Length-tension-velocity relationships studied in short consecutive segments of intact muscle fibres of the frog.

Length changes of consecutive, 0.5-0.8 mm long segments of frog single muscle fibres were studied by photoelectric recording of opaque markers placed on the fibre surface. There was a marked redistribution of segment length during an ordinary isometric contraction (fixed fibre ends) at both 2.15 and 2.6-2.8 microns sarcomere length. This length redistribution can explain the tension 'creep' that occurs during standard isometric contractions on the descending limb of the length-tension relation. Length clamp of individual segments eliminated tension creep completely. Active force of length-clamped segments was investigated within the range 2.20-3.65 microns sarcomere length. The descending limb of the length-tension relation (determined in segments where no tension creep occurred) was not strictly linear but had a slightly sigmoid shape. Active force was reduced to zero at a sarcomere length close to 3.65 microns. While isometric force varied only moderately between different segments, the velocity of unloaded shortening (V0) was found to vary greatly (by 22-50%) along the length of a fibre. V0 did not correlate with the passive resistance to a length change, the isometric force or the cross-sectional area of the individual segments. Local differences of the internal milieu and/or coexistence of myosins of different kinetic properties within a single fibre may account for the observed differences in V0.