a-d: Isotonic afterload contractions: relation between shortening, velocity, work and stretching force. Fig. 4a shows four experiment units (1, 5, 8, 11) from a sequence of eleven units with complex isometric-isotonic contractions. The top field shows the contraction forces and the bottom field the shortenings. The vertical line with zigzag arrow indicates the timing of the electric stimulus. High contraction force leads to small shortening and vice versa. Fig. 4b shows the shortenings (Δl), Fig. 4c shows the shortening velocities (v) (time derivative of the shortening) and Fig. 4d shows the work W = F*Δl, where Δl equals the shortening and F the counterforce. Graphs of two different pre-stretching lengths are shown.