the basics - muscle signaling and turnover

Despite its rigid appearance, muscles react to many stimuli and undergo constant remodeling to meet physiological demands and environmental challenges. External stimuli and challenges include activation of muscle receptors, like the adrenergic receptors, action of growth hormones, exposure to pathogens or cellular stresses like disease induced changes. Cross striated muscles also contain several stretch-sensing and strecth-responsive elements within the Z-disc, I-band and M-band. Their stretch-dependent activation leads to the expression of muscle genes to meet physiological demands.

Curiously (and rather thankfully in case of the heart), the sarcomeres within cross-striated muscles are able to regenerate while maintaining their functionality. This fact indicates that muscles developed highly specialized maintenance mechanisms that enables them to dispose of damaged proteins and substitute them with newly synthesized replacements while staying functional.