Every movement your body makes depends on a microscopic chemical balance within individual cells.
Researchers at the University of Wisconsin-Madison developed a high-sensitivity method to analyze these differences in single human muscle fibers. Their work identified 37 unique protein variations that help explain how our muscles function at the molecular level.
Mapping the molecular diversity of human muscle
Human movement relies on the specialized makeup of our skeletal muscles. These muscles consist of thousands of individual fibers, each with its own characteristics.
Muscle cells can be categorized as slow Type I fibers or fast Type IIA and IIX types. Slow fibers support posture and endurance, while fast fibers provide the quick bursts of power needed for sprinting or jumping. The specific balance of these types in your body determines how you move and maintain your balance.
Muscle proteomics is the study of all proteins within these fibers. Traditional research often looks at muscle as a single mass, which hides the important differences between individual cells by using a “bottom-up” approach, which breaks proteins into small fragments called peptides for analysis. While this helps identify which proteins are present, it destroys information about proteoforms—a specific version of a protein that results from genetic variation or changes made after the protein is built.
These variations often dictate how muscles function; however, developing a way to see these variations has been a significant hurdle.
“High sensitivity top–down proteomics studies at the single cell level remain challenging due to the limited protein content of a single cell and the complexity of proteoform abundance as a response to environmental and physiological factors,” said the authors of the latest study.
Previous work has focused on rodent models or large tissue samples. The new study aimed to overcome these difficulties by developing a method to look at individual human muscle fibers at the proteoform level.
High-sensitivity techniques for single-fiber muscle proteomics
The team used skinned fibers from the vastus lateralis, a large thigh muscle, which involved removing the cell’s outer membrane with chemicals. By stripping away this barrier, they isolated the sarcomere—the internal structure that creates physical force. This technique allowed the researchers to measure how a fiber contracts before they analyze its molecular makeup.
Extracting proteins from a single human cell is difficult, as the samples are so small. The team optimized a new protocol that dissolves the fibers completely without the need for mechanical grinding or extreme heat. They then used liquid chromatography and mass spectrometry to identify the proteins. This “top-down” approach kept the proteins intact and allowed the researchers to identify specific versions of each protein without losing the details of their modifications.
This high-sensitivity approach allowed the team to identify with extreme precision.
The method successfully identified 37 different protein variations, including troponins and myosin light chains. The analysis showed that these proteins varied significantly between individuals and between fibers within the same donor.
Troponins
A group of proteins that act like a switch, starting muscle contraction whenever calcium is present in the cell.
Myosin light chains
Small proteins that provide structural support to the muscle’s “motors,” helping to regulate the speed and force of a contraction.
The team also detected specific modifications, such as the monophosphorylated versions of alpha-tropomyosin. These small chemical changes are important for muscle contraction.
Alpha-tropomyosin
A rod-shaped protein that controls the interaction between muscle filaments, essentially blocking or allowing the contraction cycle to happen.
Applying protein analysis to clinical muscle health
By documenting variability, the study provides a more accurate picture of the diversity within human muscle tissue.
The new method also helps scientists to connect a cell’s chemical structure directly to its physical performance. Instead of guessing how a fiber might behave based on its general type, researchers can now see the exact proteoforms responsible for its strength or speed. This link between form and function is essential for understanding how human bodies adapt to different physical demands, and it moves the field of muscle proteomics toward a more precise, individualized science.
The researchers validated their method using a small group of six healthy donors, and future work is needed to determine if these patterns hold true across larger and more diverse groups of people. The technical requirements for this type of analysis are also high: handling the tiny amount of protein found in a single human cell requires high-resolution equipment and a high level of expertise.
Despite these challenges, the study establishes a foundation for future research into aging and muscle disease. Understanding how individual fibers change as people age, or when they are sick, is necessary for finding new ways to maintain muscle health.
“This optimized protocol provides a sensitive and accessible analytical platform for future quantitative studies of single human muscle fiber heterogeneity in biological, disease, preclinical, and clinical studies,” said the authors.
Reference: Wilson MC, Gao Z, Lopez JR, et al. Top–down proteomics of skinned human muscle fibers reveals proteoform‐resolved fiber‐to‐fiber variability. J Mass Spectrom. 2026;61(3):e70040. doi: 10.1002/jms.70040