Your Guide to Leg Muscle Anatomy – Plus How to Get the Most Out of Your Leg Exercises

Your Guide to Leg Muscle Anatomy – Plus How to Get the Most Out of Your Leg Exercises

If you're slouching while reading, the next sentence might scare you: "Sitting is the new smoking; it's just as insidious," warns Marc Hamilton, Ph.D., professor of biology and biochemistry at the University of Houston.

Hamilton makes clear that so many Americans are turning their leg muscles—and therefore their bodies—to mush. "You've seen the flat line on an EKG when all the doctors rush in? That's what happens to your leg muscles when you sit," he adds. As he speaks, I remember a job I had at a digital agency: When I showed up at the New York office on my first day, half the employees were at their computers. Because they didn't have chairs. The office consisted mostly of makeshift waist-height desks that we set up at bar height. (More: 3 Crucial Exercises to Combat Desk Job Body)

It turns out my fellow hipsters were onto something. "Talking on the phone or filing while standing isn't exercise for anyone, but compared to sitting, it increases your metabolism a little," says Hamilton. In case you were wondering, "light office work" while sitting burns 96 calories per hour for the average 140-pound woman, as opposed to 147 calories standing, according to a widely accepted compendium of physical activity. But more importantly, "when we sit for long periods of time, hundreds of 'bad' genes are activated, including those that stimulate muscle loss," adds Hamilton. (See also: 5 Ways to Build Bigger, Stronger Glutes That Have Nothing to Do With Squats)

Intrigued, I head to Kaiser Permanente Los Angeles Medical Center to see firsthand how opting for a chair and a laptop all day takes a toll on my leg muscles. Once there, Barry R. Chi, MD, chief of physical medicine and rehabilitation, wires my leg muscles to surface electrodes connected via several long cables to an electromyography (EMG) machine. I re-enact a day in the life of my legs by sitting, standing, walking (in both heels and flats), tiptoeing, and jogging. We top this off with squats and lunges as a benchmark to measure daily muscle activity.

True to the EKG analogy, the leg muscle readings on the EMG monitor are actually flat lines when I sit in a chair - it's like I'm not even there. But something happens when I stand in front of the monitor: it fills with electrical activity. “You may not feel anything, but your leg muscles are now supporting your entire body weight, and all of your large muscles of the body are now engaged in isometric contractions,” says Dr. Chi and points to the raised lines. “Standing for two hours can be the equivalent of a 2-mile run,” he explains.

Interestingly, my quadriceps and hamstrings show greater impact when standing or walking in heels than in flats, but Dr. Chi is quick to warn about long-term side effects of wearing heels, such as: B. Back pain.

How genetics changes your leg muscles

The length of your legs is fundamentally a matter of genetics – and that could mean way back in the family tree. In general, women are slightly longer than men: The latest body measurement statistics from a SizeUSA study conducted by TC², a nonprofit clothing industry resource, show that the average 18- to 45-year-old woman's legs (determined by crotch height) account for about 45 percent of her total height, versus 44 percent for the average man in the same age group. (More: Is It Actually Harder to Lose Weight When You're Short?)

Leg muscles are a different story. They depend a lot on your genes and what you do with them. The second half of this equation will be discussed later - i.e. diet, exercise, couch sitting habits - but for now a quick lesson on the anatomy of the leg muscles.

Everyone has the same major leg muscles: quadriceps, hamstrings, adductors, shins and calves. However, within these larger muscle groups there are several smaller muscles, each with its own unique function(s) - adduction, flexion, extension, rotation. For example, semitendinosus, semimembranosus, and biceps femoris are all part of the hamstrings. It's worth noting that while the adductor muscles are located on the inner thigh and help move your leg toward the midline of your body, the abductors are not just "outer thigh muscles," but rather muscles in the glutes that help rotate your hip. This story will stick to the muscles beneath the prey. (But here's a guide to your glutes if you're interested.)

But there is a wide range of sizes and muscle builds among people that even experts debate. “Muscle fibers in humans have evolved so that most of us have a leg anatomy with a majority of slow-twitch fibers, which give us our endurance during long runs,” said Daniel Lieberman, Ph.D., professor of human evolutionary biology at Harvard University. “We are built more for endurance, whereas chimpanzees have more fast-twitch fibers,” he explains. With less powerhouse fast twitch fibers, people are at a disadvantage when it comes to speed. “As a species, we are terrible sprinters,” says Lieberman. "Cheetahs can run 25 meters per second. The fastest human, [Jamaican world champion sprinter] Usain Bolt only runs 10.4 meters per second," he adds. (See also: Everything you need to know about slow and fast twitch muscle fibers)

It turns out that the quads, or quads, are the real wildcards 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., former professor of physical therapy at the University of Alabama at Birmingham. On the other hand, the muscles of elite marathon runners can contain up to 90 percent slow-twitch fibers. The average person's quads, or even those of swimsuit models or hulking bodybuilders, are more of a 50-50 mix of the two.

