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Related Experiment Video

Updated: Jan 12, 2026

In Vivo Functional Assessment of Rat Masseter Muscle Following Surgical Creation of a Volumetric Muscle Loss (VML) Injury
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Rat superficial masseter operates at long lengths during biting.

Nicolai Konow1, Brandon Reder2, Daniel Bartlett2

  • 1Department Biological Sciences, University of Massachusetts Lowell, Lowell, MA, 01854, USA. Nicolai_Konow@uml.edu.

Scientific Reports
|October 31, 2025
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Summary

Mammalian jaw muscles operate at longer lengths than previously thought, especially during hard food biting. This finding challenges existing models and impacts our understanding of jaw muscle mechanics and evolution.

Keywords:
Force-length operationLength-tension relationshipMusculoskeletal function.

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Area of Science:

  • Biomechanics
  • Mammalian Physiology
  • Skeletal Muscle Research

Background:

  • Previous studies suggested jaw muscles operate at short muscle lengths on their force-length curve.
  • This consensus was based on twitch contractions or bite point force measurements.
  • Activation intensity is known to truncate a muscle's optimal length (LO).

Purpose of the Study:

  • To test the hypothesis that LO of rat superficial masseter (SM) decreases with activation intensity.
  • To investigate if high-force biting involves muscle shortening from longer lengths on the force-length curve.
  • To map in vivo jaw muscle function onto its force-length relationship.

Main Methods:

  • In vivo measurement of muscle activation, strain, and force during biting on varied food hardness.
  • In situ measurement of the force-length relationship for each muscle.
  • Mapping of in vivo data onto the in situ force-length curve for six rat SM muscles.

Main Results:

  • Submaximal LO was approximately 12% shorter than twitch LO.
  • Rat SM bite forces averaged 4.1 ± 3.9 N/cm2, reaching up to 10.6 N/cm2.
  • Muscle length operation ranged from 7% below LO to 27% beyond, particularly during hard food biting.

Conclusions:

  • Jaw muscles operate at longer, potentially unstable lengths than previously believed.
  • This challenges the established understanding of jaw muscle mechanics and function.
  • Findings have broad implications for craniofacial evolution, muscle mechanics, and jaw musculoskeletal health.