A hard workout floods your bloodstream with chemical signals from muscle itself. These myokines travel to the liver, fat, and brain, and different types of exercise seem to trigger different messages.
The Post Workout Signal
Draw blood right after a hard training session and one molecule stands out: interleukin-6, or IL-6, a small protein muscle releases when it contracts.
Acute exercise can raise circulating muscle-derived IL-6 by up to 100-fold compared with resting levels. That spike triggers anti-inflammatory signals like IL-10 and IL-1ra while quieting TNF-alpha, a molecule linked to chronic inflammation.
The rise does not last. IL-6 climbs during effort, then settles back within hours, though its downstream effects keep working afterward.
Here is the twist: the same IL-6 that helps during exercise behaves differently when chronically elevated. A brief burst seems to improve blood sugar handling. Constant elevation through illness or inactivity may instead push toward insulin resistance. Timing, not just the molecule, seems to carry the meaning.
Muscle as an Endocrine Organ
IL-6 is one clue among hundreds. Contracting muscle releases a whole family of proteins called myokines into circulation, prompting researchers to describe skeletal muscle as a principal endocrine organ, alongside glands like the thyroid.
A few named myokines show the range of destinations. Irisin, discovered in 2012, is released during exercise and fasting and communicates with fat tissue. IL-6 signals to the immune system and liver. BAIBA helps coordinate the response in fat tissue.
Resistance training and cardio appear to trigger different, only partly overlapping sets of these chemical messengers. Combining strength work with cardio may broaden the range of signals muscles send through the body, offering a wider chemical conversation than either mode alone.
