Full - 1999 - Templates and anchors: neuromechanical hypotheses of legged locomotion on land

Citation

Full RJ, Koditschek DE. Templates and anchors: neuromechanical hypotheses of legged locomotion on land. J Exp Biol. 1999 Dec;202(Pt 23):3325-32. PUBMED

10 Word Summary

Biological systems are complicated, but can be explained through abstraction.

Abstract

Locomotion results from complex, high-dimensional, non-linear, dynamically coupled interactions between an organism and its environment. Fortunately, simple models we call templates have been and can be made to resolve the redundancy of multiple legs, joints and muscles by seeking synergies and symmetries. A template is the simplest model (least number of variables and parameters) that exhibits a targeted behavior. For example, diverse species that differ in skeletal type, leg number and posture run in a stable manner like sagittal- and horizontal-plane spring-mass systems. Templates suggest control strategies that can be tested against empirical data. Templates must be grounded in more detailed morphological and physiological models to ask specific questions about multiple legs, the joint torques that actuate them, the recruitment of muscles that produce those torques and the neural networks that activate the ensemble. We term these more elaborate models anchors. They introduce representations of specific biological details whose mechanism of coordination is of interest. Since mechanisms require controls, anchors incorporate specific hypotheses concerning the manner in which unnecessary motion or energy from legs, joints and muscles is removed, leaving behind the behavior of the body in the low-degree-of-freedom template. Locating the origin of control is a challenge because neural and mechanical systems are dynamically coupled and both play a role. The control of slow, variable-frequency locomotion appears to be dominated by the nervous system, whereas during rapid, rhythmic locomotion, the control may reside more within the mechanical system. Anchored templates of many-legged, sprawled-postured animals suggest that passive, dynamic self-stabilization from a feedforward, tuned mechanical system can reject rapid perturbations and simplify control. Future progress would benefit from the creation of a field embracing comparative neuromechanics.

Notes

• Terminology:
  • Templates - reduced order model of system
  • Anchor - link between template and physiological system (a more complicated description)
  • Curse of Dimensionality - (Bellman) each additional dimension causes an exponential increase in "space".
• Why use a template?
  • As a means for framing hypotheses.
    • Is CoM related postural control? Use anchor as means to test validity of template.
  • Templates should have experimentally refutable hypotheses.
  • A means of comparing "goals" and "control strategies".
  • Examples of templates are:
    • Inverted pendulum -> walking
    • Spring-loaded inverted pendulum -> running
• Why use an anchor?
  • Explain causal relationship between biology and template.
  • Allow incorporation of multiple templates.
• How much information can be uncovered? What about complex behaviors (such as chaos)? Do these go into the template or anchor?
• Steps for implementing Template-Anchor
  • Make a simple model based on intuition/observation
    • Pose hypothesis
    • Does model explain/predict behavior?
  • Formulate an anchor - a bridge between biology and the model
    • Revisit the hypothesis and ask finer grained questions.
• In terms of our own system what is our goal?
  • Control: What does this mean for a standing task?
    • Stability, CoM above BoS, regulating dynamics.
• Mechanical feedback
  • Interesting concept but doesn't change things as far as control analysis is concerned. It does point to a change in how things (robots/machines) could be designed. Spend time getting the mechanics to solve the problem instead of wrapping a controller around it to force it to have the desired dynamics.
  • Uses the term "pre-flexes" for the mechanical "feedback" of muscle.
• Why is this paper important?
  • It provides a framework for hierarchical modeling of complex systems.