FIGURE 1.

Subset of experimental records. Muscle length, muscle stress, and associated heat signal were simultaneously recorded. The left panel shows records of combined isometric and work-loop contractions, with the former mode interposed with latter, from an initial length at L_o. Work-loop contractions with the five decreasing afterloads are concomitant with increasing extent of shortening and decreasing output of heat. The middle panel shows records at an initial length of 0.9 L_o. The right panel shows records of two isometric contractions at 0.81 L_o and 0.77 L_o. Electrical stimulation was paused between interventions required for the reduction of initial length. Insets show expanded traces for two individual twitches at steady state, indicated by the arrows. The work-loop twitch consists of four phases: (1) isometric contraction, (2) isotonic shortening, (3) isometric relaxation, and (4) passive re-stretch back to initial muscle length.