Abstract
Introduction: 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.