Predicting myosin heavy chain isoform from postdissection fiber length in human skeletal muscle fibers

Experimental techniques in single human skeletal muscle cells require manual dissection. Unlike other mammalian species, human skeletal muscle is characterized by a heterogeneous mixture of myosin heavy chain (MHC) isoforms, typically used to define "fiber type," which profoundly influences cellular function. Therefore, it is beneficial to predict MHC isoform at the time of dissection, facilitating a more balanced fiber-type distribution from a potentially imbalanced sample. Although researchers performing single fiber dissection report predicting fiber-type based on mechanical properties of fibers upon dissection, a rigorous examination of this approach has not been performed. Therefore, we measured normalized fiber length (expressed as a % of the length of the bundle from which the fiber was dissected) in single fibers immediately following dissection. Six hundred sixty-eight individual fibers were dissected from muscle tissue samples from healthy, young adults to assess whether this characteristic could differentiate fibers containing MHC I ("slow" fiber type) or not ("fast" fiber type). Using receiver operator characteristic (ROC) curves, we found that differences in normalized fiber length (114 +/- 13%, MHC I; 124 +/- 17%, MHC IIA, P < 0.01) could be used to predict fiber type with excellent reliability (area under the curve = 0.72). We extended these analyses to include older adults (2 females, 1 male) to demonstrate the durability of this approach in fibers with likely different morphology and mechanical characteristics. We report that MHC isoform expression in human skeletal muscle fibers can be predicted at the time of dissection, regardless of origin. NEW & NOTEWORTHY A priori estimation of myosin heavy chain (MHC) isoform in individual muscle fibers may bias the relative abundance of fiber types in subsequent assessment. Until now, no standardized assessment approach has been proposed to characterize fibers at the time of dissection. We demonstrate an approach based on normalized fiber length that may dramatically bias a sample toward slow twitch (MHC I) or fast twitch (not MHC I) fiber populations.