The heat of shortening during the plateau of tetanic contraction and at the end of relaxation.

1. The net thermal effect of shortening on the heat production of the sartorius muscle of Rana temporaria has been studied at 0 degrees C in tetanic contractions of either 1.2 or 2.0 sec analysed up to the end of relaxation.2. Three types of contractions have been compared: isometric at the standard length (l(o)); isometric at a shorter length (l(o) minus 4.27 mm); and rapid shortening at constant velocity between those two lengths during the plateau of tetanic activity, the work being collected by an ergometer of the Levin-Wyman type. After the end of the shortening, stimulation was continued long enough to allow a complete recovery of tension before relaxation started.3. In those conditions, shortening heat appears clearly by comparison with the isometric contractions, and most of its effect persists up to the end of relaxation. However, a reduction of the net thermal effect, of the order of 15-25%, is observed during relaxation.4. The net supplement of heat is the same whether the shortening occurs in the beginning (0.4 sec) or towards the end (1.2 sec) of the contraction period; the reduction of the effect during relaxation is more important in the pelvic part than in the tibial part of the muscle.5. This small reduction of the thermal effect of the shortening is fully accounted for by the fact that the tension redeveloped after the end of the movement remains always below the tension developed in a tetanus at the same final length, but isometric from the start.6. As the greater part of the net thermal effect of the shortening persists at the end of relaxation, shortening heat cannot be part of a cyclic process reversed during relaxation; it must derive from some chemical source of energy.