Everything you’ve ever wanted to know about muscles

Building them, tearing them, repairing them, eating them.
a blond woman with lots of muscles flexes for the camera
Whether you're looking to get swole or just looking to get informed. DepositPhotos

This post has been updated. It was last published on January 1, 2019.

Whether or not you’ve resolved to get into shape this January, Muscle Month is here to teach you a thing or two about stretching, contracting, lifting, tearing, gaining, and so much more.

Welcome to PopSci’s Muscle Month! We’re kicking off the season with an FAQ on all things muscle-related, based on popular internet searches and queries from members of our staff. Got a question we didn’t answer? Let us know on Twitter.

What exactly is a muscle?

Muscles form when specialized long and tubular cells, known as myocytes, band together in a process called myogenesis. These fibers are distributed throughout our bodies and come in many different shapes, sizes, and forms, says David Putrino, a physical therapist at the Icahn School of Medicine at Mount Sinai in New York and director of Rehabilitation Innovation for the Mount Sinai Health System.

The human body has three types of muscle cells: Skeletal, smooth, and cardiac. Smooth muscles line the inside of all our hollow organs like the intestines and the stomach. That’s except for the heart, which contains cardiac muscles (hence the name). Both cardiac and smooth muscles are involuntary, meaning we can’t tell them to tense or relax. Instead their movement is regulated by a precise neural dance formulated by our autonomic nervous system.

The muscles that most of us are familiar with, says Putrino, are the skeletal muscles. They include well-known ones like the hamstrings, quadriceps, calves, and illusive external and internal obliques. “Each skeletal muscle is a discrete organ that connects with our skeleton,” says Putrino, “and is responsible for allowing our bodies to move.”

So, just to clarify, is the heart an organ or a muscle?

The heart is an organ, albeit a complex one. Again, it contains a distinct type of muscle called a cardiac muscle, which is only found in the heart, and makes up the main tissue within it, says Putrino.

What about the brain?

The brain is not a muscle at all. It’s an organ made up of neural tissue. However, you can still “work out” your brain, so to speak, to strengthen certain cognitive functioning, like memory and attention.

Okay, so how many muscles do we have? And what’s the biggest one? What about the smallest?

Approximately 639 muscles make up the human body. Interestingly, says Putrino, some of us have one or two unique and unusual accessory muscles, “so the number can go up or down based on individual variations in our anatomy.”

The aptly-named gluteus maximus is the largest one in the body. (“To be indelicate, this is your ‘butt muscle,’” Putrino, says, or your “glutes” for short.) You can thank your gluteus maximus for a movement known as the hip extension which allows us humans to perform basic functions like walking and standing—as well as more extreme endeavours like running.

The smallest muscle in the human body is the stapedius muscle, says Putrino. But never judge anything by its size: This tiny beast sits inside the ear and stabilizes the smallest bone in the body, the stapes, which is responsible for allowing us to hear the world around us.

Speaking of tiny muscles, does it really take more muscles to frown than it does to smile?

Of course everyone wants to think that it takes far more muscles to grimace than grin. (Happiness abound!) But the truth is, no one has ever really done a study to prove it, says Putrino. It also depends on how you define a frown or a smile, he says. A deep frown will definitely take more muscles to pull off than a faint smile. But when it comes to a typical frown compared to a similarly average smile, it’s hard to say.

Anecdotes asides, “I’m going to have to call myth on this one,” Putrino says.

What’s the most muscular animal?

That’s a tough one, says Putrino. “If we’re talking about just strength, beetles are definitely the strongest animals in the world and gorillas are the strongest mammals.” If we consider muscle mass, then snakes take the crown for having the highest muscle-to-body ratio. Taken another way, he says, elephants are thought to have the most individual muscles. “An elephant’s trunk alone has over 40,000 muscles.”

And all animals have similar types of muscles, right? Actually, what is the difference between light and dark muscle? Do humans have both?

