Research from the University of Michigan reveals how the flow of water within muscle fiber affects the speed of muscle contraction. Almost all organisms use muscles for movement, and it is known that muscles, like all other cells, contain about 70% water. However, scientists still do not know what sets the performance limits of muscles. Previous research has focused on the molecular level of muscles, neglecting the fact that muscle fibers are three-dimensional and full of fluid.

Physicist Suraj Shankar from the University of Michigan and L. Mahadevan, a professor of physics at Harvard University, created a theoretical model that shows the role of water in muscle contraction. They found that the way fluid moves through muscle fiber determines the speed of contraction.

They also discovered a new type of elasticity called peculiar elasticity, which allows muscles to generate force using three-dimensional deformations. This phenomenon is visible when muscle fiber contracts longitudinally, causing transverse bulging as well.

This framework can be applied to many other cells and tissues, which are also mostly composed of water, and can be applied to ultra-fast movements of single-celled microorganisms. Their findings, published in the journal Nature Physics, could impact the design of soft actuators, fast artificial muscles, and shape-changing materials, which currently have slow contraction speeds because they are externally activated.

Scientists visualize each muscle fiber as an active sponge that squeezes itself, a sponge-like material full of water, which can contract and squeeze using molecular motors.

"Muscle fibers consist of many components, such as proteins, cell nuclei, organelles like mitochondria, and molecular motors like myosin, which convert chemical fuel into motion and drive muscle contraction," said Shankar. "All these components form a porous network surrounded by water. So, it is appropriate to describe muscles as active sponges."

The squeezing process requires time to move water, so researchers hypothesized that this water movement through muscle fiber sets the upper limit of muscle fiber twitch speed.

To test their theory, they modeled muscle movements in various organisms, including mammals, insects, birds, fish, and reptiles, focusing on animals that use muscles for rapid movements. They found that muscles that produce sound, such as the rattling in a rattlesnake's tail, do not depend on fluid flow. Instead, these contractions are controlled by the nervous system and are more determined by molecular properties.

In smaller organisms, such as flying insects that flap their wings several hundred to a thousand times per second, these contractions are too fast to be directly controlled by neurons. Here, fluid flows are more important.

"In these cases, we found that fluid flows within muscle fiber are important and that our active hydraulic mechanism likely limits the fastest contraction speeds," said Shankar. "Some insects, like mosquitoes, seem close to our theoretically predicted limit, but direct experimental testing is needed."

They also found that when muscle fibers act as active sponges, the process also causes muscles to act as active elastic engines. When something is elastic, like a rubber band, it stores energy while resisting deformation. Imagine holding a rubber band between two fingers and pulling it back. When you release the rubber band, it also releases the energy stored while it was stretched.

But when a muscle converts chemical fuel into mechanical work, it can produce energy like an engine, violating the conservation of energy law. In this case, muscles exhibit a new trait called "peculiar elasticity," where the response to squeezing in one direction is not reciprocal. Unlike a rubber band, when muscles contract and relax along their length, they also bulge transversely, and their energy is not the same.

"These results contradict the prevailing view that focuses on molecular details and neglects the fact that muscles are long and fibrous, hydrated, and have processes on multiple scales," said Shankar. "Our results suggest a revised view of muscle function that is essential for understanding their physiology. This is also crucial for understanding the origins, scope, and limits underlying various forms of animal movement."

Source: University of Michigan