In skeletal muscle, steady-state force is consistently greater following active stretch than during a purely isometric contraction at the same length (residual force enhancement; RFE). Similarly, when deactivated, the force remains higher following active stretch than following an isometric condition (passive force enhancement; PFE). RFE and PFE have been associated with the sarcomere protein titin, but skeletal and cardiac titin have different structures, and results regarding RFE in cardiac muscle have been inconsistent and contradictory. Therefore, the purpose of this study was to determine if cardiac muscle exhibits RFE and PFE. Skinned fibre bundles (n=10) were activated isometrically at a sarcomere length of 2.2um and actively stretched by 15% of their length. The resultant active and passive forces were compared to the corresponding forces obtained for purely isometric contractions at the long length. RFE was observed in all fibre bundles, averaging 5.5±2.5% (ranging from 2.3 to 9.4%). PFE was observed in nine of the ten bundles, averaging 11.1±6.5% (ranging from -2.1 to 18.7%). Stiffness was not different between the active isometric and the force enhanced conditions, but was higher following deactivation from the force-enhanced compared to the isometric reference state. We conclude that there is RFE and PFE in cardiac muscle. We speculate that cardiac muscle has the same RFE capability as skeletal muscle, and that the most likely mechanism for the RFE and PFE is the engagement of a passive structural element during active stretching.

在骨骼肌中，主动拉伸后的稳态力始终比在相同长度的纯等距收缩过程中的力大（残余力增强； RFE）。类似地，当停用时，在主动拉伸后，力保持比在等轴测条件下更高（被动力增强； PFE）。RFE和PFE与肌节蛋白的肌动蛋白相关，但骨骼肌和心脏的肌动蛋白具有不同的结构，关于心肌的RFE的结果前后矛盾。因此，本研究的目的是确定心肌是否表现出RFE和PFE。皮纤维束（n = 10）以2.2um的肌节长度等距激活，并主动拉伸其长度的15％。将所得的主动力和被动力与在长距离上纯等距收缩获得的相应力进行比较。在所有纤维束中均观察到RFE，平均为5.5±2.5％（范围为2.3％至9.4％）。在十个束中的九个中观察到PFE，平均为11.1±6.5％（范围从-2.1到18.7％）。刚性等距条件和力增强条件之间的刚度没有区别，但是与等距参考状态相比，从增强的力中停用后的刚度更高。我们得出结论，心肌中存在RFE和PFE。我们推测，心肌具有与骨骼肌相同的RFE能力，并且RFE和PFE最可能的机制是在主动拉伸过程中被动结构元件的接合。在所有纤维束中均观察到RFE，平均为5.5±2.5％（范围为2.3％至9.4％）。在十个束中的九个中观察到PFE，平均为11.1±6.5％（范围从-2.1到18.7％）。刚性等距条件和力增强条件之间的刚度没有区别，但是与等距参考状态相比，从增强的力中停用后的刚度更高。我们得出结论，心肌中存在RFE和PFE。我们推测，心肌具有与骨骼肌相同的RFE能力，并且RFE和PFE最可能的机制是在主动拉伸过程中被动结构元件的接合。在所有纤维束中均观察到RFE，平均为5.5±2.5％（范围为2.3％至9.4％）。在十个束中的九个中观察到PFE，平均为11.1±6.5％（范围从-2.1到18.7％）。刚性等距条件和力增强条件之间的刚度没有区别，但是与等距参考状态相比，从增强的力中停用后的刚度更高。我们得出结论，心肌中存在RFE和PFE。我们推测，心肌具有与骨骼肌相同的RFE能力，并且RFE和PFE最可能的机制是在主动拉伸过程中被动结构元件的接合。刚性等距条件和力增强条件之间的刚度没有区别，但是与等距参考状态相比，从增强的力中停用后的刚度更高。我们得出结论，心肌中存在RFE和PFE。我们推测，心肌具有与骨骼肌相同的RFE能力，并且RFE和PFE最可能的机制是在主动拉伸过程中被动结构元件的接合。刚性等距条件和力增强条件之间的刚度没有区别，但是与等距参考状态相比，从增强的力中停用后的刚度更高。我们得出结论，心肌中存在RFE和PFE。我们推测，心肌具有与骨骼肌相同的RFE能力，并且RFE和PFE最可能的机制是在主动拉伸过程中被动结构元件的接合。