The tension felt in your muscles when you perform various stretches—like touching your toes—may be caused by fluid in your muscle fibers, according to new research from Brown University. Muscle fibers found in animals and humans are full of incompressible fluid and wrapped in a web of collagen and connective tissue. As a muscle stretches, this sheath lengthens and compresses, narrowing in diameter. Since the fluid in the muscle fiber does not compress, the stretching of the collagen and connective tissue tightens around it and creates tension.
Lead author of the study, David Sleboda, worked with professor Thomas Roberts to create a model and to analyze the behavior of bullfrog muscles in regards to muscle tension. The team measured the lengthwise stretch and tension in both the model and bullfrog muscle fibers with varied levels of fluid. Both the model and actual muscle fibers showed that a higher fluid volume correlated with increased tension for a given stretch.
Sleboda commented on the findings, asserting “Our study provides the first empirical evidence of fluid influencing muscle tension.” The experiments showed that the amount of fluid within the muscle fibers directly influences how much tension there are in the muscles. Sleboda also explained how the tension is created, stating “The fundamental problem here is a conflict of volumes. The mesh sleeve can change volume but the fiber is a constant volume. Eventually the two are going to run into each other and that’s where you see the muscle tension really shoot up.” The researchers also acknowledge other causes of muscle tension, such as kinks in the collagen mesh, as well as a protein in muscle fibers called tintin, but maintained that the fluid in muscle fibers also contributes to this tension.