Extracellular matrix content and remodeling markers do not differ in college-aged men classified as higher- and lower-responders to resistance training.

Abstract

We determined if skeletal muscle extracellular matrix (ECM) content and remodeling markers adapted with resistance training or were associated with hypertrophic outcomes. Thirty-eight untrained males (21+/-3 yrs.) participated in whole-body resistance training (10 weeks, 2x weekly). Participants completed testing (ultrasound, peripheral quantitative computed tomography (pQCT)) and donated a vastus lateralis (VL) biopsy one week before training and 72 hours following the last training bout. Higher responders (HR, n=10) and lower responders (LR, n=10) were stratified based on a composite score considering changes in pQCT-derived mid-thigh cross-sectional area (mCSA), ultrasound-derived VL thickness, and mean fiber cross-sectional area (fCSA). In all participants, training reduced matrix metalloprotease (MMP)-14 protein (p<0.001) and increased satellite cell abundance (p<0.001); however, VL fascial thickness, ECM protein content per myofiber, MMP-2/-9 protein content, tissue inhibitor of metalloproteinase (TIMP)-1/-2 protein content, collagen-1/-4 protein content, macrophage abundance, or fibro-adipogenic progenitor cell abundance were not altered. Regarding responder analysis, MMP-14 exhibited an interaction (p=0.007), and post hoc analysis revealed higher protein content in HR versus LR prior to training (p=0.026) and a significant decrease from pre-to-post-training in HR only (p=0.002). In summary, basal skeletal muscle ECM markers are minimally affected with 10 weeks of resistance training, and these findings could be related to not capturing more dynamic alterations in the assayed markers earlier in training. However, the downregulation in MMP-14 in college-aged men classified as HR is a novel finding and warrants continued investigation, and further research is needed to delineate muscle connective tissue strength attributes between HR and LR.