Human Anatomy Muscles: How Muscles Are Named & Why
Size
When it comes to size, muscles are large or small, short or long, or wide.
The largest muscle in a related group of muscles is often referred to as maximus or magnus. An example that you're familiar with is the gluteus maximus. Gluteus is Latin for your rear end -- or more politely, your buttock. Thus, gluteus maximus identifies the largest muscle in your butt--the one you feel when you squeeze the cheeks of your butt together. Another example is the adductor magnus, which is the large muscle running down the inner thigh that pulls the leg back in from the side. You can feel this muscle if you balance against a table, putting your hand against the inside of the opposite thigh and then resisting as that hand tries to push the leg out to the side.
Minimus, not surprisingly, refers to the smallest of a group. Thus gluteus minimus identifies the smaller butt muscle located underneath its maximus big brother.
Longus, as you might suspect, refers to the longest of a group--as in the adductor longus, which is thinner than the adductor magnus and runs essentially parallel to it.
Brevis identifies the shortest of a group. The adductor brevis runs across the thigh to assist in pulling the thigh in towards your body's midline as opposed to down the length of your inner thigh as do the adductor magnus and minimus.
In Latin, the word "latus" means "side." Thus latissimus identifies the largest muscle "in width" in a group. Latissimus dorsi is the name of the large muscles that run from under your arms, across your "sides," and then across the middle of your back. Bodybuilders refer to these as their "lats."
 
So again, quick review, when it comes to size, the key identifiers are maximus or magnus, minimus, longus, brevis, and latissimus.
Shape
There are really only four shapes that you need to concern yourself with when it comes to naming muscles: trapezoids, triangles, saw tooth, and flat.
A trapezoid is like a rectangle, but with only two sides that are parallel as opposed to all four. For simplicity you can think of it as an oddly shaped rectangle. The trapezius is the large distorted rectangle that sits on the upper portion of the back and is rotated 45% so that one corner attaches to each shoulder, one at the top of the neck, and the fourth corner to the middle of the back. I actually think it looks more diamond shaped, but I didn't discover it, so I didn't get to name it.
The fourth letter in the Greek alphabet is delta, which is drawn in the shape of a triangle Δ. Deltoid, then, refers to the triangular shaped muscle that sits on the top of the shoulder.
The Latin verb for saw is "serrare." In anatomy, then, serratus means saw-toothed in shape. Serratus anterior is the name of the muscle that holds your scapula (shoulder blade) to your ribs. It gets its saw-like appearance from the pattern it makes is it attaches to each individual rib.
The French word for flat is "plat." Think of the word plateau. The platysma muscle is the broad, thin, flat muscle that has its origin across the middle of your chest, runs up across the front and sides of your neck and ends at your chin. Its primary role is to hold the other muscles of the neck in place and help with facial expressions. It comes into play when you tense your jaw and neck.
Orientation of the Muscle Fibers
Orientation refers to how the muscle fibers line up to the midline of the body. This makes it pretty simple. They can only be parallel, perpendicular, or diagonal to that line.
Rectus is the word used to identify those muscles whose fibers run parallel to the midline. The Latin word for "right" is "rectus." Rectus abdominis, for example, identifies the six-pack abdominal muscle that runs down the front of the stomach--with its fibers running parallel to the spine. I have to admit, getting parallel out of rectus is a bit of a stretch. Its original root, as I mentioned, is in the Latin word for "right or true." Think of the word rectitude which identifies the quality of being virtuous or straight. Well, from that you get "rectus" as identifying a muscle that runs straight and true -- or dare I say: parallel. Don't blame me. Nobody ever said all doctors were Latin scholars. These are the famous "six-pack" muscles. They get that name because, when well developed, the muscle gets segmented by bands of cartilage that give the muscle the appearance of having six separate sections.
Oblique comes from the old French and means "at an angle." Thus we have both the external and internal obliques of the abdomen, which run one on top of the other, with their fibers running at an angle to the midline and perpendicular to each other. These are the muscles just to the outside of your six-pack, and these are the muscles you work when you do sit ups and touch each elbow to the opposite leg.
Transverse means crosswise or perpendicular to the midline of the body. Thus we have the transversus abdominis. From the name, you can pretty much guess that they're located in the abdomen and which direction their fibers run. In fact, these muscles are located in the sides of the abdomen, underneath the obliques. And the fibers do indeed run across the body. And it's these muscles you feel when you stretch from side to side.
Mechanical Action of the Muscle
Here we're talking about what the muscle does. Does it flex, extend, turn up, turn something down, lift it up, lower it, rotate around a joint, move body parts away from a midline or pull them back towards the midline, make an area rigid, or close an opening? The action that a muscle performs is often used in naming that muscle.
