The length dependence of work production in rat papillary muscles in vitro

The influence of length on work production was investigated for rat papillary muscles using the work loop technique. Active and passive length-force relationships were first determined under isometric conditions and the length for maximum force production (Lmax) was derived. Starting from different lengths within the physiological range, a series of work loops was generated using the stimulation phase shift, strain amplitude and cycle frequency previously found to be optimal for power output at 37 degrees C. The relationship between muscle length and net work was used to determine the length at which work output was maximal (Lopt). In order to examine the dynamic passive properties of the muscles, unstimulated muscles were subjected to the same regime of sinusoidal oscillation as used for the active loops. From the hysteresis loops, lengthening work (work done to extend the passive muscle), passive shortening work (work returned during shortening) and net energy loss (hysteresis) could be measured. The decline in net work production at lengths greater than 95% Lmax could largely be attributed to the rapid and non-linear increase in muscle stiffness and the increase in net energy loss over this range of lengths. The physiological significance of the length-work relationship is considered and the mechanical properties of active and passive papillary muscles are discussed with reference to sarcomere length and cardiac muscle ultrastructure.