Hind limb extensor muscle architecture reflects locomotor specialisations of a jumping and a striding quadrupedal caviomorph rodent

Abstract

Muscle architecture is an important factor in determining muscle function. The physiological cross-sectional area (PCSA) is directly proportional to the force-generating capacity of a muscle, while fibre length determines the capacity for a muscle’s length change. For a given muscle volume, both parameters cannot be maximised at the same time, and therefore, specialisation in accordance with specific functional demands is widely accepted. Building on this, the architecture of selected hind limb extensor muscles of two caviomorph rodent species of similar body size but with differing locomotor modes were analysed and compared. Individual fascicles of fixed cadavers were carefully removed during stepwise dissection. After removal of each fascicle, the left-behind groove within the muscle belly was digitised to capture the length and orientation of the removed fascicle. Pennation angle, muscle volume, and anatomical cross-sectional area were determined, and finally, PCSA and force-generating capacity were approximated for a hip extensor (M. biceps femoris), a knee extensor (M. vastus lateralis), and an ankle extensor (M. triceps surae). Muscle architecture appeared to reflect locomotor specialisation of a jumping (Chinchilla chinchilla) in comparison with a striding quadrupedal (Cavia porcellus) species, but considerable variability of the limited specimens analysed was found. With the biceps femoris as an exception, analysed specimens of Chinchilla had relatively more voluminous and thus metabolically expensive hind limb extensors with both a greater capacity for length change and for force generation. These results are in agreement with a greater demand for powerful hind limb extension during launches and provide further evidence that muscle architecture is adapted to differing functional demands in closely related species.