The distinguishing factor between light and dark muscle (or “meat,” once we’re thinking of it as edible flesh) is the presence of a protein called myoglobin, Putrino says. Myoglobin is functionally similar to hemoglobin in blood; both contain iron and are responsible for carrying oxygen to the muscles (myoglobin) and throughout the bloodstream (hemoglobin). In fact, a high presence of myoglobin in the blood is a sign of extreme muscle injury, like rhabdomyolysis, which can be life-threatening.

The high amount of myoglobin and subsequent oxygen in our muscle cells allows us to hold our breath for extended periods of time.

Just like other animals, humans have both light and dark muscle. The color depends on the oxidation state of the iron atom bound to the muscle. But overall, according to Putrino, “muscles that require less oxygen to work have less myoglobin and appear lighter.” These muscles, found around our trunk and in our core, “generate low, but stable amounts of force for long periods of time every day.” On the other hand, muscles that need far more oxygen to work, like the ones in our limbs, have more myoglobin and therefore have a darker appearance.

Right, but how do red and white meat differ?

Read above, but swap “meat” for “muscle” which are synonymous in this case. Animals eat other animal’s muscles for food. (Though it’s not always necessary, and us humans can, on average, afford to eat less of it.)

How do I build more muscle? I want to look swole this year.

There are technically two ways one could gain muscle: Increase the number of muscle cells you have or increase the size (length, width, or both) of the ones you’ve already got.

Unfortunately, we stop growing new muscles cells soon after birth, so if you are old enough to read this article, your only option is to increase their size—that’s what people mean when they say ‘build muscle.’

“Resistance training (lifting weights) is the most common way of rapidly building more muscle, but all exercise will build muscle,” says Putrino.

When you lift weights, run, swim, or even walk briskly, this adds tension to your muscles, causing them to tear slightly. Our bodies rebuild these tears by adding either sarcomeres, which are fibrous proteins inside muscle cells, or myofibrils—chains of rod-like units also in muscle cells that give them their striped appearance. More myofibrils will increase the mass of a muscle cell while more sarcomeres will increase the length.

The more tension you place on your muscles, the more likely the muscle cells will tear, repair, and get stronger. Just make sure you give yourself time to recover, or the healing process that leads to muscle growth will never actually occur.

But I need to eat more protein to build more muscle. Right?

Sort of.

Nutrition is also key to building more muscle, and supplementing your daily diet with protein is a common way to facilitate muscle building with exercise, says Putrino. But you also need plenty of calories, and carbs help support strength training and muscle growth, too. Even if you’re working out harder, adding protein powder to your diet won’t necessarily give your body what it needs. You’ll need to figure out a well-balanced diet that works best for your body; there are no shortcuts to getting fit.

three women use foam rollers together in an exercise studio
Foam rollers are popular, but do they work? DepositPhotos

Every time I exercise or lift weights my muscles ache. Why?

Sadly, science hasn’t nailed this one down yet.

Clinicians call this phenomenon delayed onset muscle soreness, or DOMS. As Putrino explains, when you exercise your muscles, the subsequent contractions cause microtraumas to your muscle cells. We build more muscle when those damaged cells repair themselves. One specific type of contraction, eccentric contraction—which happens when your muscles tense and lengthen at the same time—is the main cause of this cellular-level trauma. For example, when you do a typical squat, your quadriceps contract and lengthen as they lower.

Technically there is a way to strengthen your muscles without this subsequent soreness, albeit it’s extremely difficult. Exercises that involve concentric contractions—ones that shorten rather than lengthen the muscle—do not cause this type of soreness. Though, says Putrino, “the amount of trouble you would have to go to in order to just target concentric muscle action is significant.” He says it would be akin to doing a bicep curl but only doing the curl phase, then having a person (or machine) bring the weights down to the start position so you can curl up again. “That being said,” Putrino says, “If you were preparing for an event and didn’t want to be sore the next day, you may want to go to the extra trouble.”

How do I get rid of muscle soreness once I have it?

That’s a highly debated topic that supports an extremely lucrative industry—foam rollers, vibrating foam rollers, cryotherapy, ice baths, the icy hot patch, etc.

