Investigating the Prevalence of Hybrid Muscle Fibers in Skeletal Muscle of Lean Adults: Myosin Heavy Chain Isoform Expression Analysis

The various muscle fiber types as they are determined by their composition of the myosin heavy chain (MHC) isoform, whether it is pure type I (slow twitch) or pure type II (fast twitch), or a hybrid fiber type, are known to impact the overall metabolic function in humans (Serrano et al., 2019). While it is recognized that active high performing individuals (such as strength athletes) have a larger distribution of type IIa muscle fibers, the distribution of hybrid fibers in muscle of lean and physically inactive human fibers remains unknown (Serrano el. Al., 2019). Hybrid fibers are thought to be fibers in the transitional phase of muscle adaptation (Williamson et al. 2001). Previous studies have shown that resistance exercise training increases specialized fiber composition as compared to hybrid variants. Given the relationship between reduced metabolic activity and quantity of transitional fibers, it is possible that sedentary individuals may have a less effective metabolic profile. This study examines the composition of MHC content in muscle fibers, with a special emphasis on describing hybrid fibers in lean, physically-inactive individuals, to improve our understanding of muscle adaptation and metabolic health. The study provides a directed analysis of hybrid fibers into I/IIa and IIa/IIx groups in physically inactive humans, which adds to our knowledge of muscle fiber specialization in response to physical activity. This research proposes to test the hypothesis that an individual's distribution of muscle fiber types is influenced by the prevalence of hybrid fiber types, when categorized using single fiber SDS-PAGE, with a specific focus on lean individuals. Through single muscle fiber SDS PAGE analysis of 1821 fibers from 11 participants, 293 were identified as hybrid fibers, with a greater prevalence of IIa/IIx hybrids compared to lean-active healthy adults. Through this investigation, we aim to contribute valuable insights into the complex interplay between muscle fiber types, potentially informing future therapeutic strategies targeting muscle function and metabolic health.