Stepwise shortening of muscle fibre segments.

Shortening dynamics were measured in single fibres of frog skeletal muscle using a system that could track the spacing between hairs mounted on the fibre surface. Segment length changes were predominantly stepwise. The objective of the study was to identify potential artifacts and check their relevance. Several possible causes of artifactual steps were evaluated quantitatively and ruled out. In addition, the surface marker method and an independent length-detection method based on light diffraction were used simultaneously. The concurrence of results confirmed that it is highly unlikely that stepwise shortening could arise out of instrument artifact. Possible mechanisms underlying the phenomenon are considered.