Morphometrical fibre parameters in fast muscle of exercised and non-exercised adult zebrafish. A: Image of the swim tunnels used for exercise training. Front tunnel: exercised zebrafish; back tunnel: non-exercised zebrafish. B-D: Images of zebrafish cross-sectional white muscle. Images correspond to representative serial transverse secions stained (B) for succinate dehydrogenase for the identification of fast, intermediate (pink) and slow muscle fibres; (C and D) for ATPase for capillary demonstration (arrows) and FCSA and FPER measures (white drawing) from a non-exercised (C) and an exercised (D) adult zebrafish. Bar represents 50 µm. Morphometric fibre parameters measured in non-exercised and exercised zebrafish were: FCSA, fibre cross-sectional area (µm²) (E); FPER, fibre perimeter (µm) (F); FD, fibre density (fibres/mm²) (G); SF, shape factor (H). Statistical significance values between non-exercised and exercised zebrafish: *P < 0.05. Values are mean ± SEM from a sample size of n = 8 for each condition. I-J: Fibre cross-sectional area histograms from fast muscle of exercised (I) and non-exercised (J) adult zebrafish. In K, the two overlapped curves are shown. Muscle fibre areas were grouped in intervals of 200 µm² and the data correspond to mean ± SEM frequency of six animals. Curves represent a log-normal regression of four parameters. Regression parameters are shown in Additional file 1. See Methods for details.