Fluctuation analysis for the average length of five sarcomeres along a fast skeletal myofibril at rest. (A) Snapshot of phase-contrast image sequence of two rabbit psoas myofibrils immersed in relaxing solution (pCa 9). Pink arrows indicate six consecutive Z-disks analyzed in B; i.e., the average length of five sarcomeres was analyzed. Imaging was performed at 30 fps. Scale bar, 5 µm. (B) Time course of changes in the average SL. (C) Histogram showing the variance of SL. Average SL, 2.16 ± 0.017 µm (mean ± SD), indicating that the precision is 17 nm under the present experimental setting.