Maximum isometric tetanic force produced by bundles of red muscle fibres from dogfish, Scyliorhinus canicula

white, red and superficial. In common with teleost fish, the different types are well segregated in the body. The white fibres make up the bulk of the body, the red fibres form a band along the lateral edge of the body and the single layer of superficial fibres is confined to a narrow band between the skin and the red fibres. Each fibre type has a distinct histological and biochemical profile which, at least for white and red fibres, seems well matched to its function during swimming. White fibres are large, with low mitochondrial density and a relatively high glycogen content. They are active during vigorous swimming, which reduces their glycogen content. During sustained slow swimming, the white fibres are not active, and the glycogen content of white fibres does not fall. In contrast, red fibres are small, with high mitochondrial density and relatively high fat content, which falls during their activity in long periods of sustained swimming (Bone, 1966; Bone et al., 1986). In this study, two questions are addressed about the contractile performance of red fibres from dogfish. Are they as strong as white fibres during isometric contraction? How much force is produced during constant-velocity movement (shortening and lengthening) at full activation? Red fibres are small and held together by extensive connective tissue; consequently, some are damaged during dissection of the small bundles of fibres required for in vitro experiments. Thus, to answer the question about strength accurately, force must be expressed relative to the crosssectional area of undamaged fibres. A simple method for identifying damaged and intact fibres has been developed, and quantitative validation of the method is reported. The mechanical and energetic performance of red fibres 1585 The Journal of Experimental Biology 205, 1585–1595 (2002) Printed in Great Britain © The Company of Biologists Limited 2002 JEB3965