Fig. S1.
Instantaneous force–SL plots for twitches in FE (red) and LC with the feedforward signal either sufficient (black) or insufficient (violet) to prevent shortening. SL changes are recorded on a horizontally mounted trabecula by the striation follower (46). The feedforward signal is an exponential calibrated on the sarcomere shortening recorded in the FE twitch. Circles are the respective Tp values according to the same color code as the traces. The black dashed line is the force–SL relation interpolated on data from Fig. 1D (13). The shaded areas are two ideal loops followed by contractions that start at SL ∼ 2.2 µm in isometric condition and become isotonic when the force attains the level corresponding to the Tp value identified by the color. In terms of the mechanical parameters of the left ventricle beat, the dashed line is the relation between end systolic pressure and end systolic volume, and the loops are two pressure–volume loops starting from the same preload (i.e., end diastolic volume) and with two different afterloads (i.e., aortic pressures). The correspondence between the stress–strain relation (i.e., tension–SL relation) of a myocyte of the ventricular wall and the pressure–volume relation of the whole ventricle has been extensively discussed in ref. 47.