How Geometry Shapes Body Movements When Dozens of Muscles Act Together

For a robot to coordinate movement across numerous motors typically requires complex control algorithms. Animals, including humans, routinely perform movements involving dozens of muscles with ease, often without conscious effort. This contrast highlights a long-standing puzzle: how animals achieve complex coordination, and how much of it is directed by the brain versus how much arises naturally from the body’s physical structure. 

To explore this question, Northwestern Engineering researchers turned to rodent “whisking,” a rapid, rhythmic behavior driven by around 30 small muscles embedded in the face. Each whisker differs substantially in length, thickness, curvature, and position, meaning that in principle different motor commands for each whisker would be needed to produce coordinated motion across the array.

Despite this apparent need for whisker-by-whisker control, new work from a team led by Professor Mitra Hartmann shows that the three-dimensional geometry of the whisker system resolves the challenge.  

“Understanding how physical structure constrains motion may inform future work in areas such as bio-inspired robotics, prosthetic design, and neural interfaces,” Hartmann said.

By reconstructing the anatomy of whisker follicles and their associated muscles and using biomechanical simulations, the researchers quantified how the system moves when all intrinsic muscles contract by the same percentage. Despite large anatomical differences between individual whiskers, the simulations revealed that uniform muscle contraction leads to nearly identical rotational movements of whisker follicles across the array. This result establishes a geometric baseline for whisker motion, a reference point for determining which features of movement arise from biomechanics and which require more detailed neural control.

“Whisking allows us to ask what the body contributes to coordination before invoking muscle-by-muscle neural control,” said Hartmann, senior author of the study. “The geometry of the system itself plays a key role. The study also shows how the spacing between whisker tips changes during movement, shaping how animals can change tactile sensory resolution as they sample their surroundings during active tactile exploration.”

Understanding how physical structure constrains motion may inform future work in areas such as bio-inspired robotics, prosthetic design, and neural interfaces. Mitra Hartmann

Chair and professor of biomedical engineering and professor of mechanical engineering at the McCormick School of Engineering, Hartmann has spent much of her career investigating how physical structure and neural control interact to shape movement and sensation. Her research bridges neuroscience, biomechanics, and robotics, with a particular focus on active sensing—how animals move their sensory organs to acquire information about the world. Using rodent whisking as a model system, her lab has combined anatomy, physiology, and computational modeling to uncover general principles of motor coordination and sensory processing. 

Hartmann and her teammates presented their latest findings in the paper “Biomechanical Simplification of the Motor Control of Whisking,” published Jan. 22 in Current Biology.

Although grounded in a specific biological system, the findings address a broader challenge faced by both biological and engineered systems: how to generate coordinated movement in systems with many actuators without requiring equally complex control signals.

The research was a collaborative effort spanning multiple departments at Northwestern University, involving students and researchers from the Interdepartmental Neuroscience program, mechanical engineering, and biomedical engineering, including former undergraduate biomedical engineering students.