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 (L_op), 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 L_op 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.