Feats of supercontractile strength: functional convergence of supercontracting muscle properties among hyoid musculature in chameleons.

The structure of sarcomeres imposes limits to the capacity of striated muscle to change length and produce force, with z-disc and myosin filament interactions constraining shortening. Conversely, supercontracting muscles, hitherto only known among vertebrates in the tongue retractor muscle (m. hyoglossus) of chameleons, have perforated z-discs that allow myosin filaments to extend through them into adjacent sarcomeres, permitting continued shortening and force development. Additional hyolingual muscles in chameleons undergo extreme length changes during feeding as well and may benefit from supercontractile properties. We compared length-tension relationship data and transmission electron microscopy images from four chameleon muscles to test for the presence of additional supercontracting muscle. We document the second known example of a supercontracting muscle among vertebrates (the m. sternohyoideus superficialis) and show that the m. sternohyoideus profundus exhibits functional convergence with supercontracting muscles by increasing the range of muscle lengths over which it can exert force through the exploitation of sarcomere length non-uniformity across its muscle fibres. Additionally, we show that chameleon supercontracting muscles may share common contractile and structural properties due to a common origin from occipital somites. These results provide important insights into the developmental and evolutionary patterns associated with supercontracting muscle and extreme muscle elongation.