Velocity of shortening of unloaded heart muscle and the length-tension relation.

A new technique permits removal of the resting force from the muscle during the course of shortening and study of the maximum velocity of shortening with almost zero load at physiological initial muscle lengths. This unloading has been performed as a sudden unloading to zero total load (zero load clamping), or according to the resting length-tension relations, or according to an arbitrary unloading intermediate between both former modes of unloading. Using these various modes of unloading to zero load during shortening, it has been demonstrated that the maximum velocity of shortening VmflI remains constant between maximum length (\max) and a length 12.5% shorter than lm a i . This unique force (zero load)-velocity (Vm n x)-length (up to 12.5* below lmQI) relation is independent of the initial muscle length over this range of lengths, independent of the time after the stimulus over the largest portion of the shortening phase, and independent of the sequence of length change and mode of unloading through which it arrived at zero load within this range of