An easy-to-understand guide to myokines, glucose, mitochondria, cellular resilience and the whole-body biology of active muscle.

Muscle has an obvious job: it contracts so we can walk, lift, run, breathe, carry and play. That is true, but incomplete.

Skeletal muscle is also a major metabolic organ. It stores and uses fuel, handles a substantial share of glucose after meals, turns over large amounts of protein and helps the body respond to changing energy demands. When muscle contracts, it also releases signalling molecules that can participate in communication with other tissues.

A working muscle is therefore both an engine and a broadcaster. It uses energy to create movement while sending information that helps coordinate the rest of the body.

 

Muscle Is an Organ, Not Just a Motor

Skeletal muscle makes up a substantial proportion of body mass, but it is not one uniform block. Whole muscles contain bundles called fascicles. Fascicles contain muscle fibres, and fibres contain myofibrils built from repeating contractile units called sarcomeres. Inside sarcomeres, actin and myosin interact to produce force.

That microscopic interaction eventually becomes a step, a squat or a sprint. Yet the contractile proteins are only part of the tissue.

Muscle Is Wrapped in a Living Matrix

Individual fibres and bundles are organised by collagen-rich connective-tissue layers called the endomysium, perimysium and epimysium. These layers help transmit force and connect muscle with tendons and the skeleton. Blood vessels deliver oxygen and nutrients. Motor nerves activate fibres. Immune cells, fibroblasts and satellite cells participate in maintenance and repair.

Muscle is therefore a neighbourhood rather than a row of isolated fibres. The contracting cells, matrix, circulation, nerves and resident cells continually influence one another.

Satellite Cells Explained | The Stem Cells That Repair Muscle explores one of the specialised cell populations involved in muscle maintenance.

Communication Runs in Both Directions

The nervous system sends commands to muscle through motor neurons. Sensory information travels back. Mechanical force moves through muscle, tendon and bone. Metabolites enter the blood. Secreted molecules act on nearby or distant cells. The old picture—brain tells muscle what to do—is only half the conversation.

What Are Myokines?

Myokines are signalling molecules produced and released by skeletal muscle, particularly in relation to contraction. Their discovery helped establish muscle as an endocrine-like organ: a tissue capable of sending chemical messages beyond its traditional mechanical role.

Skeletal muscle is not a conventional endocrine gland like the thyroid or pancreas. Its secretory behaviour changes with contraction, training status, exercise intensity, duration, energy availability and the local tissue environment.

The Muscle Secretome Is Larger Than Myokines

Researchers use the word secretome for the collection of molecules released by a cell or tissue. Muscle can release proteins, peptides, metabolites, extracellular vesicles and other factors. Some act within muscle itself. Others may enter circulation. Terms such as myokines, myometabolites and exerkines can overlap, so not every exercise-related molecule in the blood came directly from muscle.

This is an important scientific caution. A molecule that changes after exercise may be biologically interesting without being proven to cause every whole-body benefit associated with exercise.

Local Messages and Long-Distance Messages

A signal released by a muscle fibre may act back on the same cell, influence a neighbouring cell or enter the circulation. Scientists describe these patterns as autocrine, paracrine and endocrine signalling. The distance travelled matters because a strong local effect does not automatically prove a meaningful effect in a distant organ.

The receiving tissue must also have the relevant receptors and be in a state that can respond. A myokine is not a general instruction broadcast equally to every cell. It is more like a message written in a biological language that only certain receivers can read under particular conditions.

Examples Under Investigation

 

Myokines Explained | How Exercising Muscles Send Signals goes deeper into individual signals and the limits of current evidence.

 

Muscle as a Glucose-Handling Organ

The body needs a continuous supply of energy even though food arrives intermittently and demand changes from sitting to sprinting. Skeletal muscle is one of the largest places where glucose can be used or stored after a meal.

Insulin and Contraction Open Different Doors

After carbohydrate-containing food is digested, glucose enters the bloodstream. Insulin helps muscle cells increase glucose uptake, where it can be used for energy or stored as glycogen.

