Skeletal muscle as an endocrine organ, and what myokines are thought to do

A tissue that was assumed to be silent

Skeletal muscle was long described in mechanical and metabolic terms, as a consumer of fuel and a producer of force. The recognition that contracting muscle releases proteins into the circulation shifted it into the category of secretory tissues. These released factors were given a collective name to distinguish them from signalling molecules originating elsewhere in the body.

Their existence provides a plausible mechanism for effects of physical activity on tissues that are not themselves contracting. That mechanistic route is the reason the concept attracted attention well beyond exercise physiology and into metabolic and neurological research generally. It also gave a name to something clinicians had long observed, namely that physical activity has effects far from the tissue doing the work.

What release actually requires

Release is tied to contraction rather than to the presence of muscle tissue, which makes activity and mass separate variables. The signals triggering release involve calcium movement, energy status within the fibre and mechanical stress on the cell. Different patterns of contraction produce different release profiles, so the type of activity influences which factors appear.

Measuring release in humans generally means sampling blood across an exercise session, which captures net concentration rather than muscle output. Because other tissues also release some of the same proteins, attributing a measured rise to muscle specifically is not straightforward.

What the candidates are thought to do

Several candidate factors have been described as influencing glucose uptake, fat handling or inflammatory signalling in distant tissues. Others have been proposed as messengers between muscle and bone, or between muscle and brain, based on receptor distribution. Most of this mapping rests on cell culture and on genetically modified animals rather than on human intervention evidence.

One widely publicised candidate turned out to be measured by antibodies of questionable specificity, and its status has been contested since. The general concept of muscle-derived signalling is well supported; the specific claims about individual factors are much less secure.

Mass, activity and what each contributes

Muscle is the largest site of glucose disposal after a meal, and that capacity relates to tissue quantity and to insulin responsiveness. Contraction can promote glucose uptake through a route that does not depend on insulin signalling in the usual way. This means activity and hormonal sensitivity contribute through partly separate mechanisms rather than through one shared pathway.

Loss of muscle mass with age is a well-documented phenomenon and it is measured through imaging or through functional testing. Distinguishing a loss of mass from a loss of quality within the remaining tissue requires imaging or strength testing rather than weight or circumference.

Reading claims about this field

The myokine concept is frequently invoked to explain why activity affects tissues far from the working muscle, which is a legitimate use. It is also invoked to sell products claiming to reproduce those signals, which is a claim of a completely different kind. Reproducing a signalling profile pharmacologically would require knowing which factors matter and in what combination, and that is not known.

The honest description is of an active research area with a solid central observation and a great deal of unresolved detail around it. Anyone with a medical condition affecting muscle, or considering a change in activity while unwell, should discuss it with a clinician first.