Muscles can produce two types of force: active and passive. Active and passive forces are dependent on the length of the sarcomere (Gordon et al., 1966). Active force is generated through cross-bridge cycling, while passive force depends virtually exclusively on the protein titin (Trombitas et al., 1998). Within a myofibril, sarcomeres are arranged in-series, thus they transmit identical forces at any instant in time. When active force production in sarcomeres differs, passive forces need to compensate for the lack of active force by elongating until passive force reaches the required magnitude. This adjustment of sarcomere length can lead to so-called sarcomere “popping” which has been proposed as a mechanism of muscle injury, residual force enhancement, and force depression (Morgan, 1990). We developed a novel technique to locally deactivate one sarcomere in an otherwise fully activated myofibril with the purpose to test if deactivated sarcomeres will “pop” when active force is eliminated. However, we found that locally deactivated sarcomeres elongated by a small amount and did not overstretch as the popping sarcomere theory would predict. Deactivated sarcomeres lengthened on average by 0.6 μm from about 2.6 to about 3.2 μm. At 3.2 μm, rabbit psoas sarcomeres have a small passive force, thereby questioning how these deactivated sarcomeres can sustain stresses exceeding 350 nN/2 μm. We propose that upon activation, sarcomeres form a linked actin-titin framework that provides increased passive force, thus preventing overstretching and damage of sarcomeres when active force is eliminated. We show the novel result that there is an in-built safety mechanism in previously activated sarcomeres that prevents them from being overstretched and damaged when deactivated. We propose that activation causes a titin-actin linkage that provides stability to sarcomeres that is not abolished immediately by deactivation and may be re-inforced by a small sarcomere stretch.
Ino et al. (Sun,) studied this question.