Fig. 2. Horizontal Z-disk displacement (x_myo) plotted versus sarcomere length (SL) for extensor digitorum longus fifth toe muscles of wild-type (circles) and desmin-null (squares) animals. Superficial regions are shown with open symbols while deep regions are shown with filled symbols. Note that x_myo increases to a greater extent as SL increases in muscles from knockout compared with wild-type animals. Error bars represent ±1 S.E.M. in either the x_myo or SL direction (N=7 wild-type muscles and N=9 desmin-null muscles). Superficial and deep regions of each muscle were sampled from 10 micrographs per region. The regression slope of the x_myo versus SL relationship was significantly different from zero in the specimens from knockout animals (P<0.005) but not in the specimens from wild-type animals (P>0.4), and analysis of covariance (ANCOVA) revealed a significant difference between the slopes of these relationships (P<0.05).