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 00 C. The a. b product from our experiments is considerably greater than the reported values for the maintenance heat rate at 00 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.

SUMMARY1. The increase in rate of heat production caused by stretch in the unstimulated frog's sartorius (stretch response) has been measured using a conventional thermopile technique.2. The rate of heat production was found constant between l0 (the distance in vivo between the tendons when the legs were in a straight line) and 1-2 1l, and rose rapidly above this length to reach 3-5 times the basal rate at 1-3 lo. Stretching to greater lengths appeared to damage the muscles.3. The stretch response is increased by several substances which increase the duration of the active state.4. Unlike the rate of heat production at lo, the stretch response is increased by procaine; while the presence of C02 greatly reduces it.5. Evidence is presented supporting the hypothesis that the stretch response is associated with the appearance of tension in the sarcolemma.

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If the K+ content of normal Ringer solution is increased, the duration of the plateau of the active state begins to rise at 8–10 mM [K]out. If the Cl− of the Ringer solution is replaced by NO3−, the active state duration begins to rise at 4–5 mM [K]out, and increases markedly with further depolarization. The observed dependence of active state duration on [K]out is similar to the known relation between resting metabolism and [K]out and is taken as evidence in support of the concept of subthreshold activation of the excitation–contraction coupling system by depolarization.