Active lengthening increased force and M3 spacing, while active shortening decreased force, stiffness, and M3/M6 spacings compared to purely isometric reference contractions in rabbit muscle fibers.
Residual force enhancement in skeletal muscle fibers is achieved without an increase in the proportion of attached cross-bridges, whereas force depression is accompanied by a decrease in attached cross-bridges.
Our purpose was to use small-angle X-ray diffraction to investigate the structural changes within sarcomeres at steady-state isometric contraction following active lengthening and shortening, compared to purely isometric contractions performed at the same final lengths. We examined force, stiffness, and the 1,0 and 1,1 equatorial and M3 and M6 meridional reflections in skinned rabbit psoas bundles, at steady-state isometric contraction following active lengthening to a sarcomere length of 3.0 µm (15.4% initial bundle length at 7.7% bundle length/s), and active shortening to a sarcomere length of 2.6 µm (15.4% bundle length at 7.7% bundle length/s), and during purely isometric reference contractions at the corresponding sarcomere lengths. Compared to the reference contraction, the isometric contraction after active lengthening was associated with an increase in force (i.e., residual force enhancement) and M3 spacing, no change in stiffness and the intensity ratio I1,1/I1,0, and decreased lattice spacing and M3 intensity. Compared to the reference contraction, the isometric contraction after active shortening resulted in decreased force, stiffness, I1,1/I1,0, M3 and M6 spacings, and M3 intensity. This suggests that residual force enhancement is achieved without an increase in the proportion of attached cross-bridges, and that force depression is accompanied by a decrease in the proportion of attached cross-bridges. Furthermore, the steady-state isometric contraction following active lengthening and shortening is accompanied by an increase in cross-bridge dispersion and/or a change in the cross-bridge conformation compared to the reference contractions.
Joumaa et al. (Sun,) reported a other. Active lengthening and shortening vs. Purely isometric reference contractions at corresponding sarcomere lengths was evaluated on Structural changes within sarcomeres (force, stiffness, and X-ray reflections). Active lengthening increased force and M3 spacing, while active shortening decreased force, stiffness, and M3/M6 spacings compared to purely isometric reference contractions in rabbit muscle fibers.