A new study from the University of Michigan reveals that the flow of water within muscle fibers may determine how quickly muscles can contract.
Muscle fibers are made up of about 70% water, and understanding how this fluid affects muscle performance has been a mystery until now.
Suraj Shankar, a physicist at the University of Michigan, and L. Mahadevan, a professor of physics at Harvard University, developed a theoretical model to explore the role of water in muscle contraction.
Their research shows that the movement of fluid within muscle fibers plays a crucial role in how fast muscles can contract.
Muscle fibers are filled with various components, such as proteins, cell nuclei, and molecular motors like myosin, which help convert chemical fuel into motion.
Shankar and Mahadevan compare muscle fibers to active sponges, which contract and squeeze themselves using these molecular motors.
They propose that the movement of water through these fibers limits how quickly a muscle can twitch.
To test their theory, the researchers studied muscle movements in a range of animals, including mammals, insects, birds, fish, and reptiles.
They discovered that for animals with very fast muscle contractions, such as those producing sound like a rattlesnake’s rattle, the speed of contraction is more influenced by the nervous system and molecular properties rather than fluid flow.
In smaller animals, like flying insects that beat their wings rapidly, the speed of muscle contractions is too fast for the nervous system to control directly.
Here, the flow of fluid within muscle fibers becomes more important. Shankar’s team found that fluid movement within the muscles limits how quickly these insects can contract their muscles.
In some insects, such as mosquitoes, their muscle contractions seem to be close to the theoretical limit predicted by the study. However, further experiments are needed to confirm these predictions.
The study also uncovered a new type of muscle elasticity called “odd elasticity.” Unlike a rubber band that stores and releases energy in a predictable way, muscle fibers behave differently. When a muscle contracts and relaxes lengthwise, it also bulges out sideways. This unique property allows muscles to generate power from repetitive deformations, functioning like a soft engine.
This discovery challenges the previous focus on molecular details alone and highlights the importance of muscle hydration and its three-dimensional structure.
By incorporating these factors, the researchers suggest a revised understanding of muscle function, which could influence how we study and design muscles and muscle-like materials in the future.
