An Anatomical Evaluation of the Myocardial Length-Tension Diagram

A method of examining a small (50 to 100 µ o.d.) strand of living cardiac muscle containing several fibers with a high power optical system is described. When the strand was stretched so that it was just taut, the muscle fibers within were found to be slack and severely buckled. An extension of 40 to 60% of this initial length was required before the fibers became straightened and aligned with the long axis of the strand. This finding can account for the fact that cardiac muscle, unlike skeletal muscle, must be subjected to considerable passive tension before its maximum active tension can develop. The resting sarcomere length (that observed when the fiber is just straightened) was 2.0 to 2.2µ. Overall strand length and sarcomere length were measured and had no accurately predictable relation. Rabbit papillary muscle fixed at a length at which the passive tension was just zero showed similar buckling of the muscle fibers. The collagen content (in the form of hydroxyproline analysis) of heart muscle was approximately 6 times that of skeletal muscle. The significance of these findings and their relation to some previously reported mechanical properties of cardiac muscle are discussed.