Muscle contraction also increases glucose uptake through partly insulin-independent pathways. Exercise effectively creates another route for glucose to enter working muscle. This helps explain why physical activity has immediate metabolic effects beyond the energy used during the session.

Muscle Glycogen Is Local Fuel

Glycogen is a branched storage form of glucose found mainly in muscle and liver. Muscle glycogen largely supports the muscle fibre that stores it. Liver glycogen has a different role: it can help maintain circulating glucose between meals and during changing demands.

During exercise, muscle glycogen can be broken down to support ATP production. The liver responds to rising demand by releasing glucose from its own stores and producing glucose through gluconeogenesis. This creates an active muscle–liver conversation coordinated by hormones, metabolites and nerves.

Lactate Is a Shuttle, Not Merely Waste

Working muscle can produce lactate. The old story treated it as a useless by-product responsible for all fatigue and soreness. Modern physiology recognises lactate as a mobile metabolic intermediate. Other muscle fibres and organs can use it as fuel, and the liver can convert lactate carbon into glucose through the Cori cycle.

Metabolism is movement: fuel and chemical information travel between cells and organs. Lactate is one of the clearest examples of that exchange.

Why Muscle Mass and Muscle Use Are Different Questions

Having more muscle can increase the size of the tissue available to store glycogen and use glucose, but simply owning muscle is not the same as using it. Contraction rapidly changes glucose transport and fuel demand. A smaller amount of regularly active muscle can send a different metabolic signal from a larger amount that is rarely challenged.

This is why metabolic discussions should include both body composition and behaviour. Strength, cardiorespiratory fitness, sitting time and recent activity all help describe what the tissue is being asked to do.

The Muscle–Fat and Muscle–Liver Axes

Adipose tissue stores energy as triglycerides. When energy demand rises, fatty acids can be released and used by tissues including muscle. The relative contribution of carbohydrate and fat depends on exercise intensity, duration, training status, diet, hormones and fuel availability.

Metabolic flexibility does not mean becoming a permanent “fat burner”. It means being able to adjust fuel use according to supply and demand. Muscle, liver and adipose tissue coordinate this flexibility through substrate exchange and chemical signalling.

The Liver Keeps Fuel Available

Between meals and during exercise, the liver helps maintain circulating glucose. It can break down stored glycogen and make glucose from substrates such as lactate, glycerol and selected amino acids. Meanwhile, muscle draws on its own glycogen and circulating fuels according to demand.

After exercise and food, the direction of traffic changes. Glucose can return to glycogen stores, amino acids can support protein turnover and fatty acids may be stored or oxidised. Metabolic health depends partly on managing these transitions rather than remaining in one permanently “fasted” or “fed” state.

Adipose Tissue Is Also a Signalling Organ

Body fat is not an inert warehouse. Adipose tissue releases adipokines and participates in appetite, inflammation, insulin sensitivity and energy regulation. Muscle–fat communication is therefore two-way. Contracting muscle changes fuel demand and signalling, while the amount, distribution and biological state of adipose tissue can influence the environment in which muscle operates.

What Is Metabolic Flexibility? | Why Your Body's Ability to Adapt Matters explains why healthy metabolism is adaptive rather than locked into one fuel.

Muscle Changes the Meaning of Body Composition

Two people at the same body weight can have different proportions of muscle, fat, bone and water, with different implications for strength and metabolic capacity. Muscle is not valuable only because it increases energy expenditure. It provides a large, adaptable tissue capable of storing glycogen, using glucose and performing work.

Body Composition Explained: Muscle, Fat, Metabolism & Why the Scale Does Not Tell the Whole Story places body weight within this wider biological context.

The Muscle–Bone Conversation

Muscle and bone are physically connected, but their conversation is more than a rope pulling on a lever. When muscle generates force, tendons transmit that load to bone. Bone cells detect strain and can adapt to repeated, appropriate loading. This conversion of physical force into cellular signals is mechanotransduction.

