Series elastic and contractile elements in heart muscle: changes in muscle length

Isometric force generation, the result of the interaction of an actively shortening contractile element (CE) with a passive series elastic component (SE), has been analyzed in heart muscle using the cat papillary muscle. The stiffness of the SE (Δ load/Δ extension or d p/d l) was shown to be a linear function of developed force of contraction or load ( P). The load-extension curve of the SE was thus exponential in form, the SE being stretched an amount equivalent to 8–10% of initial muscle length during the development of maximum isometric force (600 g/cm 2 ). The load-extension curve of the SE was also obtained from the curve relating initial velocity of shortening to time after stimulation. This latter relation and the linear relation of SE stiffness to force were found to be independent of initial muscle length. These results were interpreted to suggest that increases in muscle length bring about an increase in the number of series elastic components as well as contractile elements arranged in parallel. The series elastic component thus cannot be relegated entirely to external attachments of the muscle (e.g., tendon), but must be closely associated with the contractile element system itself.