Flexor muscles decrease the angle at a joint. The flexor pollicis longus is the muscle in the forearm that pulls on the thumb and bends or flexes it inward toward the palm. We already know that longus means it's the longest muscle in its group -- and, in fact, it runs the full length of the forearm from the elbow to the thumb. And "pollicis" is Latin for thumb. Thus, the name tells us that it's the long muscle that flexes the thumb.
Extensors are the muscles that counter flexors. They increase the angle at a joint. The extensor pollicis longus, therefore, is the long muscle in the forearm that straightens out the thumb once it's been bent inward.
Pronators turn limbs so that they face downwards or backwards. If you hold your arm out in front of you, palm up, it's the pronator teres muscle that allows you to turn the arm at the elbow so that the palm is now facing downwards. The Latin word "pronus" means "face down" -- as in lying prone. And "teres" is Latin for "rounded or cylindrical," which refers to the shape of the muscle.
The counter to a pronator is a supinator. The musculus supinator, for example, is the muscle in your forearm that turns your palm back facing up after you've pronated it down. Supinator comes from the Latin word supinum, which means "lying on your back."
Levators, as is obvious from the word, are muscles that lift things up. The levator ani is the muscle that pulls the anus up at the end of the digestive tract. If that muscle starts to weaken, you can feel the anus starting to push out and down -- very uncomfortable.
Depressors, as would be expected, are the opposite of levators; they push or pull things downward. The depressor labii inferioris is the muscle located below (or inferior) to the lips that pulls the lower lip down.
Rotators, as the name implies, produce a circular movement around a joint -- the rotator cuff in the shoulder being the most obvious example.
Abductor muscles move bones away from a midline in the body. The term is used both generically to describe the action of any muscle that moves away from the midline (the gluteus medius, for example, is an abductor in that its action is to pull the thigh out from the midline) and as part of the formal name of a handful of muscles such as the abductor pollicis brevis, which pulls the thumb away from the palm. 
Adductors move the bones back towards the midline as we saw with both the adductor longus and the adductor brevis, which are located in the inner thigh and that we looked at previously.
Tensor muscles make things rigid. The tensor fascia lata muscle in the leg tightens and gets rigid to support the knee.
Sphincters close openings, as does the anal sphincter.
Number of Points of Origin
A small number of muscles are named after the number of points of attachment they have at their points of origin. These would be:
The biceps brachii in your arm that has two points of origin. Brachii is Latin for branches -- thus, your biceps muscle has two branches at its origin.
 
The triceps brachii in your arm has three points of origin.
And the quadriceps in your leg has four points of origin.
Origin and Insertion
An even smaller number of muscles are named after the parts of the body where they start and end -- their origin and insertion. The sternocleidomastoid muscle, for example, originates from both the sternum and clavicle (breastbone and collarbone) and inserts into the mastoid bone (just below the ear).
Named by Function
And a small number of muscles are named after their function.
The risorius is a facial muscle that is crucial for the expressions of smiling and laughter. Humans are the only animals that have a well developed risorius.  Risorius comes from the Latin word "risus," which means "laugh."
Masseter comes from the Greek word for "chew," and that's its function in the human jaw.
The sartorius muscle (from the Latin word for "tailor") runs from the outer hip, across the thigh, and ends at the inner knee. This muscle pulls the leg up at the knee while simultaneously turning it inward. It is used to cross the legs in the manner of an old time tailor sitting on the floor and sewing hides together--hence the name.
 
Location
And finally, some muscles are named by where they are found in the body.
The temporalis muscle is named after the temporal bone (your temple) on top of which it is located.
Likewise, the zygomatic bone is your cheekbone, and the zygomaticus muscles are located over your cheekbone.
Review of the major muscle groups
Now that we know how muscles are named, let's quickly work through the body and take a look at the major muscle groups
Upper body front
Pectoralis major muscles originate in the sternum and clavicle. They attach under the arm and pull the arms up across the body and rotate them inward. These are your pecs. They are worked by butterflies and bench presses.
The deltoid originates in the clavicle, with insertion in the arm below the shoulder. The deltoid elevates the arm when it contracts.
Biceps brachii and brachialis work in tandem, attaching in the forearm, and lifting the forearm.
Rectus abdominis is a paired muscle that runs down either side of the abdomen. It is supported by tendons that give the muscle the appearance of being segmented -- thus the six pack.
Thigh muscles
Adductor magnus and longus, which pull the legs together at the midline
Rectus femoris, which helps you kick
Vastus lateralis and vastus medialis, which along with the rectus femoris help extend the knee and form a cage around it.
 
Back
The trapezius and latissimus dorsi, joined by a mass of tendons, pull across the back.
Under the scapula are the Infraspinatus, teres minor, and teres major, which all serve to stabilize the shoulder blades so that the shoulders can move.
Gluteus maximus and the gluteus minimus hidden underneath the maximus are primarily stabilizing muscles. They support the pelvis and the trunk where it sits on the femur (the thigh bone).  These muscles are most noticeable when standing on one leg. Without special exercise--squats, lunges, and backward leg lifts--they tend to lose tone as we age. This is the saggy butt often associated with aging.
