Passive Force Augmentation in Actively Stretched Myofibrils and Sarcomeres

Abstract

Single skeletal muscle myofibrils were stretched actively and passively (pCa2+ = 3.5 and 8 respectively) from optimal length to beyond myofilament overlap (4.0μm) and individual sarcomere lengths and the associated stretch forces were measured. Actively stretched myofibrils produced much more force than passively stretched myofibrils at all sarcomere lengths, including when all sarcomeres were beyond myofilament overlap (Figure 1). In order to confirm that cross bridge based acto-myosin forces were not present at sarcomere lengths beyond myofilament overlap, passively and actively stretched myofibrils were deactivated and activated, respectively at 5μm. In both cases, no change in force was observed, indicating that active cross bridge forces were not present at long (5μm) length. Based on these results, we suggest that there is a dramatic passive force augmentation in passively stretched myofibrils and the individual sarcomeres.