Your Muscles Have a Secret Job

Most people learn the obvious job of muscle first. Muscles contract, pull on bones and make you move. Your quadriceps help you stand, your calf muscles push you forward, and several muscles you have probably never heard of stop you from collapsing sideways while doing either. All true. But once muscle starts working, it begins doing jobs that are much harder to see.

A resting muscle and a contracting muscle are not biologically doing the same thing. Start walking, cycling, climbing stairs, swimming, dancing or doing squats and your muscles suddenly need more fuel and blood flow. Their cells change how they handle glucose, and working muscles begin releasing chemical signals that communicate with other parts of the body. Scientists now recognize skeletal muscle as an endocrine and secretory organ. Your muscles are not just motors.

Five minutes matters

Lunch gives us a good way to see this happen. Carbohydrates are digested into glucose, which enters your bloodstream and needs somewhere to go. Skeletal muscle is one of your body's largest places for putting that glucose to work. But glucose cannot simply wander into a muscle cell because it happens to be passing by. It needs a way through the cell membrane.

Muscle cells have glucose transporters called GLUT4. Think of them as doors that allow glucose to move from the bloodstream into muscle cells. Muscle contraction signals more GLUT4 transporters to move into position, rapidly increasing the muscle's ability to take glucose from the blood. Exercise also leaves muscle more sensitive to insulin afterward, so its effects on glucose control continue after the activity stops. Think about that: your muscles can still be responding to activity after you are back sitting in class.

This is why even a few minutes of activity after eating is biologically meaningful. Researchers have compared long periods of uninterrupted sitting with those same periods interrupted by short bouts of muscular activity. When people get up and use their muscles for even a few minutes at a time, their bodies handle the rise in glucose after eating better than when they remain sitting. Five minutes is not being compared with five minutes of sitting. A few active minutes are interrupting a much longer period in which large muscles would otherwise remain mostly inactive.

Those short bouts can happen repeatedly across a day. A five-minute walk, taking the stairs, walking somewhere instead of remaining seated or getting large muscles working for a few minutes all interrupt muscular inactivity. Each time muscles contract, their demand for fuel increases and glucose transport changes. The metabolic effect does not end the instant you sit down again.

Muscle and bone respond too

Muscular strength and endurance improve because the body responds to repeated demands. When muscles repeatedly produce force, muscle tissue adapts and the nervous system becomes better at recruiting and coordinating it. You can actually become stronger before there is a dramatic change in muscle size because your brain and nerves are becoming better at using the muscle you already have.

Bones respond to movement too. Running, jumping, resistance activities and many sports put forces through bone, and bone is living tissue that responds to those forces. During the growing years, repeated loading helps build stronger bones. A jump therefore involves considerably more biology than briefly leaving the floor and trusting gravity to handle the return trip.

Flexibility contributes differently. Regular stretching increases the range of motion available at particular joints. It should not be credited with all the metabolic and cardiovascular effects of activities that make large muscles work harder. The four health-related fitness components are a team, not four identical players.

Your heart, lungs and muscles become a team

Cardiorespiratory endurance involves much more than having “good lungs.” Your lungs bring oxygen into your body, your heart pumps blood, your blood vessels deliver it, and working muscles use that oxygen to help release energy from fuel. Regular activity gives this entire system a reason to adapt. Your heart becomes better at moving blood, circulation improves and muscles become better at using oxygen.

That explains why the same physical task eventually feels easier. A flight of stairs that once left you breathing hard can become much less demanding after you become fitter. The staircase did not become shorter or finally decide to cooperate.

That also helps explain why people say regular activity gives them “more energy.” Exercise obviously uses energy, so it does not magically fill an invisible battery. Greater fitness means many ordinary activities require a smaller portion of what your body is capable of doing. The same day can simply take less out of you.

Then your muscles start sending messages

Here is where muscle becomes much more interesting than a motor. Contracting muscles release chemical messengers called myokines. These signals are part of the way active muscle communicates with tissues including bone, blood vessels, the liver, fat tissue and the brain. Working muscle is not simply receiving instructions. It is sending messages of its own.

There is no single magical “exercise chemical” travelling around fixing everything. Scientists have discovered many signals produced during activity, and they are still learning exactly what individual ones do. Some are involved in metabolism, inflammation and tissue adaptation, while others participate in communication between the active body and brain. You certainly do not need to memorize their names. The important discovery is that active muscle communicates differently from inactive muscle.

A muscle resting quietly while you sit through three episodes of a show is still doing important work. But it is not using fuel or sending exercise-related signals in the same way as contracting muscle. One is mostly waiting for its next assignment. The active one has opened the fuel department, increased glucose transport, contacted several other organs and apparently scheduled a meeting with the brain.

Your body responds to what you repeatedly ask of it

Cardiorespiratory endurance, muscular strength, muscular endurance and flexibility are therefore more than PE measurements. They describe different capacities within a body that adapts. Some responses begin within minutes: muscles take up more glucose, circulation changes and chemical messages start travelling. Others develop through repeated activity: stronger muscles and bones and greater cardiovascular capacity.

One active day does not determine your health, and neither does one inactive day. What matters more is the pattern that develops over time. Walking, cycling, dancing, swimming, climbing, active games and resistance activities all give your body different reasons to adapt. You have plenty of room to find activities you actually want to return to.

And the effects do not stop at muscle, bone, glucose or the heart. Some of the messages produced by an active body become part of a conversation involving your brain. That helps explain why physical activity is connected with attention, learning, sleep and mental well-being.