The force-length relationship of mechanically isolated sarcomeres

The sarcomere force-length relationship is arguably the most basic property of skeletal muscle force production. It has been accepted as textbook knowledge and is in direct support of the sliding filament and cross-bridge theories of contraction. However, the sarcomere force-length relationship has never been measured directly. Here, we show results of two experiments elucidating the force-length properties of mechanically isolated sarcomeres. We demonstrate that sarcomere forces are greatly dependent on sarcomere lengths for purely isometric conditions, but can take on essentially any steady-state value depending on an individual sarcomere's contractile history. Therefore, we conclude that steady-state isometric forces in isolated sarcomeres do not only depend on sarcomere lengths (or equivalently actin-myosin overlap) but depend crucially on a sarcomere's contractile history. These results have direct implications for our understanding of the molecular mechanisms of muscle contraction.