Skeletal muscle is an extremely plastic tissue: it rapidly responds to a variety of stimuli ranging from physical inactivity and disease that decrease muscle mass and quality, to exercise training that can promote hypertrophy and reshape its morphology and cellular composition.
The ability of skeletal muscle to adapt to various conditions relies on the function of different cells composing its microenvironment. Besides the multinucleated myofibers of different types, satellite cells, mesenchymal stem cells such as fibro-adipogenic progenitors, fibroblasts, endothelial cells, pericytes and immune cells contribute to tissue maintenance, post-developmental growth and recovery from injury.

This Research Topic covers the skeletal muscle microenvironment changes associated with different stimuli such as exercise, hypoxia, different dietary patterns, aging, obesity, injury, systemic diseases, bed-rest, critical illness and physical inactivity, as well as the mechanisms underlying such adaptations. Articles presenting new methods to assess skeletal muscle adaptations to physiological and pathological conditions are also welcome.

To expand the current knowledge on how skeletal muscle adapts to various conditions, we welcome human and animal model studies addressing, but not limited to, the following themes:
• Morphological, molecular and functional myofiber changes in response to aging, disease and modifiable life-style factors, such as exercise training and diet;
• Proliferation, differentiation and crosstalk of/between mononuclear cells residing in skeletal muscle (satellite cells, mesenchymal stem cells/fibro-adipogenic progenitors, endothelial cells and immune cells) during muscle adaptation, degeneration and regeneration;
• Remodeling of the skeletal muscle capillary network in response to different stimuli;
• Role of metabolic and molecular signaling pathways in adaptation and maintenance of the skeletal muscle microenvironment, muscle mass and muscle quality;
• Molecular, cellular and metabolic response of myofibers and mononuclear cells residing in skeletal muscle to critical illness and bed-rest, and its association with clinical outcomes;
• Molecular and cellular characterization of the skeletal muscle microenvironment in different cohorts, from high-level athletes to patients with different metabolic and orthopedic conditions;
• Impact of aging on the skeletal muscle microenvironment and its ability to adapt to different stimuli;
• Novel methods to assess skeletal muscle morphology, molecular biology, cellularity and function.

Original research, brief research reports, review articles, and methods articles are welcome to enrich this collection.

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Plasticity of the Skeletal Muscle Microenvironment From the Critically Ill Patient to the High-Level Athlete