Predicting axial muscle-fibre strains in larval zebrafish.

The fast axial muscle fibres of teleosts enable rapid starts and high-speed swimming. Their arrangement in densely packed nested helices, combined with shear deformation, has been proposed to reduce variance in muscle-fibre-aligned strains across transverse cross-sections, enabling similar contributions to power generation. In zebrafish (Danio rerio), a helical pattern is already present at hatching, but its effect on strain variance remained unclear. Previous studies analysed only the anal region and used distortion-prone vibratome sections. Here, we predicted muscle-fibre-aligned strain distributions as a function of lateral curvature using in vivo measurements of muscle-fibre orientation along the anterior 70% of the axial muscles of 4 days post-fertilisation larval zebrafish. Using a bending-beam model, we compared strain distributions for hypothetical longitudinal fibre orientations and the measured helical arrangement, and examined how added-shear deformation and modest median-plane compression alter these distributions. Relative to longitudinal fibres, the helical arrangement reduced muscle-fibre-aligned strains in lateral regions but had little effect on strain variance across transverse sections. Incorporating shear into the helical arrangement, however, consistently reduced strain variance irrespective of body curvature, with the strongest effects anteriorly where helicity is most developed. Allowing median-plane compression further reduced strain variance on the concave side but increased it on the convex side. To assess functional relevance, we predicted maximum mean fibre strains and strain rates along the body from previously recorded swimming kinematics. We conclude that a helical muscle-fibre arrangement with added shear reduces strain variance and supports feasible strain rates during vigorous swimming. These results probably generalise across bony fish larvae.