Shortening properties of two biochemically defined muscle fibre types of the Norway lobster Nephrops norvegicus L.

Mechanical properties of myofibrillar bundles from single chemically skinne d fibres from the superficial abdominal flexor muscle of the Norway lobster Nephrops norvegicus were measured, and the protein content of these fibres was analysed by SDS-PAGE. Two slow fibre phenotypes (S-1, S-2) were distin guished on the basis of their myofibrillar protein assemblages. Data from 9 S-1 and 8 S-2 fibres obtained at similar sarcomere length demonstrate sign ificant differences between the fibre types in maximal tension (N cm(-2), S -1: 10.5 +/- 3.9; S-2: 3.1 +/- 0.8), in the delay of the peak of stretch ac tivation (ms, S-1: 122 +/- 18; S-2: 412 +/- 202), in fibre stiffness (N cm( -2) per nm half sarcomere, S-1: 0.36 +/- 0.19; S-2: 0.09 +/- 0.03) and in m aximal shortening velocity (fibre length s(-1), S-1: 0.53 +/- 0.10; S-2: 0. 27 +/- 0.06). Furthermore, the maximal power output of the type S-1 fibres was about five times larger than that of S-2 fibres. The power output was m aximal at lower loads in S-1 fibres (relative load = 0.37 +/- 0.04) than in S-2 fibres (relative load = 0.44 +/- 0.05). This study represents a compre hensive investigation of two slow muscle fibre types which are thought to b e specialized for slow movements (S-1 fibres) and for the postural control of the abdomen (S-2 fibres).