The compliance of contracting skeletal muscle.

1. The method of controlled releases was used to obtain tension-extension curves in toad (Bufo bufo) sartorii under a variety of conditions at 0 degrees C.2. The curves obtained were approximately linear over a considerable range of force (0.4P(0) to P(0)) if the releases were given from the plateau of tetanic tension. The slope of this linear region was little affected by changes of release velocity in the range 10-120 mm/sec.3. Such changes as did occur with alterations in release velocity could be quantitatively accounted for in terms of the internal shortening predicted by A. V. Hill's two-component model.4. As the muscles were stretched above l(0), we found that the maximum stiffness of the tetanized muscles fell in much the same way as the maximum developed force, P(0).5. In another series of experiments we found a rapid change in the overall shape of the tension-extension curve during the early phase of force development in an isometric tetanus. The stiffness of the muscle increased with increasing developed force during this period.6. The force-velocity curve in these muscles was measured by two methods, both giving a similar result. Surprisingly, toad muscle appears to have about the same intrinsic speed as frog muscle at 0 degrees C. The a.b product from our experiments is considerably greater than the reported values for the maintenance heat rate at 0 degrees C in these muscles.7. The probable site of the variable compliance in active muscle is discussed. It seems most likely that this is within the A-band, perhaps in the cross-bridges themselves.