Marked irregular myofiber shape is a hallmark of human skeletal muscle ageing and is reversed by heavy resistance training

Abstract Background Age‐related loss of strength is disproportionally greater than the loss of mass, suggesting maladaptations in the neuro‐myo‐tendinous system. Myofibers are often misshaped in aged and diseased muscle, but systematic analyses of large sample sets are lacking. Our aim was to investigate myofiber shape in relation to age, exercise, myofiber type, species and sex. Methods Vastus lateralis muscle biopsies (n = 265) from 197 males and females, covering an age span of 20–97 years, were examined. The gastrocnemius and soleus muscles of 11 + 22‐month‐old male C57BL/6 mice were also examined. Immunofluorescence and ATPase stainings of muscle cross‐sections were used to measure myofiber cross‐sectional area (CSA) and perimeter. From these, a shape factor index (SFI) was calculated in a fibre‐type‐specific manner (type I/II in humans; type I/IIa/IIx/IIb in mice), with higher values indicating increased deformity. Heavy resistance training (RT) was performed three times per week for 3–4 months by a subgroup (n = 59). Correlation analyses were performed comparing SFI and CSA with age, muscle mass, maximal voluntary contraction (MVC), rate of force development and specific force (MVC/muscle mass). Results In human muscle, SFI was positively correlated with age for both type I (R2 = 0.20) and II (R2 = 0.38) myofibers. When subjects were separated into age cohorts, SFI was lower for type I (4%, P 80) compared with old. The increased SFI in old muscle was observed in myofibers of all sizes. Within all three age cohorts, type II myofiber SFI was higher than that for type I myofiber (4–13%, P