Physical limits to muscle-driven motion – Evolutionary Biomechanics Group

Scaling of musculoskeletal performance

Animals large and small use muscle to move – but can they all achieve the same relative performance? We combine results from dimensional analyses with empirical data on muscle physiology and anatomy to explain (and ideally predict!) the variation of animal performance with size. By linking physical models with animal size, we hope to identify opportunities for non-trivial musculoskeletal adaptations that distinguish animals across body sizes and locomotor mode.

3D Kinematics

Insects seem to move effortlessly in a variety of habitats. They can walk upright, climb up vertical walls, stick to inverted surfaces, and are robust to the loss of one or even multiple limbs. We track points of interest using markerless pose estimation and reconstruct the data in 3D for further mechanical and statistical analysis. Why do insects move the way they do?

Dimensionless numbers for muscle

Striated muscle looks more or less the same, whether an animal swims, runs, crawls, or flies; whether it is smaller than the tip of a sharp pencil or heavier than 15 school buses; whether it first appeared millions or only thousands of years ago. Does this shared design have shared implications? Starting from first principles, we seek to identify dimensionless numbers that enable fair comparison of muscle performance across animal size, physiological make-up, and environmental setting.

Read A theory of physiological similarity in muscle-driven motion

Insect recording stations

Obtaining 3D kinematics data of insects remains difficult and is at best tedious – insects generally do not cooperate.

In one of the lab’s many episodes of Insects vs Engineers, we developed a planar omnidirectional treadmill to simplify and speed up the recording of 3D kinematics data. The treadmill design allows observation of natural locomotion on a flat surface; insects can freely select speed and walking direction, and yet they still don’t get away – we now can record continuous gait cycles over long time periods. Take that, insects!

Robotics-informed biology

Insects can be annoyingly stubborn, and they refuse to let themselves be trained. As a consequence, we can only observe the performance for a limited subsection of the total kinematic space. What if they moved their limbs differently – would they perform better, worse, or just the same? Answering these questions is important to understand the evolutionary pressures that have shaped terrestrial insect locomotion. We use “simple” robotic insect models and their “digital twins” to gain full control over kinematics and dynamics, so that we can map out performance spaces, and observe which regions of these spaces are occupied by insects.

Effect of surface wettability on pad adhesion

How well can insects cling to surfaces of different properties?

On rough surfaces, such as rocks or tree bark, insects can employ not only their pads but also their claws to help them attach. On smooth surfaces, however, claws become useless, and insects have to rely on intermolecular forces. What role does surface chemistry play in the pad-surface interaction? Variations in surface chemistry lead to differences in the ability of contacting liquids to spread – different surface chemistry usually implies different wettability. To test whether surface chemistry influences adhesion, we produce smooth surfaces of varying wettability by Chemical Vapour Deposition (CVD), characterise them by goniometry and Atomic Force Microscopy, and perform single pad adhesion force measurements using a custom-made fibre-optic force sensor.

Scaling of pad efficiency

How do climbing animals safely attach to surfaces across large difference in body size?

Textbook scaling theory predicts that larger animals are increasingly challenged: the force that adhesive patches needs to sustain depends on body volume, but the sticky area depends on body surface area. One way to overcome this problem would be a disproportionate increase in pad area. However, animal footpads only become disproportionally larger across very distant groups; within closely related clades, they appear to become more sticky instead! What is the source of this enhanced efficiency? We explore this problem theoretically, and through experimental work which focusses on the physical properties of the insect pad secretion.

We test whether the surface tension and viscosity of the secretion vary systematically with size, by combining Interference Reflection Microscopy and dewetting measurements on individuals spanning almost 3 orders of magnitude in mass. Learning how natural adhesive organs overcome stress concentrations and scaling issues to maintain their attachment performance may inform the next generation of artificial adhesives.

Read: Shear-sensitive adhesion enables size-independent adhesive performance in stick insects | The physical properties of the stick insect pad secretion are independent of body size

Muscle morphology & bite forces

To cut fragments from vegetation, leaf-cutter ants need to generate bite forces that exceed their body weight by almost a factor of 1000. How do these small insects manage to generate such large forces?

We approach this question in several ways: First, we directly measure bite forces. Second, we take a closer look inside the head capsule, using micro-computed tomography, and then quantify the size and geometry of the several thousand individual muscle fibres using custom-written 3D reconstruction algorithms! Third, we develop theoretical models that directly predict bite performance from morphology and first principles biomechanics.

Read: A biomechanical model for the relation between bite force and mandibular opening angle in arthropods | Strong positive allometry of bite force in leaf-cutter ants increases the range of cuttable plant tissues | Morphological determinants of bite force capacity in insects: a biomechanical analysis of polymorphic leaf-cutter ants

Mandible joint kinematics

Insect mandibles are “multi-purpose” tools: they are used for cutting, masticating, digging, material transport, prey capture and defense. In contrast to this functional complexity, the kinematic space of most insect mandibles is thought to be fully characterised by a simple hinge joint. Although widespread, this assumption is based solely on morphological inference; quantitative kinematic analysis are lacking.

We developed a recording setup and kinematic analysis framework to rigorously test the “hinge joint hypothesis” in leaf-cutter ants. At large opening angles, mandibles yaw, consistent with a simple hinge joint. But at small opening angles, mandibles can both yaw and pitch, suggesting multiple degrees of freedom, and that insect mandibles may be capable of more complex motion than traditionally assumed.

Read: Three-dimensional kinematics of leaf-cutter ant mandibles: not all dicondylic joints are simple hinges

limits to muscle-driven motion

Muscle is

Dimensionless numbers to characterise muscle

The variation of muscle performance with animal size