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 μm 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 μm 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 μm. 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.
KeywordsLength Change Isometric Force Sarcomere Length Single Muscle Fibre Slack Length
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