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Fig. 5. Isometric twitch length–tension curves derived from active twitch
forces measured from the dl1 muscles. The dl1 muscles
operate exclusively on the ascending limb of their length–tension curve.
(A) Isometric twitch length–tension curve based on in situ
measurements from the intact thoraces of seven moths. (B) Isometric twitch
length–tension curves measured from mechanically isolated dl1
muscles from four moths. (C) Length–tension curves representing pooled
data from intact thorax preparations (heavy line) and from isolated muscle
preparations (light line). All curves in A and B are 2nd order polynomials fit
to individual data sets. Each curve in C was obtained by averaging the
polynomial coefficients from the individual curves within each of the two data
sets shown in A and B. The solid vertical line in each plot indicates the mean
value of operational length (Lop), measured relative to
the anatomical rest length of the dl1 muscles in six tethered
flight preparations. The broken vertical lines (a, mean minimum muscle length;
b, mean maximum muscle length) indicate the average bounds of muscle length
changes during tethered flight. Muscle length for active twitch force
measurements in the intact thorax preparations (A) was also referenced to the
anatomical rest length. This common length reference allowed us to map the
in vivo muscle length changes measured in tethered flight onto the
twitch length–tension curve measured in intact thorax preparations (A).
The vertical lines indicating Lop and the mean range of
length changes derived from measurements in tethered flight and from the
intact thorax preparation are replicated in B and C. Although the intact
thorax and isolated muscle preparations produced different curves, both
indicate that active twitch force is substantially lower than maximal over the
range of in vivo muscle lengths.
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