Hamstrings: semitendinosus and biceps femoris (not to be confused with the biceps brachii in your arm) -- to flex the lower leg at the knee. (Semitendinosius comes from the Latin words for "half tightly stretched" and refers to normal tightness of the calf muscles.)
Calf muscles: gastrocnemius, with its two heads -- medial and lateral. (Gastrocnemius comes from the Greek and Latin words for calf.)
Face and Neck
Sternocleidomastoid muscles flex and rotate the head.
Omohyoid muscles are used primarily in chewing and swallowing.
Sternohyoid helps control the hyoid bone, which plays a key role in swallowing and speech.
Thyroid cartilage.
Conclusion
Let's look back at a couple of key pieces of information that we explored today and see what principles we can draw from them that might be useful in optimizing our own health.
Developing Muscles in Balance
First, early in the newsletter we explored in some detail that all muscle movement is based on the principle of levers, activated by contracting muscles -- and that to get movement in opposite directions, we need opposing pairs of muscles that pull the levers in opposite directions when they contract. Thus, biceps bend our arms up and triceps straighten them out. This brings us to our first key concept: balance.
When you work one set of muscles--say the biceps--you need to work their counterparts--in this case, the triceps. This is not just a matter of looking pretty for bodybuilding competitions; it's a question of long term fitness. When muscles are not developed in balance:
It stresses bones in only one direction, which causes bones to build in an unsound manner. Remember, bones build in response to stress--both gravitational and exercise induced. All muscles attach to bones and pull on those bones, using those bones as levers. If, for example, you continually work your biceps without developing your triceps, your muscles are overwhelming pulling on your arm bones in only one direction. This causes the bone to develop strength on one side, and weakness on the other. (We'll soon explore how bones build in response to stress, which is why weight bearing exercise is so important.) Prolonged, uneven stress on bone is a prescription for serious bone curvature or even a predisposition for breakage down the road.
It also should be noted that developing muscles that are out of balance is a prescription for chronic muscle aches and pains as the over developed muscles continually pull and tweak their underdeveloped counterparts.
And it's bad for spinal alignment. If your core muscles are not developed equally (or not at all), they will pull the spine in one direction or the other, rather than stabilize it in the proper alignment. This guarantees chronic back pain and even disc problems in time.
Muscle Cages
Our muscles also tend to work as protective cages in several areas of our body. These cages not only protect those sensitive areas from damage, they also support and stabilize those areas to protect them from going out of alignment and chronic pain. Two key areas stand out: your lower back and your knee.
The core muscles of your stomach (rectus abdominis, transversus abdominis, and the obliques) and your back (the latissimus dorsi) not only protect your internal organs and hold them in place; they also literally hold your spine in correct alignment. If those muscles weaken or develop an imbalance, you can pretty much guarantee, you'll be dealing with chronic back pain.
As for your knee, a cage of muscles and their tendons runs from the thigh to the lower leg protecting the knee from instability and injury. Damage to these muscles and tendons requires extensive physical therapy; otherwise the knee will never recover its full strength. Medial and lateral ligaments also help stabilize the knee, as do anterior and posterior ligaments -- all of which can be injured. The real protection for your knee, though, is this cage of tendons that come from the muscles of your thigh. The stronger those muscles are the stronger that cage is. And keeping in mind that painful knees are one of the biggest reasons people have difficulty walking as they age, keeping those muscles toned and in balance makes all the sense in the world. And this is especially important after any injury--no matter your age. For example, when you are injured and become immobile, the huge muscles of the thigh, such as the quadriceps are the first muscles to atrophy. Without extensive physical therapy, they will never fully recover, which means that your knee is now more prone to injury because its cage is compromised.
Next Newsletter
In the next issue, we'll conclude our examination of human musculature by talking about the principles of exercise. You will want to read this newsletter if you are concerned about your health. Muscles are not just for good looks. They:
Give us strength.
Make bones stronger.
Hold your spine in line and your organs in place.
Protect you from falling. The stronger your legs muscles are, the less likely you are to be unsteady and fall as you age.
Protect you even if you do fall. Muscle is dense tissue. If your hip is surrounded by toned muscle, when you fall, it cushions your hip bone and protects it from breaking.
Keep you independent longer as you age.
Specifically, we'll explore the different types of muscle fibers we have and how certain exercises can selectively change the balance of those fibers in our muscles -- emphasizing one kind of fiber over another. It's what allows one set of exercise to develop a group of muscles that are suitable for a bodybuilder, while another set makes the same group of muscles suitable for a sprinter. And for those of us who are not professional athletes, understanding muscle fibers and how to exercise them is what allows us to tune our bodies to maximize the lifestyle that we actually lead -- assuming we're not couch potatoes.