But according to Putrino, the best way to get rid of DOMS caused by exercise is a combination of active stretching and—stay with us here—more exercise. But your recovery exercise should be light in comparison to the workout that made you sore; you want to get your body moving while still giving it time and energy to recover. Take a walk, go for a swim, do some yoga. This facilitates the removal of waste products and the influx of fresh blood and nutrients. Massage and heat treatments, like warm baths or saunas, have also been known to help, he says.

What causes muscle spasms?

When we say “muscle spasm,” we could mean a hundred different things, explains Greg Nuckols, who holds both degrees in exercise science and three all-time world records in powerlifting (his site, Stronger by Science, is a nerdy weightlifter’s goldmine). “It’s kind of a catch-all term for general muscular pain where the direct cause is unknown.” It might be a cramp happening in a weird muscle that’s hard to stretch out, like one of the muscles coming off your spine. But in other cases, Nuckols says, it can be less physical: “A lot of times people get what they perceive to be muscle spasms after they have some other type of injury and they get kind of habituated to feeling pain in that region.”

If stretching the muscles in that area doesn’t ever seem to help the pain, Nuckols says, it’s probably more that you’re expecting to be in pain than the presence of actual tissue damage. “A lot of pain is based on perceptions and expectancy and things completely divorced from actual tissue damage. You can have tissue damage but not pain, and you can have pain without tissue damage.” That means sometimes the spasm is all in your head, which isn’t to say that it’s not real, just that the source of the pain isn’t your muscles—it’s your brain.

But really, Nuckols says, “people talk about spasms as if they know what they’re talking about, but we don’t really know what we’re talking about yet. They’re still quite poorly understood.”

Okay, so what causes muscles to cramp?

People used to think cramps were basically a product of sweating—you get dehydrated and depleted of electrolytes, and that somehow causes a cramp. But that’s not actually true. “I don’t want to say there’s no evidence linking dehydration to cramping, but there’s just unbelievably weak evidence for that hypothesis,” Nuckols explains.

Far more likely is that it’s due to poor neurological control over your muscles as you get increasingly tired and damaged during exercise (don’t worry, the damage is actually what helps you build more muscle!). As you’re moving, your spinal cord and muscles constantly send and receiving signals that they have to integrate in order to function properly. But when you’re fatigued, all those neurological signals can start to get criss-crossed. The golgi tendon organ, for instance, is supposed to prevent your skeletal muscles from contracting too hard—it’s like the emergency shutoff button. In the middle of a tough workout, though, your spinal cord can end up sending way too many signals for a muscle to contract. If those wires get crossed or the golgi tendon organ fails, you suddenly have a cramp. “What is actually going on in a cramp is just involuntary muscle contraction in muscles that are supposed to be under voluntary control,” says Nuckols. “Electrolyte imbalances and dehydration potentially play a very small role, but it seems to be generally due to acute neuromuscular fatigue from high levels of exercise.”

What causes muscle twitching?

First off, a quick vocab lesson: the technical term for a muscle twitch is a benign fasciculation. A fasciculation is just a fancy word for a twitch, and the benign part means it’s harmless. Nuckols says we don’t completely understand what’s going on, but it has a lot to do with the same process as cramping. Most of the muscles in your body aren’t completely relaxed at any given moment, which means there’s always a bunch of signals being integrated and sent out in order to maintain posture and go about your life. You’re not consciously controlling that, but it’s happening all the time. “That’s a very finely tuned process,” Nuckols explains, “so a fasciculation is most likely a hiccup in that process.” That shouldn’t be surprising given how many millions of signals are shooting around our bodies all the time. “Frankly, it’s surprising that it happens as infrequently as it does.”

If my muscle is twitching, does that mean I need to eat a banana to get more potassium?

Sorry, but no. Potassium probably has nothing to with it. In fact, Nuckols says if you have enough of an imbalance between your sodium and potassium levels to affect muscle contraction you probably have a bigger problem—and besides, low potassium doesn’t cause twitching. People with high blood pressure sometimes get put on diuretics to manage it, and that causes their bodies to excrete more potassium. “As a way to test that hypothesis, you’d expect people on diuretics to manage blood pressure to have way more twitches than other people,” Nuckols explains, “but they don’t, so it’s probably not due to potassium.”

