Figure 1. Variables of contraction and relaxation determined in this study. (Top left) Force traces of an isometric twitch before and during exposure to 10⁴M thiopental, DF- peak developed force. (Bottom left) Rate of increase and decrease of force of the isometric twitch. +dF/dt maximal rate of force development;-dF/dt- the maximal rate of force decline. (Top center) Shortening traces of an isotonic twitch against the preload of L^maxonly, control and 10 sup -4 M thiopental. DL = peak shortening. (Bottom center) Velocity trace of the isotonic twitch. +V the peak velocity of shortening; V - the peak velocity of lengthening. (Right) Shortening (top) and velocity (bottom) traces of a zero-load-clamped twitch, control, and 10 sup -4 M thiopental, where load was rapidly decreased from preload to zero at the onset of the sweep. At the stimulus, muscle shortening proceeds against zero load. MUVS the maximal unloaded velocity of shortening. The small vertical arrow in each upper panel represents the timings of the electrical stimulus.