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1. Abrupt alterations in load (load clamps) have been imposed on cat papillary muscle during isotonic shortening and relaxation of afterloaded twitch and tetanic contractions, to assess the maximal force potential for a given contractile state. 2. These load clamps were accompanied by an initial fast lengthening reflecting an undamped series compliance. Even when exceeding isometric twitch and tetanic force, these loads could be borne for a considerable time, accompanied by a slower lengthening after the initial extension of the series compliance. At sufficiently high loads the muscle was pulled out very rapidly; a maximal supra-isometric force potential was defined as the load the muscle could bear momentarily and this was measured at times throughout contraction and relaxation. 3. This maximal force potential was determined at different initial muscle lengths. Depending on the instantaneous loading conditions, various length-force relations were obtained from : (a) peak force values of isometric twitches at different starting lengths, (b) the shortest length reached during after loaded isotonic twitches, and (c) the forces obtained in overloaded isotonic twitch contractions. 4. These results are consistent with a crossbridge model in which the delayed lengthening during isotonic overloading is due to back rotation and detachment of attached crossbridges and in which the inital phase of spontaneous isotonic relaxation is governed by the same mechanism.