Trabecu la stress-length work-loops at steady-state. (A) Exemplar recordings from a trabecula show simultaneous measurement of length, stress, and heat rate. In the work-loop protocol, muscle length was set at L_o (preload), and the muscle was subjected to a series of 3-Hz work-loop contractions at progressively diminishing afterloads, with the minimum afterload in the vicinity of passive force. Contractions at each afterload were interspersed by isometric contractions. A single work-loop twitch at steady-state is shown in each inset, where the four phases of the work-loop contraction are (1) isometric contraction, (2) isotonic shortening, (3) isometric relaxation, and (4) relengthening. Baseline heat rate is indicated by the horizontal broken line. (B) Twitches from an exemplar trabecula undergoing eight different afterloaded work-loop contractions superimposed. A work-loop contraction of a sufficiently high afterload, i.e., greater than the peak isometric stress, essentially resulted in an isometric twitch. The lowest achievable afterload was in the vicinity of the passive stress. (C) The changes of muscle length corresponding to each of the eight afterloaded work-loop contractions in B are superimposed. (D) A parametric plot of the stress in B and the length in C reveals the eight afterloaded work-loops. Active stress is the stress above passive stress, whereas afterload occupies the range of total stress.