Bone also releases signalling molecules, sometimes called osteokines, while muscle releases myokines. Researchers are studying how these chemical signals may contribute to two-way communication between the tissues.

The most established practical message remains mechanical: muscle contractions and impact or resistance loading provide information that bone is required. Protein or collagen cannot reproduce that signal from the dining table.

Mechanotransduction Explained: How Movement Tells Your Body to Build Muscle, Bone and Connective Tissue and The Muscle–Bone Connection: How Strong Muscles Help Build Strong Bones Throughout Life explain the mechanical and cellular relationship.

The Muscle–Brain Axis

Exercise begins in the nervous system. The brain plans movement, motor neurons recruit muscle fibres and sensory systems report what happened. Over time, practice can improve coordination and motor-unit recruitment before visible muscle growth occurs.

The conversation may also include circulating metabolites and exercise-induced signalling molecules. Lactate can cross into the brain and act as fuel and signal. Exercise changes growth-factor biology, vascular function and neuroplasticity. Some myokines are being investigated for possible links with cognition and mood.

It would be too simple to say that one myokine “causes” the mental benefits of exercise. The brain response reflects blood flow, autonomic activity, sleep, social context, metabolic changes, learning and many signals acting together. The strength of the muscle–brain concept lies in the network, not a single molecule.

Movement Is Information for the Nervous System

Every new movement gives the nervous system information about force, position, balance and timing. Repetition can make neural pathways more efficient. Challenging balance, coordination and skill therefore adds something different from simply producing metabolic work.

This helps explain why an active life benefits from variety. Walking supports rhythmic aerobic work. Resistance training demands force. Dancing, ball sports and martial arts add timing and coordination. Each asks a slightly different question of the muscle–brain system.

 

The Muscle–Immune Conversation

Exercise changes immune-cell movement, cytokines, blood flow and tissue repair. Contracting muscle-derived IL-6 offers a striking example of context. A temporary exercise-related rise can participate in fuel mobilisation and anti-inflammatory signalling networks, while chronically elevated IL-6 in other settings may be associated with inflammation and disease.

The molecule has not changed its name, but the source, timing and surrounding signals change its meaning. Biology is a sentence, not a single word.

Regular activity is associated with healthier immune regulation, whereas prolonged inactivity and excess visceral fat can contribute to a different inflammatory environment. Exercise is not an immune “boost” that makes the system simply stronger. A well-regulated immune system must activate, resolve and tolerate appropriately.

What Is Inflammaging? | Understanding Age-Related Inflammation explores how low-grade inflammatory patterns can interact with ageing, activity and metabolic health.

The Gut–Muscle Axis

Muscle does not communicate with the gut in one simple pathway, yet the relationship is increasingly important. The gut digests and absorbs amino acids, carbohydrate, fats, vitamins and minerals used throughout muscle metabolism. Gut microbes transform dietary substrates into metabolites that may influence immune and metabolic signalling.

The conversation also runs back towards the gut. Exercise changes blood flow, motility, stress hormones and immune activity. Moderate regular activity is associated with different microbiome patterns from inactivity, although training, diet, age, medication and environment are difficult to separate in human studies.

This does not mean one probiotic or one food can “activate” the gut–muscle axis. It means muscle function sits inside a larger system involving dietary quality, digestion, microbial metabolism, inflammation and physical activity.

The Gut-Muscle Axis explores this emerging field without turning correlation into certainty.

Muscle, Mitochondria and Cellular Resilience

Muscle contractions require ATP. Mitochondria produce much of this usable cellular energy, especially during sustained activity. Training challenges the energy system and can stimulate adaptations in mitochondrial number, structure, enzymes and quality control.

This is why active muscle is not merely larger muscle. Muscle quality also reflects mitochondrial function, blood supply, nerve input, fat infiltration, connective tissue and the ability to produce force relative to size.

The Muscle–Mitochondria Connection: How Muscle Supports Energy, Metabolism & Healthy Ageing and Mitochondria Explained: The Complete Guide to Cellular Energy, Metabolism and Whole-Body Health provide the deeper cellular story.

