<scp>MALICoT</scp> : A pilot cross‐sectional study analyzing the effects of power training and age on endomysium content and fiber area in the soleus muscle of adult males

The cross-sectional Master Athletic Laboratory Study of Intramuscular Connective Tissue (MALICoT, DRKS00015764) examined effects of athletic exercise and age on endomysium content in human soleus muscle. Forty-three clinically healthy male participants were grouped into young (20-35 years) nonphysically active controls (n = 12), young power-trained athletes (n = 10), older (60-75 years) nonphysically active controls (n = 11), and older power-trained athletes (n = 10). Biopsy specimens from the left soleus muscle of each participant were cryo-sectioned for (i) routine histological staining for myopathological evaluation, (ii) deep learning-based image analysis of H&E- and MHC-stained sections, (iii) laminin-γ-1/collagen IV, collagen I, and collagen III immunofluorescence image analysis, and (iv) quantitative proteomics. Myopathological evaluation revealed normal skeletal muscle in 26 participants, while 11, 4, 1 and 1 biopsies showed unspecific myopathological changes, chronic neurogenic atrophy, type-II fiber atrophy, and unspecific myositic changes, respectively. Analysis of H&E-, MHC-, laminin-, and collagen-stained sections revealed a 1.3-fold higher level of "mean fiber area" in young power-trained and aged unathletic participants. No significant effect was detected in the endomysium content. Proteomic analysis revealed no group-specific differences except that plasma membrane calcium-transporting ATPase 2 was less abundant in samples from aged power-trained athletes. Overall, neither athletic exercise nor age had an effect on the content or composition of the endomysium in the soleus muscle of male humans.