If you really are deficient in potassium, you’re more likely to experience generalized muscle weakness than a twitch.

What is a muscle knot?

The short answer here: we’re not really sure. Some experts think they’re something called myofascial trigger points, which are basically super-tight, contracted areas of muscle causing pain in the area. But other experts think they’re totally psychosomatic. One critique of the trigger point theory in the journal Rheumatology noted that we don’t seem to be able to find consistent physical evidence for muscle knots on medical scans. If they’re made of some kind of fibrous tissue, which is what they feel like from the outside, we should be able to find evidence of them. But we don’t. They suggest that a better explanation might be nerve inflammation. An inflamed neuron can make a region feel more sensitive even if there’s nothing more going on than an errant group of pain receptors firing.

Is it true that muscle weighs more than fat?

A pound of muscle and a pound of fat weigh the same, of course, but it is true that muscle is denser than fat. “But I think people often overestimate how large the difference is,” Nuckols says. There’s a picture floating around social media ostensibly showing five pounds of fat versus five pounds of muscle, he explains, and the hunk of fat is roughly twice the size of the muscle. “That’s just completely inaccurate,” he says. “Muscle is roughly 10 to 15 percent denser than fat.”

So if you start lifting and you see the numbers on the scale rising, it could very well be due to an increase in muscle with a simultaneous loss of fat. But if you’re rapidly packing on pounds, chances are good you’re adding some fat to the mix as well.

Is your tongue a muscle?

Heck yes, it is. “Tongues are really cool,” Nuckols says. See, most of your muscles are arranged in one of two patterns. Parallel muscles have fibers running from origin to insertion, which means they go from one tendon to another. Your biceps, for example, originate up by your shoulder and insert on the other side of your elbow, with all the muscle fibers running in parallel. Pennate muscles, like your quads or your triceps, have a central tendon running through them with all the fibers running into that tendon at an angle. “Your tongue, on the other hand, is what’s called a muscular hydrostat,” Nuckols explains. “That basically means it’s muscle that’s under conscious control that isn’t attached to bone.” It has fibers running every which way that can relax and contract separately, which is why you can manipulate your tongue in so many ways instead of just contracting in one direction. “It actually works very similarly to octopus arms,” says Nuckols. “Octopus arms aren’t attached to a central skeleton, but they’re very dextrous and mobile, and they function on the same principles that your tongue does.”

So your tongue is basically like a little octopus arm inside your mouth (neat!), and it is most definitely a muscle.

What is muscular dystrophy?

There are many types of muscular dystrophy, since the term refers to any condition that causes muscle weakness by preventing proper muscle formation or function. The most common type is called Duchenne muscular dystrophy, which affects a protein called dystrophin (though not all dystrophies have anything to do with dystrophin). To understand why it matters, we have to dig a little deeper into how your muscles exert a force. Basically, your muscles attach to tendons, and your tendons attach to bones. As the fibers contract inside a muscle, they exert a force directly on the connective tissue that attaches to the tendons, and then the tendons can apply torque to your bones. Dystrophin is one of the main proteins that attach muscle fibers to connective tissue. People with Duchenne muscular dystrophy have a mutation in that protein that prevents proper adhesion, which means their muscles may contract just fine, they just can’t exert much of a force on the tendons.

But there’s also another layer. “Muscles have to have stress to maintain themselves and stay healthy,” Nuckols explain, “so if you can’t transmit force through a muscle well, that process gets messed up. It’s a whole cascade that ultimately makes the muscles smaller and weaker.”

Other forms of muscular dystrophy affect similar proteins in muscle fibers and connective tissue that prevent the whole system from working together. They’re all genetic in nature, but they don’t all show up in children and they don’t all affect the same musculature.

This article has been updated to reflect the fact that you can strengthen your muscles without them becoming sore, it’s just very difficult to do.