The Origin of Force Increase in Actively Stretched Single Muscle Fibres

Abstract

Theoretically, at long sarcomere lengths (SLs) beyond myofilament overlap, only passive forces, produced by passive structural elements, are possible. However, previous studies showed a dramatic increase in force above the passive force when myofibrils were actively stretched beyond actin and myosin filament overlap. It has been suggested that titin might produce this increase in force, possibly by increasing its stiffness in a variety of ways. In this study, I used rabbit psoas single muscle fibres to investigate whether the increase in force observed after active stretch to a sarcomere length beyond myofilament overlap was caused by a passive component (titin) alone, or possibly some remnant cross-bridges that continue to contribute force in some unknown manner in muscle fibres. The results indicate that both passive and active components are contributing to this force increase in actively stretched single muscle fibres at an average sarcomere length beyond myofilament overlap.