Curated by RSF Research Staff
The emergent physics of animal locomotion
By: Simon Sponberg --- an assistant professor of physics and of biological sciences at Georgia Tech in Atlanta.
Moths flitting between flowers on a moonlit night and cockroaches scurrying underfoot are dynamical systems. Like many other animals, they get around with a seeming ease and agility that we humans find hard to replicate in systems we create. It may seem that we ought to know everything there is to know about animal locomotion. But we have yet to meet Richard Feynman’s provocative standard, “What I cannot
create, I do not understand.” The failure is not due to a limitation of our engineering abilities; rather, it reflects the difficulty of puzzling out how movement emerges from the physical and physiological systems of organisms. We cannot yet emulate the motility seen in nature nor derive the behavior.
The mystery is not new. In his prescient What Is Life? (1944), Erwin Schrödinger wrote about the possible limits of reductionism in explicating the physics of life and recognized that new principles might arise from examining life at the organism scale:
From all we have learnt about the structure of living matter, we must be prepared to find it working in a manner that cannot be reduced to the ordinary laws of physics. And that not on the ground that there is any “new force” or what not, directing the behaviour of the single atoms within a living organism, but because the construction is different from anything we have yet tested in the physical laboratory.
Today a physics approach to complex systems and a biomechanics approach to living systems are being integrated in the physics laboratory.
NEUROMECHANICS, A SCIENCE OF MOVEMENT
All animals can move. They (and some plants, fungi, and prokaryotes) have evolved diverse movement strategies that rely on generating stable and maneuverable dynamics even when the world is uncertain, slippery, compliant, or flowing.
When moving, animals must acquire, process, and act on information from neurons, muscles, the body, and the external environment. Figure 1 illustrates in schematic form how those systems interact. Neural feedback includes reflexes akin to the response that occurs when stretch receptors in muscles detect that one of our knee tendons has been tapped at the doctor’s office. Such feedback also encompasses the visual and vestibular (inner ear) signals and the other ways we sense our bodies and our environments. All those signals are processed by sensory neurons and alter the brain’s activation of muscles. However, the forces that muscles produce are not simple functions of neural commands: Muscles are also state dependent. Change the strain on a muscle by suddenly extending the joint and the force will change, even without a change in the neural activation. State dependence is not surprising considering that muscle is a complex material made of a fluid-filled, hierarchical tissue composed of millions of microscopic motor proteins arranged into an active crystal.
Neuromechanics considers how movement arises through the interplay of multiple physiological systems and their interactions with the environment around the animal. Understanding how those systems control movement remains elusive in part because no one animal can serve to test all neuromechanical hypotheses. Researchers can, however, turn to the idea that “for such a large number of problems there will be some animal of choice, or a few such animals, on which it can be most conveniently studied,” as August Krogh wrote in his 1929 American Journal of Physiology article, “The progress of physiology.”
To fulfill Schrödinger’s hope for new laws of organisms, we can discover themes in individual well-suited exemplars, as Krogh’s principle would encourage. But we also can systematically relate the dynamics of movement across many species and explicitly incorporate the shared evolutionary history of organisms. We can also hope, with experimental input, to determine general principles that span organisms and that could translate even to nonbiological systems such as robots
Continue reading at: Physics Today: the emergent physics of animal locomotion