title: Joint torques in a freely walking insect reveal distinct functions of leg joints in propulsion and posture control.
abstract: Determining the mechanical output of limb joints is critical for understanding the control of complex motor behaviours such as walking. In the case of insect walking, the neural infrastructure for single-joint control is well described. However, a detailed description of the motor output in form of time-varying joint torques is lacking. Here, we determine joint torques in the stick insect to identify leg joint function in the control of body height and propulsion. Torques were determined by measuring whole-body kinematics and ground reaction forces in freely walking animals. We demonstrate that despite strong differences in morphology and posture, stick insects show a functional division of joints similar to other insect model systems. Propulsion was generated by strong depression torques about the coxa-trochanter joint, not by retraction or flexion/extension torques. Torques about the respective thorax-coxa and femur-tibia joints were often directed opposite to fore-aft forces and joint movements. This suggests a posture-dependent mechanism that counteracts collapse of the leg under body load and directs the resultant force vector such that strong depression torques can control both body height and propulsion. Our findings parallel propulsive mechanisms described in other walking, jumping and flying insects, and challenge current control models of insect walking.
authors.affiliation: Department of Biological Cybernetics, Faculty of Biology, Bielefeld University, Bielefeld 33615, Germany Cognitive Interaction Technology Center of Excellence, Bielefeld University, Bielefeld 33615, Germany cdallmann@uni-bielefeld.de.
authors.forename: Chris J
authors.identifier: 0000-0002-4944-920X
authors.initials: CJ
authors.lastname: Dallmann
authors.affiliation: Department of Biological Cybernetics, Faculty of Biology, Bielefeld University, Bielefeld 33615, Germany Cognitive Interaction Technology Center of Excellence, Bielefeld University, Bielefeld 33615, Germany.
authors.forename: Volker
authors.identifier: 0000-0001-9239-4964
authors.initials: V
authors.lastname: Dürr
authors.affiliation: Department of Biological Cybernetics, Faculty of Biology, Bielefeld University, Bielefeld 33615, Germany Cognitive Interaction Technology Center of Excellence, Bielefeld University, Bielefeld 33615, Germany josef.schmitz@uni-bielefeld.de.
authors.forename: Josef
authors.identifier: 0000-0003-2054-9124
authors.initials: J
authors.lastname: Schmitz
mesh_terms: D000818:Animals; D001696:Biomechanical Phenomena; D005121:Extremities / Q000502:physiology*; D007313:Insecta / Q000502:physiology*; D007596:Joints / Q000502:physiology*; D019415:Torque; D016138:Walking / Q000502:physiology*
keywords: biomechanics; ground reaction force; insect; joint torque; motor control; walking