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Abstract
Fish skulls are complex kinetic systems with movable components that are powered by
muscles. Cranial muscles for jaw closing pull the mandible around a point of rotation
at the jaw joint using a third-order lever mechanism. The present study develops a
lever model for the jaw of fishes that uses muscle design and the Hill equation for
nonlinear length-tension properties of muscle to calculate dynamic power output. The
model uses morphometric data on skeletal dimensions and muscle proportions in order
to predict behavior and force transmission mediated by lever action. The computer
model calculates a range of dynamic parameters of jaw function including muscle force,
torque, effective mechanical advantage, jaw velocity, bite duration, bite force, work
and power. A complete list of required morphometrics is presented and a software program
(MandibLever 2.0) is available for implementing lever analysis. Results show that
simulations yield kinematics and timing profiles similar to actual fish feeding events.
Simulation of muscle properties shows that mandibles reach their peak velocity near
the start of jaw closing, peak force at the end of jaw closing, and peak power output
at about 25% of the closing cycle time. Adductor jaw muscles with different mechanical
designs must have different contractile properties and/or different muscle activity
patterns to coordinate jaw closing. The effective mechanical advantage calculated
by the model is considerably lower than the mechanical advantage estimated from morphological
lever ratios, suggesting that previous studies of morphological lever ratios have
overestimated force and underestimated velocity transmission to the mandible. A biomechanical
model of jaw closing can be used to interpret the mechanics of a wide range of jaw
mechanisms and will enable studies of the functional results of developmental and
evolutionary changes in skull morphology and physiology.