Abstract
Human muscle fibers are generally classified by myosin heavy chain (MHC) isoforms characterized by slow to fast contractile speeds. Type I, or slow-twitch fibers, are seen in high abundance in elite endurance athletes, such as long-distance runners and cyclists. Alternatively, fast-twitch IIa and IIx fibers are abundant in elite power athletes, such as weightlifters and sprinters. While cross-sectional comparisons have shown marked differences between athletes, longitudinal data have not clearly converged on patterns in fiber type shifts over time, particularly between slow and fast fibers. However, not all fiber type identification techniques are created equal and, thus, may limit interpretation. Hybrid fibers, which express more than one MHC type (I/IIa, IIa/IIx, I/IIa/IIx), may make up a significant proportion of fibers. The measurement of the distribution of fibers would necessitate the ability to identify hybrid fibers, which is best done through single fiber analysis. Current evidence using the most appropriate techniques suggests a clear ability of fibers to shift between hybrid and pure fibers as well as between slow and fast fiber types. The context and extent to which this occurs, along with the limitations of current evidence, are discussed herein.
Highlights
Skeletal muscle contains a heterogeneous make-up of different fiber types that exist on a continuum of slow to fast, thereby making their function task-specific
Even amongst fibers of the same type, there are structural and functional characteristic differences; Myosin heavy chain (MHC) type is a somewhat crude but useful method to classify fiber types based on the relationship between MHC type and fiber function
The original biopsy site, and other factors involved with tissue extraction and processing it iscan possible that different single muscle fibers could be compared from pre- to postinfluence the downstream analysis and inferences
Summary
Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations. Type IIx fibers, or fast glycolytic fibers, possess the fastest twitch speeds but are highly fatigable [1] These characteristics differentiate the respective utility of each fiber type in a variety of real-life contexts [2] and, are an area of great interest in both healthy and diseased populations. The ability of fiber types to transition from slow to fast and vice versa has been an ongoing topic of contention and investigation. It is well-established that an individual’s muscle fiber type composition can be relatively predictive of sports performance, with a greater proportion of type I fibers predictive of success in slower, longer distance events [3]. We discuss the limitations of current research, implications for practical application, and future directions for study
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