Proteostasis: Maintaining the Protein Workforce

Muscle contains enormous quantities of protein. Those proteins are continually made, folded, used, damaged, repaired and broken down. Proteostasis describes the network that maintains this functional protein environment, including synthesis, folding, quality control and degradation.

Autophagy and Cellular Recycling

Autophagy is one of the systems cells use to enclose and recycle selected damaged or unnecessary components. The ubiquitin–proteasome system handles many individual proteins, while autophagy can process larger structures and aggregates. These pathways are normal maintenance, not switches that are either completely on or off.

Exercise can influence cellular clean-up and mitochondrial quality-control pathways, but more stress is not always better. Adaptation requires a challenge that the person can recover from.

Anabolism and Catabolism Are Partners

Anabolism builds molecules and structures. Catabolism breaks them down. Fitness culture often treats building as good and breakdown as bad, but healthy muscle needs both. Damaged proteins must be removed before replacements can work well. Stored fuel must be broken down before its energy can be used. Training itself temporarily increases disruption before recovery builds capacity.

The goal is not permanent growth. It is regulated turnover: build when useful, dismantle what is damaged, recycle what can be reused and maintain enough reserve for the next challenge.

 

Muscle as a Reserve and Resilience Organ

Muscle is a reservoir of amino acids as well as a movement and metabolic organ. During illness, injury or prolonged inadequate intake, muscle protein can be broken down to provide amino acids for other priorities. This can be lifesaving in the short term but costly when losses are large or recovery is incomplete.

Resilience does not mean never being challenged. It means maintaining enough reserve to respond, recover and continue functioning. Functional muscle contributes to the capacity to rise from a chair, carry groceries, stabilise joints, recover from bed rest and return to ordinary life after disruption.

Muscle Recovery Explained | Why Recovery Builds Strength & Supports Healthy Ageing explains why repair is part of adaptation rather than time away from progress.

Why Muscle Matters at Every Stage of Life

Childhood and Adolescence

During growth, muscle supports movement skill, play, sport, posture and the forces that help shape bone. Children need varied food, enough energy, age-appropriate protein and opportunities to run, jump, climb, carry and learn physical skills. The goal is development and confidence, not adult bodybuilding culture.

Adulthood

In adulthood, muscle helps meet the physical demands of work, parenting, recreation and training. Regular resistance exercise can build reserve before later-life decline begins, while aerobic activity supports mitochondrial and cardiovascular capacity.

Pregnancy and Postpartum

Pregnancy changes body mass, posture, circulation and energy demand. Postpartum recovery can involve sleep disruption, healing and rebuilding strength. Exercise and nutrition need to suit the individual stage and clinical guidance, but muscle remains central to everyday capacity.

Midlife and Later Life

With age, muscle mass, strength and power can decline, and muscles may become less responsive to small doses of protein or weak training stimuli. These changes are not an instruction to stop. Progressive resistance exercise, adequate protein and enough recovery remain powerful ways to support function.

Protein Throughout Life: Why Your Protein Needs Change With Age explains why growth, adulthood, pregnancy, training and healthy ageing create different protein contexts.

How to Support Muscle as a Metabolic Organ

Use Muscle Regularly

Resistance training provides a direct force signal. Walking, cycling, swimming, running, sport and active transport challenge energy systems and mitochondria. A varied routine can combine strength, aerobic fitness, balance, power and mobility according to age and ability.

Progress the Signal

The body adapts to demand. Repeating the same easy stimulus forever may maintain familiarity without continuing to build capacity. Progress can come from resistance, repetitions, range, speed, duration, terrain or skill. It should be gradual enough to recover from.

Eat Enough Energy and Protein

Muscle repair and adaptation require energy. Protein supplies amino acids for turnover and synthesis, while carbohydrate can support glycogen and training quality. Vitamins, minerals, fats and plant foods contribute to the wider metabolic environment.

High-Protein Foods: The Foundation of Muscle, Healthy Ageing & Recovery Nutrition offers practical meal-building guidance.

