Stretch-induced force enhancement and stability of skeletal muscle myofibrils

The main purpose of the experiments presented in this chapter was to test the hypothesis that the stretch-induced force enhancement commonly observed in skeletal muscle is associated with sarcomere length instability. Single myofibrils isolated from the rabbit psoas muscle were attached to a nanolever pair for force measurement at the one end, and to a glass needle for controlled displacements at the other end. The image of the striation pattern was projected onto a linear 1024-element photodiode array, which was scanned (20 Hz) to produce a dark-light pattern corresponding to the A- and I-bands, respectively. Starting from a mean SL of approximately 2.55 microm, stretches of a nominal amplitude of 4 to 10% of SL, at a nominal speed of 100 nm x sec(-1) were applied to activated myofibrils (pCa2+ = 4.75). Following stretch, the isometric, steady-state force was greater by 10.9% to 45.9% than the force produced before stretch, and was greater than the force predicted at the corresponding final length. Passive force could not account for the force enhancement. Sarcomere lengths along the activated myofibrils were non-uniform, but remained constant before stretch or during the extended isometric period after stretch. Further, sarcomeres never stretched to a length beyond thick and thin filament overlap. It is concluded that sarcomeres are stable, and therefore the increased force observed after stretch must be a sarcomeric property, not associated with continuous length changes of unstable sarcomeres, as had been assumed in the past.