s fused tetanus, while a short (ca. 0.5 mm) segment of the fibre was held at constant length. The segments were defined by opaque markers of hair that were placed on the fibre surface. The distance between two adjacent markers (one segment) was monitored by means of a photo-electric recording system. The length of a given segment could be controlled to within 0.2% of the segment's length by adjusting the over-all length of the fibre by means of an electromagnetic puller and servo system. 2. Segments producing constant force (no 'tension creep') during length-clamp recording were studied at different striation spacings within the following ranges of sarcomere length: 2.20-3.70, 1.90-2.45 and 1.50-2.20 microns. The absence of tension creep suggested (Edman & Reggiani, 1984 a) that the sarcomere pattern remained stable within the length-clamped segment during contraction at different lengths. 3. The tetanic force of a given length-clamped segment was consistently found to increase, as the sarcomere length was reduced from 2.20 to 1.98-2.02 microns, the mean increase in force being 6.9 +/- 0.4% (S.E. of mean, thirty-two segments). By further decreasing the sarcomere length active force was reduced. 4. The increase in force-producing capability between 2.20 and 2.00 microns sarcomere length was further explored by recording the maximum rate of force redevelopment, dF/dtmax, after a quick release during the plateau of a fixed-end tetanus. dF/dtmax varied with sarcomere length between 2.20 and 2.00 microns in the same way as the isometric force of a short, length-clamped segment, increasing by ca. 10% over this range. This finding provides further support for the view that the fibre's capacity to produce force is not constant between 2.20 and 2.00 microns sarcomere length. 5. The descending limb of the length-tension relation extended between 2.00 and approximately 3.65 microns sarcomere length. Its middle, straight portion (between 2.30 and 3.30 microns sarcomere spacings) extrapolated to zero tension at 3.49 microns sarcomere length. The upper and lower portions of the descending limb were slightly curved (at sarcomere lengths less than 2.30 and greater than 3.30 microns, respectively) giving the descending limb a symmetrical sigmoid appearance.(ABSTRACT TRUNCATED AT 400 WORDS)