Muscle Is a Gland, Too: The Myokines Your Body Releases When You Move

For most of medicine's history, muscle was treated as plumbing. Pulleys and levers, a scaffold for movement, a place to store protein for a rainy day. You trained it the way you'd service a machine — to make the machine work better. What nobody expected was that the machine would turn out to be talking.

In the last two decades, exercise physiology has quietly rewritten muscle's job description. Skeletal muscle — the largest organ in most people's bodies, roughly 40% of body mass — is now understood to be an endocrine organ: a gland that manufactures and releases hundreds of signaling molecules into the blood every time it contracts. These molecules are called myokines, and they are one of the more honest arguments for why movement does so much, in so many places, at once.

The discovery that changed the job description

The turning point came around the year 2000, in a Copenhagen lab led by Bente Klarlund Pedersen. Her group was chasing a puzzle: interleukin-6, a molecule usually associated with inflammation, spiked dramatically in the blood during prolonged exercise — sometimes a hundred-fold — yet the people producing it were getting healthier, not sicker. The answer was that the IL-6 wasn't coming from immune cells reacting to damage. It was being secreted by the working muscle itself, on purpose, as a signal.

That reframed muscle from passive tissue into an active secretory organ, and it opened a field. The umbrella term Pedersen's group coined — myokines — now covers a long and growing catalog of proteins, peptides, and other messengers released during contraction. A 2020 review in Endocrine Reviews laid out the map; a February 2026 paper in Cells went further, describing an "endocrine code" — the idea that muscle doesn't just shout one signal but sends context-dependent combinations, timed and targeted like a language.

Muscle crosstalk

A contracting muscle sends mail across the body

A contracting muscle sends mail

The reach is what makes it interesting. Myokines don't just act locally. They travel, and they have addresses. Reviews describe muscle-derived signals reaching the brain, bone, fat tissue, liver, pancreas, the vascular system, and even skin — a web of "crosstalk" in which the muscle you contract in your legs may be shaping metabolism in an organ you never think about.

Some of these signals are metabolic housekeeping: helping working muscle draw glucose and free fatty acids for fuel, and nudging the liver and fat tissue to cooperate. Here the story is genuinely well supported. Muscle-derived IL-6, released during contraction, behaves quite differently from the persistently elevated IL-6 that tracks with chronic low-grade inflammation — a useful reminder that the same molecule can mean opposite things depending on where and why it appears.

The muscle–brain line, with honest caveats

The signal that captures the most attention runs from muscle to brain. Contraction appears to raise levels of molecules like cathepsin B and BDNF (brain-derived neurotrophic factor), which are linked in the lab to the birth of new neurons in the hippocampus and to synaptic plasticity — the machinery of learning and memory. It's a satisfying mechanism for something we already observe: that people who move tend to hold onto cognitive sharpness.

But this is exactly where a longevity writer has to slow down. Much of the muscle–brain evidence comes from mice on treadmills, not humans, and the most famous myokine of all — irisin — has spent a decade mired in argument over whether the assays used to measure it were even detecting the right thing. The direction is promising. The certainty is not there yet. Anyone selling you a supplement that "activates your myokines" is selling ahead of the science.

Why this reframes aging

Here is the part that matters for longevity. If muscle is a gland, then the age-related loss of muscle — sarcopenia — is not just a loss of strength. It is the slow shutting-down of an endocrine organ. As muscle mass and activity decline, the secretome it produces changes too, and the review literature on aging and sarcopenia regards this quieting of myokine signaling as part of the problem, not a side effect.

It reframes the daily choice, as well. The instinct with aging is to protect: sit more, spare the joints, conserve. But muscle only sends its mail when it works. A tissue that isn't asked to contract isn't just getting weaker — it's going quiet, and the rest of the body stops hearing from it. This is one reason maintaining muscle across the decades keeps showing up as a marker of healthy aging, alongside signals like grip strength.

What actually triggers the signal

The practical translation is almost annoyingly simple. The trigger for myokine release is contraction — muscle doing work — which means there is no shortcut around actually using it. You cannot swallow the signal. You can only earn it, repeatedly.

Both broad kinds of training seem to matter, in complementary ways. Resistance work builds and defends the muscle mass that does the secreting; strength training after 40 is less about aesthetics than about keeping the gland intact. Aerobic work, including easy Zone 2, drives the metabolic myokine response session after session. Consistency beats intensity here, because the signal is not stored — each bout is a fresh dose. If you want to see the effect of that consistency show up as trends over months, the Agen Band tracks the recovery and activity signals that accompany it, and a simple longevity protocol is mostly a way to keep earning the dose.

The bottom line

Muscle is not the machine we thought it was. It is a machine that also happens to be a gland — the largest one you own, and one of the few you can grow on purpose. That doesn't make exercise a medicine, and it doesn't license the tidy causal stories the wellness industry loves to tell. What it does is offer a plainer, more durable reason to keep moving: every contraction is a message, and the body was built to listen. Stop sending, and something goes quiet that no pill has yet learned to replace.