Morphological differences in myofibre size and shape: A comparative study of the soleus, gastrocnemius, triceps brachii and vastus lateralis in humans and mice.

Certain skeletal muscles are specialized for their functional roles, yet direct comparisons of cellular morphology of distinct muscles beyond fibre type distribution are limited. This study investigated myofibre morphology in predominantly slow, fast and mixed fibre muscles in humans and mice, with the aim of establishing reference values for muscle-specific myofibre size and shape. Nine healthy young men (Age: 26 ± 1 years, BMI: 23 ± 1 kg/m²) had muscle biopsies taken from soleus, triceps brachii and vastus lateralis muscles. Additionally, the soleus and gastrocnemius muscles were harvested from 7 male C57BL/6 mice. Muscle samples were analysed by ATPase (human) or immunofluorescence (mouse) stainings of fibre type specific cross-sectional area, perimeter and Shape Factor Index (SFI; fibre perimeter²/4 × π × fibre cross-sectional area). In humans, type I fibres had 30%-40% larger CSA and 4%-7% higher SFI in soleus (1.54 ± 0.06) compared to triceps brachii (1.47 ± 0.05) and vastus lateralis (1.43 ± 0.04). Type IIa fibres SFI were 10%-11% higher in soleus (1.61 ± 0.08) compared to triceps brachii (1.45 ± 0.04) and vastus lateralis (1.45 ± 0.08). Soleus type I fibres were more heterogeneous in terms of size and shape compared to other muscles. Analyses of mouse muscle showed a similar pattern, in that CSA and SFI were higher in type I and IIa fibres of the soleus compared to the gastrocnemius. These findings suggest a consistent morphological characteristic of soleus fibres across species, with potentially important implications for future biomedical research.