A recent study from the University of Michigan suggests that the flow of water within muscle fibers plays a crucial role in determining how quickly muscles can contract.
Most animals rely on muscles for movement, and it’s well-known that muscles, like other cells, contain around 70% water. However, the factors that influence muscle performance limits have not been fully understood. Past research on muscle function has mainly focused on the molecular level, overlooking the three-dimensional structure of muscle fibers filled with fluid.
Physicist Suraj Shankar from U-M and L. Mahadevan, a physics professor at Harvard University, developed a theoretical model to explore the impact of water flow on muscle contraction speed. They discovered that the movement of fluid within muscle fibers dictates how rapidly a muscle can contract.
They also identified a unique type of elasticity in muscles, known as odd elasticity, which allows muscles to generate power through three-dimensional deformations. This phenomenon explains why muscles bulge perpendicularly when contracting lengthwise.
The implications of this research extend beyond muscle physiology and could influence the design of soft actuators, fast artificial muscles, and shape-morphing materials, as mentioned in a publication in Nature Physics.
Shankar emphasized that viewing muscle as a complex, hierarchically structured material rather than just a collection of molecules reveals new insights into muscle contraction mechanisms.
The researchers likened muscle fibers to self-squeezing active sponges, where various components such as proteins, mitochondria, and molecular motors interact in a water-filled porous network. This squeezing action, driven by water movement, sets a constraint on how rapidly muscles can twitch.
By studying muscle movements in diverse organisms, the researchers observed that for animals requiring rapid muscle contractions (like flying insects), fluid flows within muscle fibers play a significant role in controlling contraction speed.
Furthermore, the study uncovered the concept of odd elasticity in muscles, where repetitive deformations enable muscles to act as soft engines and generate power. This property challenges traditional views on muscle function and underscores the importance of considering muscle physiology at various scales.
