Passive force enhancement in single myofibrils

The purpose of this study was to gain further insight into passive force enhancement by testing whether passive force enhancement occurs in single myofibrils. Myofibrils (n=6) isolated from rabbit psoas muscle were fixed at a sarcomere length of 2.4 mu m, and then stretched passively and actively to a sarcomere length of 3.4 mu m. Passive force after deactivation of the myofibrils was increased after active compared to passive stretching. Therefore, passive force enhancement, previously observed in muscle and fiber preparations, also occurs in single myofibrils. Passive force enhancement in myofibrils ranged from 86 to 145% of the steady-state force observed after passive stretch. Because titin is the main source of passive force in myofibrils, we propose that titin might be responsible for passive force enhancement observed in myofibrils. We propose that this might occur through an increase in stiffness when calcium concentration increases upon activation.