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1. Rat superficial masseter operates at long lengths during biting

   https://doi.org/10.1038/s41598-025-21953-z  

   Konow, Nicolai; Reder, Brandon; Bartlett, Daniel; Jenness, Devin; Patel, Trushti; Moore, Jeffrey R; Brocklehurst, Robert J (December 2025, Scientific Reports)

   Abstract The operating length ranges of mammalian jaw muscles have been estimated using twitch contractions or force measurements at the bite point, prompting a consensus that jaw muscles operate at short lengths on their force-length (FL) curve. However, since activation intensity truncates muscle optimal length (L_O), we hypothesized thatL_Oof rat superficial masseter (SM) would decrease with activation intensity, with high-force biting involving muscle shortening from long lengths on the FL curve. We measured muscle activation, strain, and forcein vivoduring biting on food with varying hardness and mapped thein vivodata from each muscle (N = 6) onto its FL relationship, measuredin situ. SubmaximalL_Owas approx. 12% shorter than twitchL_O, and SM bite forces averaged 4.1 ± 3.9 N/cm²(mean ± S.D.) and reached 10.6 N/cm², corresponding to muscle activation and food hardness. Length operation ranged from 7% belowL_O(ascending FL plateau), to 27% beyond (descending limb). The finding that jaw muscles operate at long, potentially unstable lengths, particularly during hard food biting significantly expands our understanding of skeletal muscle function, with broad implications for craniofacial evolution, muscle mechanics and control, and healthy as well as pathological function of the jaw musculoskeletal system. 

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2. The relative contributions of multiarticular snake muscles to movement in different planes

   https://doi.org/10.1002/jmor.21591  

   Tingle, Jessica L.; Jurestovsky, Derek J.; Astley, Henry C. (June 2023, Journal of Morphology)

   Abstract Muscles spanning multiple joints play important functional roles in a wide range of systems across tetrapods; however, their fundamental mechanics are poorly understood, particularly the consequences of anatomical position on mechanical advantage. Snakes provide an excellent study system for advancing this topic. They rely on the axial muscles for many activities, including striking, constriction, defensive displays, and locomotion. Moreover, those muscles span from one or a few vertebrae to over 30, and anatomy varies among muscles and among species. We characterized the anatomy of major epaxial muscles in a size series of corn snakes (Pantherophis guttatus) using diceCT scans, and then took several approaches to calculating contributions of each muscle to force and motion generated during body bending, starting from a highly simplistic model and moving to increasingly complex and realistic models. Only the most realistic model yielded equations that included the consequence of muscle span on torque‐displacement trade‐offs, as well as resolving ambiguities that arose from simpler models. We also tested whether muscle cross‐sectional areas or lever arms (total magnitude or pitch/yaw/roll components) were related to snake mass, longitudinal body region (anterior, middle, posterior), and/or muscle group (semispinalis‐spinalis, multifidus, longissimus dorsi, iliocostalis, and levator costae). Muscle cross‐sectional areas generally scaled with positive allometry, and most lever arms did not depart significantly from geometric similarity (isometry). The levator costae had lower cross‐sectional area than the four epaxial muscles, which did not differ significantly from each other in cross‐sectional area. Lever arm total magnitudes and components differed among muscles. We found some evidence for regional variation, indicating that functional regionalization merits further investigation. Our results contribute to knowledge of snake muscles specifically and multiarticular muscle systems generally, providing a foundation for future comparisons across species and bioinspired multiarticular systems. 

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3. Fluoromicrometry reveals minimal influence of tendon elasticity during snake locomotion

   https://doi.org/10.1242/jeb.249259  

   Tingle, Jessica L; Garner, Kelsey L; Astley, Henry C (March 2025, Journal of Experimental Biology)

   ABSTRACT Multiarticular muscle systems are widespread across vertebrates, including in their necks, digits, tails and trunks. In secondarily limbless tetrapods, the multiarticular trunk muscles power nearly all behaviors. Using snakes as a study system, we previously used anatomical measurements and mathematical modeling to derive an equation relating multiarticular trunk muscle shortening to postural change. However, some snake trunk muscles have long, thin tendinous connections, raising the possibility of elastic energy storage, which could lead to a decoupling of muscle length change from joint angle change. The next step, therefore, is to determine whether in vivo muscle shortening produces the postural changes predicted by mathematical modeling. A departure from predictions would implicate elastic energy storage. To test the relationship between muscle strain and posture in vivo, we implanted radio-opaque metal beads in three muscles of interest in four corn snakes (Pantherophis guttatus), then recorded X-ray videos to directly measure muscle shortening and vertebral column curvature during locomotion. Our in vivo results produced evidence that elastic energy storage does not play a substantial role in corn snake lateral undulation or tunnel concertina locomotion. The ability to predict muscle shortening directly from observed posture will facilitate future work. Moreover, the generality of our equation, which uses anatomical values that can be measured in many types of animals, means that our framework for understanding multiarticular muscle function can be applied in numerous study systems to provide a stronger mechanistic understanding of organismal function. 

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

   https://doi.org/10.1098/rspb.2025.0078  

   Schneider, Nikole G; Henchal, Nicholas A; Diaz_Jr, Raul E; Anderson, Christopher V (March 2025, Proceedings of the Royal Society B: Biological Sciences)

   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. 

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5. One size does not fit all: diversity of length–force properties of obliquely striated muscles

   https://doi.org/10.1242/jeb.244949  

   Thompson, Joseph T.; Taylor-Burt, Kari R.; Kier, William M. (April 2023, Journal of Experimental Biology)

   ABSTRACT Obliquely striated muscles occur in 17+ phyla, likely evolving repeatedly, yet the implications of oblique striation for muscle function are unknown. Contrary to the belief that oblique striation allows high force output over extraordinary length ranges (i.e. superelongation), recent work suggests diversity in operating length ranges and length–force relationships. We hypothesize oblique striation evolved to increase length–force relationship flexibility. We predict that superelongation is not a general characteristic of obliquely striated muscles and instead that length–force relationships vary with operating length range. To test these predictions, we measured length–force relationships of five obliquely striated muscles from inshore longfin squid, Doryteuthis pealeii: tentacle, funnel retractor and head retractor longitudinal fibers, and arm and fin transverse fibers. Consistent with superelongation, the tentacle length–force relationship had a long descending limb, whereas all others exhibited limited descending limbs. The ascending limb at 0.6P0 was significantly broader (P<0.001) for the tentacle length–force relationship (0.43±0.04L0; where L0 is the preparation length that produced peak isometric stress, P0) than for the arm (0.29±0.03L0), head retractor (0.24±0.06L0), fin (0.20±0.04L0) and funnel retractor (0.27±0.03L0). The fin's narrow ascending limb differed significantly from those of the arm (P=0.004) and funnel retractor (P=0.012). We further characterized the tentacle preparation's maximum isometric stress (315±78 kPa), maximum unloaded shortening velocity (2.97±0.55L0 s−1) and ultrastructural traits (compared with the arm), which may explain its broader length–force relationship. Comparison of obliquely striated muscles across taxa revealed length–force relationship diversity, with only two species exhibiting superelongation. 

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