The problem is that many people are often so afraid of having big thighs and calves that they neglect strength training their legs. But actually, bulky legs are mainly due to fat. "Our legs can go from a shank steak to a marbled rump roast without us even knowing it," says Vonda Wright, MD, a double-board-certified orthopedic surgeon based in Orlando, Florida. "It's a snowball effect when we start accumulating fat, and it affects muscle function and strength," she continues. (See: Fire Up Your Lower Body With This Beginner-Friendly Workout)

Body composition of your leg muscles

If you were assigned female at birth, your hormones have signaled that fat cells should be stored around your butt and thighs since puberty, ultimately serving as an energy reserve for pregnancy and breastfeeding. "Women tend to gain fat in very specific parts of the body, most often from the waist to the knee," explains Andrew Da Lio, MD, professor and chief of plastic and reconstructive surgery at the University of California, Los Angeles. The most common of these parts is the outer thigh, he says.

There are two layers of fat in the legs: a superficial layer and a deeper layer, explains Dr. Da Lio. In the superficial layer, cellulite is found when fat penetrates between the tissues that connect the skin to the underlying muscle. Gain too much of the deeper leg fat and it can actually start to infiltrate your leg muscles, says Dr. Wright. The good news? This deeper layer is also usually the first layer of fat to shrink during exercise. (See more: Try this no-equipment leg workout if you can't make it to the gym)

How to strengthen your leg muscles

Last fall, I tried an experiment with the help of the Risk Factor Obesity Program lab at the UCLA Center for Human Nutrition. I did every exercise I routinely avoid at the risk of my legs looking like Schwarzenegger's: dozens and dozens of squats and lunges every week, combined with the stair climber and cycling classes. And a funny thing happened: According to the lab's DEXA (Dual-Energy X-ray Absorptiometry) body scanner, I lost 10 percent of the fat on each thigh in four weeks. After eight weeks of sticking to a low-calorie diet, I had lost more than an inch on each thigh.

"You can change the composition of your leg muscles - the ratio of fat to lean mass. Increasing your strength and endurance will lead to a change in the appearance of your legs," says Dr. Wright. And there was my proof in the form of the x-ray-like DEXA readouts that showed the grayish halo that is the fat on my thighs was shrinking.

But here's the kicker: The darker center, consisting of my quads and hamstrings, wasn't bursting at the seams after those countless squats. In fact, it pretty much stayed there, which is the moral of this story. If I hadn't done these repetitions during the diet, my muscles would probably have shrunk a little and so would my metabolism.

Stronger legs can indeed be a secret to maintaining a healthy body. "Increasing the strength and endurance of your legs generally makes it easier to exercise and exercise, resulting in more physical activity throughout the day. You burn more calories overall," says McCarthy. In fact, a study from the University of Alabama at Birmingham found that women who continued to lose weight a year after the diet had much greater leg strength than those who didn't. (Also: The Best Leg Day Exercise Trainers Want You to Add Them to Your Workouts)

But what about your ankles?

The region between your calves and ankles is defined not by muscles, but by the Achilles tendon that connects the two. For some, this area tightens dramatically from a well-toned calf muscle, while for others it gradually decreases. And then there are those whose lower legs appear to fall in a straight line without any indentation, inspiring the unflattering and completely body-shaming label: cankles.

“Candles are essentially a visual effect,” says Dr. Wright. "Models often look like they have ankles because their legs are tubes from knee to ankle. It's all relative," she says. (More importantly, weak ankles and ankle mobility affect the rest of your body.)

In order for the calf to look rejuvenated, the calves need to have some muscle size. Still, many are hesitant to strengthen their calf muscles because they fear they will become fatter and create a cankle effect. "That's a myth. Knee joints don't come from muscles, because up to the ankle they're just tendons," explains Dr. Wright. It's a matter of genetic roulette, fat accumulation and body composition.

Okay, now that you've gotten a lesson on how and why leg muscles look and behave, here's a breakdown of what exactly your leg muscles are and where to find them.

The anatomy of the leg muscles

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

Foreleg muscles

When it comes to the front of your legs, there are two muscle groups – the anterior thigh muscles (i.e. your thigh) and the anterior calf 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 forms your shinbone and helps you flex your ankle to move your foot toward your knee. The peroneus longus runs along the outside of your front lower leg.

Lateral leg muscles

Your inner leg muscles or inner thigh muscles are known as the 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 and rotating the femur.

Hind leg muscles

The hamstrings (at least below the glutes) make up your hamstrings and calves. The three hamstring muscles—biceps femoris, semitendinosus, and semimembranosus—are responsible for bending your knee and extending your hip. The calf muscles include the gastrocnemius, the top of your two, which provides power to your feet with every step, and the soleus, which lies below the gastrocnemius. The tibialis posterior is a very small muscle deep in the calf that helps stabilize your foot.