Abstract

SummaryBackgroundLimb movements are generally driven by active muscular contractions working with and against passive forces arising in muscles and other structures. In relatively heavy limbs, the effects of gravity and inertia predominate, whereas in lighter limbs, passive forces intrinsic to the limb are of greater consequence. The roles of passive forces generated by muscles and tendons are well understood, but there has been little recognition that forces originating within joints themselves may also be important, and less still that these joint forces may be adapted through evolution to complement active muscle forces acting at the same joint.ResultsWe examined the roles of passive joint forces in insect legs with different arrangements of antagonist muscles. We first show that passive forces modify actively generated movements of a joint across its working range, and that they can be sufficiently strong to generate completely passive movements that are faster than active movements observed in natural behaviors. We further demonstrate that some of these forces originate within the joint itself. In legs of different species adapted to different uses (walking, jumping), these passive joint forces complement the balance of strength of the antagonist muscles acting on the joint. We show that passive joint forces are stronger where they assist the weaker of two antagonist muscles.ConclusionsIn limbs where the dictates of a key behavior produce asymmetry in muscle forces, passive joint forces can be coadapted to provide the balance needed for the effective generation of other behaviors.

Highlights

  • Most animal movements are driven by muscle contractions, but there is a substantial body of evidence that passive forces, originating in muscles, tendons, or other tissues, interact with active forces to generate limb movements in both vertebrates and invertebrates [1,2,3,4,5,6,7,8,9,10,11,12,13]

  • We have previously shown that the resting angle of the locust hind leg femur-tibia (FT) joint is dominated by the extensor tibiae muscle, suggesting strong passive forces in the extensor tibiae muscle and weaker passive forces in the flexor tibiae muscle [8]

  • We show that passive flexions of the hind leg tibia are driven by forces arising within the joint itself, not in the muscles or tendons; whereas extensions are driven by passive forces of the extensor tibiae muscle

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Summary

Background

Limb movements are generally driven by active muscular contractions working with and against passive forces arising in muscles and other structures. Results: We examined the roles of passive joint forces in insect legs with different arrangements of antagonist muscles. In legs of different species adapted to different uses (walking, jumping), these passive joint forces complement the balance of strength of the antagonist muscles acting on the joint. We show that passive joint forces are stronger where they assist the weaker of two antagonist muscles. Conclusions: In limbs where the dictates of a key behavior produce asymmetry in muscle forces, passive joint forces can be coadapted to provide the balance needed for the effective generation of other behaviors

Introduction
Results
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Experimental Procedures

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