**1. Introduction**

In addition to the universal physiological, metabolic and regulatory challenges of almost all cellular entities in the body, muscle tissues have to constantly adapt to a variety of biological

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issues related to high energy demand, elevated levels of cellular stress and enormous physi‐ cal strains during excitation–contraction–relaxation cycles [1]. The survival of skeletal muscle fibres therefore depends heavily on (i) a high degree of physiological adaptability, (ii) a unique level of tissue plasticity, (iii) efficient molecular chaperoning to prevent proteotoxic insults and (iv) a sophisticated repair machinery that can counter-act frequent cellular injuries [2]. A crucial stabilizing element that is intrinsically involved in this continuous maintenance of contractile tissues is the extracellular matrix (ECM) [3]. On the one hand, the complex layers of the muscle ECM provide the physical structure for force transmission between contracting fibres and their surrounding tissue environment [4], and on the other hand the ECM functions as an embed‐ ding medium for essential supportive components of muscles such as capillaries and motor neurons [5].

The composition and organization of the ECM adapts considerably in response to changed functional or structural demands during myogenesis, fibre maturation and exercise-induced changes [6]. During the natural aging process and in association with a variety of muscular disorders, a hyperactive connective tissue may trigger myofibrosis with a detrimental impact on muscle elasticity and fibre contractility [7]. Physiological or pathological changes in the muscle ECM frequently mirror the different phases of altered muscle structure and function [8]. The main component of the ECM is represented by collagen, which exits in a large number of isoforms that connect with proteoglycans, matricellular proteins and adhesion receptors to form an elaborate extracellular network and tight cell–matrix interactions [9].

This chapter provides an overview of the molecular components of the ECM from skeletal muscle and describes the proteomic concept of the ECM complexome. The formation, matu‐ ration and flexibility within the various layers of the ECM in developing, maturing and adapting skeletal muscles is outlined, as well as the crucial role of myofibrosis in neuromus‐ cular pathology.
