Fig. 9. Upper panel: Schematic representation of a trachealis cell and the intracellular arrangement of contractile units at 0.75 L_{in situ} (left-hand rectangle), and the three possible rearrangements of the contractile units after the cell has been stretched and fully adapted at 1.5 L_{in situ}. Assuming that the kinetics of actomyosin crossbridge interaction are not affected by the contractile unit reconfiguration, all three models predict no change in isometric force with length doubling; all three models also predict an increase of 67% in shortening velocity, muscle power output, rate of ATP consumption and myosin thick filament density with length doubling [consistent with observations made by Kuo et al. (Kuo et al., 2003), and data in Fig. 8]. In all models, the number of contractile units in series has increased by 67% with length doubling. In Model A, the increase in cell length is associated with the appearance of non-overlap zones between the thick and thin filaments, and no change in the thick filament length. In Model B, the increase in cell length is associated with the appearance of non-contractile elements (instead of non-overlap zones) in series with the contractile units, and no change in the thick filament length. Although the non-contractile elements are placed at the ends of the cell in the drawing, they can be anywhere in the cell as long as they are in series with the contractile units. In Model C, the increase in cell length creates neither non-overlap zones nor non-contractile elements; instead, the thick filament length is increased by 20% and the number of contractile units in parallel decreased by 20%. Lower panel: Predictions by the models regarding the relationship between maximally shortened length and isotonic load. The data points are redrawn from Fig. 7. The model predictions are based on the linear regression line (solid) for the data collected at 0.75 L_{in situ}. The maximally shortened length under zero-load is assumed proportional to the number of dense bodies in series plus any serially connected non-contractile elements.