Overview

Broadly, my research uses mechanical principles to understand how animal form relates to function.
 

Mechanical principles apply real constraints to organisms, which they manage and exploit through adaptation (evolution) and plasticity (development and learning).

Currently, I am studying how patterns in locomotion emerge from optimization processes. A prevailing hypothesis posits that organisms often choose locomotion patterns minimizing effort (metabolic energy expenditure), while fulfilling some relevant task- e.g. foraging, migrating or tracking. Through modelling and experiment, I have been putting this hypothesis to the test.

 

Current Research

Minimally-constrained model of legged locomotion

 

Conceptual models in locomotion vary from extremely abstract (e.g. a single point mass on a stick) to extremely complex (e.g. high-fidelity OpenSim musculoskeletal models). I'm developing a model in the balance; sufficiently complex to make precise predictions about behaviour (e.g. walk-run transition speeds, preferred gaits) but simple enough to understand underlying mechanisms, and applicable to a broad range of organisms.

I describe the latest model in PLOS Computational Biology. Consisting of four "pistons" connected to a rigid body, it accurately predicts walking at slow speeds and trotting at intermediate speeds and the walk-trot transition speed for dogs. Importantly, this model has no springs, suggesting that elastic tendons are not a prerequisite for the economy of these strategies.

 

The Pitch-Translation Tradeoff Hypothesis

Why do mammals trot? This "two-beat" gait involves expensive up-down oscillation of the body, that could be mitigated with distributed "four-beat" gaits. Yet trotting is the slow-running gait of choice for most mammals - though some, like giraffes, wildebeest and elephants, prefer not to trot.

I have been exploring whether the pitch moment of inertia resolves this conundrum. Pitching the body is expensive, and can be avoided with a trot. But if your body naturally resists pitching (e.g. a giraffe's long neck), then you can use alternatives to trotting.

I have validated this theory in a modelling framework (preprint, talk) and am looking to perform experiments and measurements to verify it in animals. 

 

Split belt SLIP

What does a bird have to do with split-belt treadmills? Seabirds fly along the surface of the ocean, where layers of wind move at different speeds. The birds take advantage of this velocity difference to extract energy from the wind, in a strategy called "dynamic soaring". A biped can similarly exploit the different belt speeds of a split-belt treadmill. We use a spring-loaded inverted pendulum (SLIP) model to determine what strategy extracts the most energy from a split belt treadmill– and whether this strategy is employed by humans

 

Reduced gravity locomotion

To determine if animals choose gaits to optimize metabolic energy expenditure, we can modify their environment to make the usual pattern suboptimal, while an unusual pattern becomes more economical.

Changing gravity is a pronounced environmental change with a reliable effect on an organism's mechanics. With simple conceptual models, we can make specific predictions about how humans and other animals will adapt for economical locomotion.

JEB paper, Dynamic Walking Poster, Summary Video, Longer Video

 

Past Research

Insect wing microstructure for efficient drop shedding

Insect wings are covered in small bumps, hairs or scales called microstructures. These microstructures often have a particular but locally-variable orientation. Why? For my undergraduate independent research project, I showed that the orientation may promote efficient drop shedding. Published in PLOS One

 
 

Wing rapid area change for fast perching

Many birds go from flying at a high speed to landing on a small branch in fractions of a second. One way they may accomplish this is by quickly changing the frontal area of their wings. This "rapid area change" takes advantage of strange added mass effects, quickly shedding the mass of the air attached to the wing like propellant firing out of a rocket nozzle. 

Published: J Fluid Mech, Bioinsp Biomim. See also my tutorial on added mass recovery 

© 2020 Delyle Polet

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