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Fig. 4. Records of tetanic contractions in a single muscle fiber at optimum
sarcomere length and along the descending (A) and ascending (B) limbs of the
length-tension relationship. Records shown in A were derived by holding a
discrete fiber segment at a constant length during the tetanus. For records
below the slack length, the segment was initially shortened under segment
length control to the desired sarcomere spacing. Records in B are standard
isometric contractions (no segment-length control) of the same fiber as in A.
For contractions at moderately reduced lengths, the fiber was allowed to
shorten freely to the desired length by taking up a pre-set slack. For
contractions below approximately 1.9 µm, the initial shortening phase was
constrained by applying a controlled ramp movement of the electromagnetic
puller. The latter manoeuvre made the fiber develop some tension during the
initial shortening phase. The upper trace in each set of records shows force;
the lower trace shows segment length recording with sarcomere spacing
indicated. A downward deflexion of segment length signal indicates shortening.
P0, measured isometric force at 2.45µm sarcomere
length; L0, segment length at 2.45µm sarcomere length.
Temperature, 24.1°C.
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