Series elasticity in heart muscle. Its relation to contractile element velocity and proposed muscle models.

The series elasticity (SE) of cat papillary muscle was measured in 6 muscles by the isotonic quick-release method, with careful correction for the compliance of the equipment. The resultant SE curve was exponential in form with an extension of 4% to 5% of initial muscle length for a preload of 0.5 g and a developed tension of 10 g; this demonstrates a stiffer SE for cardiac muscle than has previously been reported. The quick-release SE extension curve was unaffected by inotropic interventions or changing the time of quick release, and was less compliant than the SE curves calculated by two different isotonic methods. These differences arise from the fact that contractile element velocity has a secondary rise during the transition from the isometric to isotonic phase of any afterloaded contraction. With increasing preload (and initial muscle length) the SE became stiffer, suggesting that a substantial part of it is in series with both the contractile and parallel elastic elements. ADDITIONAL