Frog Fibres: What Muscle Architecture Can Tell Us About Anuran Locomotor Function.

Muscle fibre architecture is an important aspect of anatomy to consider when estimating muscle properties. How fibre architecture varies across species specialising in different locomotor functions is not well understood in anurans, due to difficulties associated with fibre extraction in small animals using traditional methods. This paper presents the first digital analysis of fibre architecture in frogs using an automated fibre-tracking algorithm and contrast-enhanced µCT scans. We find differences in hindlimb muscle fibre architecture between frogs specialising in different locomotor modes, as well as examples of many-to-one mapping of form to function. The trade-off between fibre length and muscle physiological cross-sectional area, and therefore contractile speed, range of motion and muscle force output, differs significantly between jumpers and swimmers, but not walker-hoppers. Where species place on this functional spectrum of fibre architecture largely depends on the muscle being examined. There is also some evidence that fibre length may be adjusted to increase contractile speed without undertaking the metabolically expensive process of growing and maintaining larger muscles. Finally, we make a detailed outline of the remaining gaps in our understanding of anuran fibre architecture that can now be addressed with this valuable digital method in future research.