Bundles of the curarized semitendinosus muscle of the frog were fixed during isotonic (afterload) and isometric contraction and the length of the A and I bands investigated by electron microscopy.

The sarcomere length, during afterload contraction initiated at 25 per cent stretch, varied depending on the afterload applied between 3.0 and 1.2 µ, i.e. the shortening amounted to 5 to 50 per cent. The shortening involved both the A and I bands. Between a sarcomere length of 3.0 to 1.7 µ (shortening 5 to 35 per cent) the A bands remained practically constant at about 1.5 µ (6 to 8 per cent shortening); the length of the I bands decreased from 1.4 to 0.3 µ (80 per cent shortening). Below a sarcomere length of 1.7 to 1.2 µ the A bands shortened from 1.5 to 1.0 µ (from 6 to 8 to 25 per cent). At sarcomere lengths 1.6 to 1.2 µ the I band was replaced by a contraction band.

During isometric contraction the A bands shortened by about 8 to 10 per cent; the I bands were correspondingly elongated.

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