The Anatomy of Leg Muscles – Full Guide
The Anatomy of Leg Muscles - Full Guide

If you’re lounging as you read this, the next sentence may scare you sit-less.

“Sitting is the new smoking; it’s just as insidious,” says Marc Hamilton, Ph.D., a physiology professor at the Pennington Biomedical Research Center in Baton Rouge, Louisiana. Hamilton is making a point about how so many Americans are letting their leg muscles — and therefore their bodies — turn to mush. “You’ve seen the flat line on an EKG when all the doctors rush in? That’s what’s happening to your leg muscles when you’re sitting.”

As he speaks, I flashback to a job I had at a digital agency. I showed up on my first day of work in the New York City office to find half the staff standing at their computers. Because they didn’t have chairs. The office consisted mostly of makeshift desks about waist-high that we would belly up to like a bar.

Turns out my hipster coworkers were onto something. “Standing while talking on the phone or filing isn’t exercise by anybody’s standard, yet, compared with sitting, it increases your metabolic rate a bit,” says Hamilton. In case you were wondering, though, doing “light office work” while sitting burns 96 calories an hour for an average 140-pound woman as opposed to 147 calories while standing, according to a widely accepted compendium of physical activity. Besides, Hamilton adds, “when we’re sitting for extended periods, hundreds of ‘bad’ genes are turned on, including ones that stimulate muscle atrophy.”

Intrigued, I head to the Kaiser Permanente Los Angeles Medical Center to see firsthand the toll that opting for a chair and a laptop all day is taking on my leg muscles. Once there, Barry R. Chi, M.D., chief of physical medicine and rehabilitation, wires my leg muscles with surface electrodes that are tethered by several long cables to an electromyography (EMG) machine. I reenact a day in the life of my legs by sitting, standing, walking (in both heels and flats), rising up on tiptoe, and jogging. We cap this off with squats and lunges as a yardstick to measure everyday muscle activity against. True to the EKG analogy, the leg muscle readings on the EMG monitor are indeed flat lines when I sit in a chair. It’s as if I’m not even there.

But something happens when I stand up in front of the monitor. It fills with electrical activity. “You may not feel anything, but your leg muscles are now supporting your whole body weight, and all of your big muscles of the body are now engaged in isometric contractions,” says Dr. Chi, pointing to the elevated lines. (Interestingly, when I stand or walk in heels my quads and hamstrings show greater surges than when I’m in flats, but Dr. Chi quickly warns against long-term high heel side effects like back pain.) “Standing for two hours can be the equivalent of going for a two-mile run,” he explains.

How Genetics Alter Your Leg Muscles

The length of your legs is basically a matter of genetics — and we may be talking way, way back in the family tree. In general, women are slightly leggier than men: The latest body-measurement statistics from the SizeUSA study, conducted by TC², a not-for-profit apparel industry resource, show that the average 18- to 45-year-old woman’s legs (determined by crotch height) make up about 45 percent of her total height versus 44 percent for the average man in the same age group.

Leg muscles are another story. They depend greatly on our genes and what we do with them. We’ll get to the latter half of that equation — diet, exercise, couch-sitting habits — later, but for now, a quick leg muscle anatomy lesson.

We all have the same main leg muscles: quadriceps, hamstrings, adductors, shins, and calves. Within those larger muscle groups, though, there are several smaller muscles, each with their own unique function(s) — adduction, flexion, extension, rotation. For example, the semitendinosus, semimembranosus, and biceps femoris are all part of the hamstrings. It’s worth noting that while adductor muscles are located on the inner thigh and help to move your leg into the mid-line of your body, abductors are not merely “outer thigh” muscles, but rather muscles located in the glutes that help rotation your hip. For the purposes of this story, we’re sticking to below the booty.

But there’s a wide range of sizes and muscle makeup among people that even experts debate. Muscle fibers in humans evolved so that most of us have leg anatomy with a majority of slow-twitch fibers, which give us our staying power during long runs,” according to Daniel Lieberman, Ph.D., professor of human evolutionary biology at Harvard University. “We’re built more for endurance, whereas chimps have more fast-twitch fibers,” he explains. With fewer powerhouse fast-twitch fibers, humans are at a disadvantage when it comes to speed. “As a species, we’re terrible sprinters,” says Lieberman. “Cheetahs can run 25 meters per second. The fastest human, [Jamaican world champion sprinter] Usain Bolt, runs only 10.4 meters per second.”

The quadriceps, or quads, it turns out, are the real wild cards of your leg muscles, as they can range from predominantly fast-twitch to the complete opposite: The quads of someone like Bolt can contain up to 90 percent fast-twitch fibers, says John P. McCarthy, Ph.D., professor of physical therapy at the University of Alabama at Birmingham. While elite marathoners’ muscles can contain up to 90 percent slow-twitch fibers. The quads of average people, or even those of swimsuit models or hulking bodybuilders, are more a fifty-fifty mix of the two.

The problem is that many women are often so afraid of getting bulky thighs and calves that they neglect to strength-train their legs. Actually, bulky legs are mainly due to fat. “Our legs can go from shank steak to marbled rump roast without our even knowing it,” says Vonda Wright, M.D., an orthopedic surgeon and professor at the University of Pittsburgh Center for Sports Medicine. “It’s a snowball effect when we start accumulating fat, and it affects the function and strength of muscle.”

