# Rat superficial masseter operates at long lengths during biting

## TL;DR

This study shows that rat jaw muscles operate at long lengths during biting, challenging the belief that they work best at short lengths.

## Contribution

The study reveals that rat jaw muscles operate at long lengths during high-force biting, challenging previous assumptions about muscle function.

## Key findings

- Rat superficial masseter muscles operate at lengths up to 27% beyond optimal during high-force biting.
- Bite forces reached up to 10.6 N/cm², influenced by muscle activation and food hardness.
- Findings suggest jaw muscles may function unstably at long lengths, impacting craniofacial evolution and muscle mechanics.

## 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 (LO), we hypothesized that LO of 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 force in vivo during biting on food with varying hardness and mapped the in vivo data from each muscle (N = 6) onto its FL relationship, measured in situ. Submaximal LO was approx. 12% shorter than twitch LO, 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% below LO (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.

The online version contains supplementary material available at 10.1038/s41598-025-21953-z.