Mechanosensing is required for the senses of touch and hearing, and impacts on cellular processes such as cell differentiation, migration, invasion and tissue homeostasis. Mechanical inputs give rise to p38- and JNK-signaling, which mediates adaptive physiological responses in various tissues. In muscle, fiber contraction-induced p38 and JNK signaling ensures adaptation to exercise, muscle repair and hypertrophy. However, the mechanism by which muscle fibers sense mechanical load to activate this signaling, as well as the physiological roles of mechanical stress sensing more broadly, have remained elusive. Here, we show that the upstream MAP3K ZAK is a sensor of cellular compression induced by osmotic shock and cyclic compression in vitro, and muscle contraction in vivo. This function relies on ZAK’s ability to recognize stress fibers in cells and the corresponding Z-discs in muscle fibers, when under tension. Consequently, ZAK-deficient mice present with skeletal muscle defects characterized by fibers with centralized nuclei and progressive adaptation towards a slower myosin profile. Our results highlight how cells in general sense mechanical compressive load, and how mechanical forces generated during muscle contraction are translated into MAP kinase signaling.