Scientists have recently uncovered new insights into muscle contraction by examining the role of internal fluid dynamics within muscle fibers. A study published in Nature Physics, conducted by researchers from the University of Michigan and Harvard University highlights that the speed and power of muscle contractions are influenced by more than just molecular motors as they also depend on the movement of fluid within the muscles.
Active hydraulics muscle contraction speed
Traditional understanding has focused on the molecular motors within muscle cells, primarily proteins like actin and myosin, which slide past each other to generate force. However, the new research suggests a more complex mechanism involving the entire three-dimensional structure of muscle fibers, including the fluid within them.
Researchers Suraj Shankar and L. Mahadevan have developed a theoretical model treating muscle fibers as “active sponges.” This model integrates the behavior of molecular motors with the muscle’s internal elastic properties and fluid flow. Shankar explains the fluid movement, termed “active hydraulics,” imposes an upper limit on contraction speed, especially in extremely fast muscles, such as those used by certain insects for flight.
The study suggests that in fast-contracting muscles, fluid flow within the fibers may be a critical factor in determining contraction speed. For example, some insects, like mosquitoes, operate near this theoretical hydraulic limit, although experimental validation is needed.
Muscles generate power through cycles of compressing and stretching
Beyond understanding muscle mechanics, this discovery could impact the development of artificial muscles and treatments for muscle disorders. A notable aspect of the study is the identification of “odd elasticity” in muscles. This nonreciprocal behavior means the force generated when a muscle stretches in one direction is not equal and opposite to the force when compressed in the opposite direction. This property enables muscles to act as tiny engines, generating power through cycles of stretching and compressing in different directions.
Shankar points out that unlike a rubber band, a muscle bulges perpendicularly when it contracts and relaxes, allowing it to generate power from repetitive deformations. The researchers’ model also explains why muscle fibers change volume during contraction, a phenomenon not accounted for in older models.
