Quantification and characterization of grouped type I myofibers in human aging

Myofiber type grouping is a histological hallmark of age‐related motor unit remodeling. Despite the accepted concept that denervation–reinnervation events lead to myofiber type grouping, the completeness of those conversions remains unknown. Methods Type I myofiber grouping was assessed in vastus lateralis biopsies from Young (26 ± 4 years; n  = 27) and Older (66 ± 4 years; n  = 91) adults. Grouped and ungrouped type I myofibers were evaluated for phenotypic differences. Results Higher type I grouping in Older versus Young was driven by more myofibers per group (i.e., larger group size) ( P  < 0.05). In Older only, grouped type I myofibers displayed larger cross‐sectional area, more myonuclei, lower capillary supply, and more sarco(endo)plasmic reticulum calcium ATPase I (SERCA I) expression ( P  < 0.05) than ungrouped type I myofibers. Discussion Grouped type I myofibers retain type II characteristics suggesting that conversion during denervation–reinnervation events is either progressive or incomplete. Muscle Nerve 57 : E52–E59, 2018