Sleep and Recover

Sleep influences appetite, metabolism, training quality and physical performance. Recovery also requires lighter periods between demanding sessions. No food or supplement can fully replace a training plan that repeatedly exceeds the body’s ability to recover.

Remember the Hierarchy

 

A Practical Week Does Not Need to Be Perfect

Supporting muscle as a metabolic organ does not require maximal training every day. A realistic week might include two or three resistance sessions, regular walking or active transport, one or more aerobic sessions and brief movement breaks during long periods of sitting. The mix changes with age, health, preference and experience.

·       Train major movement patterns progressively rather than chasing soreness.

·       Break up long sitting periods with walking or simple movement where practical.

·       Include protein-containing foods across regular meals.

·       Use carbohydrate to support activity and replenish glycogen according to demand.

·       Protect sleep and include easier days after demanding sessions.

·       Seek qualified guidance when pain, illness, pregnancy, medication or a health condition changes what is appropriate.

Myth vs Fact

 

Frequently Asked Questions

Why is muscle called a metabolic organ?

Because it stores and uses fuel, disposes of a substantial amount of glucose, stores glycogen, turns over protein and adapts its energy systems to demand.

Is muscle really an endocrine organ?

It has endocrine-like functions because contracting muscle releases signalling molecules. It is not a conventional endocrine gland, and muscle–organ communication also occurs through many non-endocrine routes.

What are myokines?

Myokines are signalling molecules produced and released by muscle, particularly in relation to contraction. Some act locally and others may contribute to communication with distant tissues.

How does muscle help control blood glucose?

Insulin and muscle contraction can both increase glucose uptake. Glucose can then be used for energy or stored as muscle glycogen.

Does building muscle automatically cure insulin resistance?

No. Greater functional muscle and regular activity can support glucose handling, but metabolic health also reflects genetics, sleep, diet, medication, liver and fat biology and other factors.

How does muscle communicate with bone?

Muscle creates mechanical force transmitted through tendons, and both tissues release signalling molecules. The strongest practical signal for bone remains appropriate loading.

Does exercise reduce inflammation?

Regular activity is associated with healthier inflammatory regulation, but acute exercise can temporarily raise certain signals. Source, timing and context determine meaning.

Can walking support muscle metabolism?

Yes. Walking increases contraction-mediated fuel use and supports aerobic capacity. Resistance exercise adds a stronger signal for strength and muscle maintenance.

Why does muscle matter in childhood?

Muscle supports growth, movement skills, play, sport and mechanical loading of bone. Children need enough food and varied movement rather than adult supplement routines.

What matters most for healthy muscle with age?

Progressive resistance exercise, regular movement, adequate energy and protein, sleep, recovery and management of relevant health conditions all matter.

Final Thoughts

Skeletal muscle does not sit silently beneath the skin waiting for the brain to request movement. It stores fuel, handles glucose, turns over protein, houses adaptable mitochondria, generates force and releases biological signals. It communicates with bone through load, with liver and fat through fuel exchange, with the brain through nerves and metabolites, and with immune cells through changing signalling networks.

The most memorable lesson is also the most practical: the body maintains what it is asked to use. Exercise is maintenance information. It tells muscle—and the systems connected to it—that strength, energy capacity and resilience are still required.

Food provides resources. Sleep provides recovery time. But movement supplies the message. Across childhood, adulthood and later life, keeping muscle active means keeping one of the body’s largest metabolic conversations alive.

References and Further Reading

·       Foundational reviews describing skeletal muscle as a secretory and endocrine-like organ.

·       Human exercise-physiology research on contraction-mediated glucose uptake, muscle glycogen and lactate shuttling.

·       Reviews of myokines, exerkines and communication between muscle, adipose tissue, liver, bone, brain and immune cells.

·       Research on muscle mitochondrial biogenesis, proteostasis, autophagy and age-related changes in muscle quality.

·       Australian physical-activity and resistance-training guidance across the lifespan.