Z disc expansion during supercontraction: a passive mechanism?

The hypothesis of active Z disc expansion during supercontraction is re-examined using larval blowfly body-wall muscles. The previous results on glycerinated supercontracting fibres are confirmed. However, further observations on whole fibres stimulated to contract artifically and on skinned muscle fibres, as well as certain theoretical considerations, have led to the postulation of a passive mechanism. This mechanism is dependent upon mechanical continuity of the Z discs across the fibre.