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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 μm, and then stretched passively and actively to a sarcomere length of 3.4 μm. 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. [ABSTRACT FROM AUTHOR]

Passive stiffness was found to be increased in mouse soleus muscles lacking desmin. Because titin is considered to be the major source of muscle elasticity, the stiffening might be explainable by titin adaptation. To test this, passive mechanical properties of single skinned fibres of soleus muscles from desmin knockout and control mice were analysed by using various extension tests. Titin expression was studied by SDS-gel electrophoresis. Absence of desmin did not modify either electrophoretic mobility of the titin band (3700 kDa) or optical density-unit ratios between bands for titin and nebulin (≅0.3) and bands for titin and myosin heavy chain (≅0.08). Elastic properties of fibres were not altered in the absence of desmin since passive tensions were similar under quasi-static (56–66 kN m–2) and dynamic (100–118 kN m–2) conditions whatever the kind of fibre. Thus, titin is unlikely to be responsible for the large increase in passive stiffness observed in whole soleus muscles when desmin is lacking. [ABSTRACT FROM AUTHOR]