Body Composition of Your Leg Muscles

Since puberty, your hormones have been signaling fat cells to be stored around your butt and thighs, ultimately to help serve as reserve energy for pregnancy and breastfeeding. “Women tend to gain fat in very specific body parts, mostly those from the waist to the knee,” explains Andrew Da Lio, M.D., professor and chief of plastic and reconstructive surgery at University of California, Los Angeles. The most common of those parts is the outer thigh, says Dr. Lio.

There are two levels of fat in the legs, a superficial layer and a deeper layer, explains Dr. Da Lio. The superficial layer is where we find cellulite when fat pushes through between the tissues that connect skin to the underlying muscle.

Gain too much of the deeper leg fat and it can actually begin to infiltrate your leg muscles, says Dr. Wright. The good news? This deeper layer is also typically the first layer of fat to shrink when we exercise.

How to Strengthen Your Leg Muscles

Last fall, with the help of the UCLA Center for Human Nutrition’s Risk Factor Obesity Program lab, I tried an experiment. I did every exercise I routinely avoid on the chance it would make my legs look like Schwarzenegger’s: dozens and dozens of squats and lunges each week combined with the stairclimber and Spinning classes. And a funny thing happened. I lost 10 percent of the fat from each thigh in four weeks, according to the lab’s DEXA (dual-energy X-ray absorptiometry) body scanner. By eight weeks, during which I also stuck to a low-calorie diet, I’d lost more than an inch from each thigh.

“You can change the composition of your leg muscles — the ratio of fat to lean mass,” says Dr. Wright. “Increasing your strength and endurance will lead to a change in how your legs look.” And there was my proof in the form of the X-ray-like DEXA readouts, which showed that the grayish halo representing the fat on my thighs was shrinking.

But here’s the kicker: The darker center consisting of my quads and hamstrings wasn’t busting at the seams after those gazillion squats. In fact, it had pretty much stayed put, which is the moral of this story. If I hadn’t done those reps while I was dieting, my muscles probably would have shrunk a little, too, and along with them, my metabolism.

Stronger legs may indeed be a secret to maintaining a healthy size. “When you increase the strength and endurance of your legs, it generally makes it easier to exercise and move around, leading to greater physical activity throughout the day. You burn more calories overall,” McCarthy says. In fact, a University of Alabama at Birmingham study found that women who maintained weight loss one year after dieting had much greater leg strength than those who didn’t. (Related: The Best Leg Day Exercises Trainers Want You to Add to Your Workouts)

Certain data from the SizeUSA study seem to back up this research. It found that the circumference of an average woman’s thigh ranged from 24 inches for active women under age 45 to 26 1/2 inches for less active women of the same ages. That’s a two-and-a-half-inch difference. Of course, this is not to say that smaller is better!

As for me, my legs are now about as lean as they were when I was a competitive distance runner. For the first time in 10 years, I fit into my favorite pair of Levi’s, the ones I bought right after college, and I finally zipped my athletic calves into a pair of tall boots.

But What About Your Ankles?

The region between our calves and ankles is not defined by muscle but rather by the Achilles tendon, which connects the two. For some women, this area cinches in dramatically from a well-toned calf muscle, while for others it slopes down gradually. And then there are those whose lower legs appear to drop in a straight line, with no indentation at all, inspiring the unflattering and totally un-woke label: cankles.

“Cankles are essentially a visual effect,” says orthopedic surgeon Vonda Wright, M.D. “Models often look as if they have cankles because their legs are tubes from the knee to the ankle. It’s all relative.”

For the calf to have the appearance of tapering, there has to be a bit of muscle size to the calves. Yet, again, many women are reluctant to strengthen their calf muscles for fear that they will thicken and produce a cankle effect. “That’s a myth,” says Dr. Wright. “Cankles don’t come from muscle, because by the time you get to the ankle, it’s all tendons.”

It’s a matter of genetic roulette, fat accumulation, and body composition. Instead of trying to get rid of your cankles, how about just go out, get moving, and have some fun with your fitness. Turns out, you might just see some more muscle definition without even attempting to target them.

Okay, now that you’ve gotten a lesson on how and why leg muscles look and act as they do, here’s a breakdown of exactly what leg muscles you’re dealing with.

The Anatomy of Leg Muscles

Take a look at the below leg muscle diagrams for a more detailed look at the front, side, and back of your leg muscles labeled.

Front Leg Muscles

When it comes to the front of your legs, there are two muscle groups — the anterior upper leg muscles (i.e. your thigh) and the anterior lower leg muscles (i.e. your shin). There are four parts of your quadriceps: rectus femoris, vastus lateralis, vastus medialis, and vastus intermedius. The tibialis anterior is the strip of muscle that makes up your shin and helps you flex your ankle to move your foot toward your knee.The peroneus longus runs down the outside of your anterior lower leg.


front view of leg muscles illustration

Side Leg Muscles

Your inner leg muscles or inner thigh muscles are known as your adductor muscles, which include the pectineus, adductor longus, adductor brevis, and gracilis. This group of leg muscles is responsible for bringing your thigh toward the center of your body, as well as rotating the thigh bone.

side view leg muscles illustration

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