Anatomy and Physiology: Acting in Concert
Acting in Concert
With almost 700 muscles, both superficial and deep, it makes sense that some of them might work together. If you remember the structure and function of the sarcomere (see The Structure of the Muscles and Muscle Cells), you may remember that muscles only work in one direction: contraction. After a contraction the muscle is at the mercy of other muscles, since it cannot elongate on its own. When the muscle is relaxed, however, the sarcomeres, and thus the muscle as a whole, can be stretched.
An easy mistake to make is to assume that one muscle is always the agonist, and the other is always the antagonist. In truth, the agonist is always the muscle that is contracting at that moment (let's say the biceps brachii), and the antagonist is the muscle that is being stretched (let's say the triceps brachii). As soon as the triceps start to contract, it becomes the agonist, and the biceps become the antagonist.
That stretching, however, requires the contraction of other muscles. For that reason, we evolved muscles on opposite sides of the bones. Remember flexion and extension, or abduction and adduction, from The Joints? The pair of muscles that make these movements are called antagonistic pairs. The muscle doing the initial movement (of the pair of movements, flexion, for example) is called the agonist, or prime mover. The muscle doing the opposite movement, in this case extension, is called the antagonist.
With about 700 muscles, it makes sense that some muscles may not work alone, especially with both superficial and deep muscles. These helper muscles, called synergists, work to stabilize a movement, and also to increase the efficiency of the prime mover. Another type of muscle is called a fixator, and its job is to stabilize the origin of the prime mover. This is especially important in terms of the scapula.
When you compare the articulations of the scapula to those of the clavicles or the pelvic bones, you will notice something interesting. The clavicle articulates with the axial skeleton at the sternum, and the pelvic bones articulate at the sacrum, but the scapula doesn't articulate with the axial skeleton at all. The only thing holding the scapula in place is the action of the fixators; without them, the scapula would be pulled away from the body whenever we did something as mundane as flexing our forearm.
A Muscle by Any Other Name ?
With so many muscles, anatomists decided to name muscles using a few simple principles. Understanding these principles makes muscle names a snap. We have covered most of the essential principals already when we learned directional terms, bone names, and specific muscle movements (The Joints).
To give you an example of how to use the names, consider the tibialis anterior. From the name you know two things: It's not only near the tibia (tibialis), but it's in front of it (anterior). Another example is the flexor carpi ulnaris: It connects the carpals (carpi) and the ulna (ulnaris), and the movement it does is flexion (flexor). Not all the muscles are so easy, but you'll be surprised at how many muscles follow these simple naming rules. There are only a few loose ends, such as muscle shape and the direction of the muscle fiber, that we will need to get out of the way before you become an expert at muscle names. The seven characteristics used to name muscles can be seen in the following table.
|Action||Muscle movement (The Joints)||Supinator|
|Fiber direction||Angle from midline||Rectus abdominus|
|Location||Bone, directional terms||Tibialis anterior|
|Number of origins||If more than one||Biceps femoris|
|Origin and insertion||Location of tendons||Sternocleido-mastoid|
|Shape||General muscle shape||Deltoid|
|Size||Big, small, long, short||Adductor brevis|
All Shapes and Sizes
Flex Your Muscles
The name of the muscle includes more than just biceps. Were you aware that you have two biceps? In your arm you have the biceps brachii (brachii means arm), and in your leg, close to your femur, you have the biceps femoris.
The gluteal muscles are used in extending your leg, which is very useful in walking. Other animals have such muscles, but none as big as yours! We are bipeds, which means the muscles of the legs are used extensively as we walk. What is the implication of this fact? Humans have big butts!
Muscle shape is often a factor in muscle names. Many of these are based on geometric shapes: The deltoid is shaped like a triangle, the rhomboideus major is rhomboid (or diamond) shaped, and the trapezius is trapezoid shaped. A nongeometric shape name is the serratus anterior, which is saw-toothed in shaped (think serrated knife).
The number of origins is also used. Most muscles have only one origin, so the exceptions usually have that fact as part of their name. Biceps, means two origins, triceps means three, and quadriceps means four origins.
In terms of muscle size, the words maximus (big), medius (medium), and minimus (small) are often used. Think of extending your leg (you can feel this when you climb stairs), and you will be thinking of the large gluteus maximus, the mid-size gluteus medius, and the smaller gluteus minimus. These muscles, as you will often find, are organized from superficial (maximus) to deep (minimus). The words major and minor are also used to mean big and small, as in the zygomaticus major and the zygomaticus minor. Remember, think in terms of groups when you hear certain muscle names: If there's a major, look for a minor; if there's a maximus, look for a minimus!
The length of a muscle is sometimes part of the muscle name. The word longus, as you would imagine, means long, as in the adductor longus. Short muscles use the word brevis, meaning short (as in brevity), as in the peroneus brevis. Well, that's the long and short of it!
Excerpted from The Complete Idiot's Guide to Anatomy and Physiology 2004 by Michael J. Vieira Lazaroff. All rights reserved including the right of reproduction in whole or in part in any form. Used by arrangement with Alpha Books, a member of Penguin Group (USA) Inc.