Muscle is not just a motor but an endocrine organ: silent at rest, it releases hundreds of signaling molecules, called myokines, when it contracts.Skeletal muscle is ~40% of body weight. At rest it sits quiet — not just 'not moving' but not signaling. The biggest paradigm shift of the last two decades: muscle is not just a motor for movement, it is an endocrine organ that releases hundreds of signaling molecules — myokines — when it contracts.
2 · Contraction · fibers signal
Contraction, through Ca²⁺ flux, a rising AMP/ ratio and mechanical stress, prompts myocytes to secrete myokines such as and irisin, messengers rather than damage products.With each contraction, Ca²⁺ flux + rising AMP/ ratio + mechanical stress activate pathways that prompt myocytes to secrete (it can rise many-fold during exercise; how it acts during exercise differs from the persistently raised IL-6 of chronic inflammation), irisin, BDNF, IL-15, and more.
Key point: these molecules are not damage products, they are contraction messengers — which is why 'no pain no gain' is wrong; walking also releases myokines.
3 · Messengers travel · myokine targets
Myokines travel in the blood to distant organs and flip many metabolic switches, though for many links the evidence comes mainly from animal studies.Myokines travel via blood to distant organs: → liver: promotes glucose output, increases fat oxidationIrisin → white fat: promotes 'browning' (heat production); mostly animal evidence, and whether it does the same in people is still debatedBDNF → brain: linked to plasticity in the hippocampus, and thought to be one possible route for exercise's effects on thinking and mood (mostly animal and small human studies)IL-15 → fat/immune: animal studies link it to fat loss and immune-cell regulationFGF21 → liver/fat: may be linked to insulin sensitivity One bout of exercise flips many metabolic switches across the body at once; for many of these links the evidence still comes mainly from animal studies.
Within 1-2 hours after exercise, prompts the liver to release IL-1ra and sTNF-R, endogenous anti-inflammatory molecules that neutralize circulating pro-inflammatory cytokines.IL-1ra is the IL-1 receptor antagonist and sTNF-R the soluble TNF receptor; both directly neutralize circulating pro-inflammatory cytokines.
This explains why people who exercise regularly carry a lighter background of low-grade inflammation, and their baseline is usually lower too; and why a single moderate bout (a brisk walk, say) already brings a short-lived improvement in insulin sensitivity in insulin-resistant people.
Exercise is medicine — its 'delivery route' is muscle contraction, its 'active ingredients' are myokines, and its 'side effects' are being stronger and more capable.