Anatomy and Physiology: Don't Wait. Rotate!
Don't Wait. Rotate!
Rotational motion is motion where the tip of the shaft of the bone stays at the same angle, but it rotates along its longitudinal (or long) axis, like a pencil rolling between your fingers.
Remember the directional terms prone (face down) and supine (face up)? If you think about anatomical position, the hands are always facing forward (toward the ventral surface); the “back of your hand” is, like the rest of your body, dorsal. If you keep those in mind, pronation and supination shouldn't be a problem. Try this. Dangle your arm down to one side and try turning your palm so it faces backward, but don't move your shoulder! That movement is called pronation.
Don't confuse pronation and supination with medial and lateral rotation. Both pairs are rotational movements, but pronation and supination are specialized. In internal (or medial) rotation and external (or lateral) rotation of the arm, the radius and ulna stay parallel, and all the movement is at the shoulder (or at the hip in the case of rotation of the leg). Pronation and supination (of the arm only) are entirely due to the radius and ulna, with no movement at the shoulder.
The opposite motion, from the palm facing backward to the palm facing forward is supination. It is important to note that at no point is there any movement at the shoulder. All of the movement is in the lower arm. In pronation the head of the radius pivots in the radial notch of the ulna, and the distal end of the radius ends up crossed over the ulna. This is the reason why anatomical position has the palms facing forward, for in this position the radius and ulna appear parallel. Figure 7.4 compares these different types of motions.
Internal rotation (sometimes called medial rotation) and external rotation (sometimes called lateral rotation) differ from pronation and supination by the location of the rotation; unlike pronation and supination, there is no movement in the lower arm. In medial rotation all the movement is in the shoulder, and the arm rotates so that, once again, the palm faced backward, but this time the radius and ulna don't cross. Think of that movement as rotating the arm toward the midline of the body, hence the name medial rotation. Reverse that motion, moving the shoulder to return to anatomical position, and you have just done lateral rotation. Once again, think of rotating the arm toward the side, putting the lateral in lateral rotation. Also note that medial rotation and lateral rotation are also possible in the legs.
Right rotation and left rotation of the head are fairly straightforward, as it were (see Figure 7.5). Why do I specify right and left? I have to! With your head being right on the midline, I can't use the words medial and lateral, now can I?
Special movements are called that simply because they don't easily fit in the other categories. You can, however, divide them according to the part of the body involved. For those of you foot fetishists, let's start from the ground up, as it were. If you stand up with both feet flat, and then swivel your foot so the soles are facing each other, inward, in other words, that is called inversion. If you move your feet so that the soles are facing outward—in an imitation of the Jerry Lewis walk?—you are doing eversion.
Now imagine you have a cat who spends half the night hacking up fur balls, and after you wake up to your alarm which was set, as usual, freakishly early, you stumble right into a puddle of vomit and hair (Eeeeewwww). Right after you put your bare foot into the gunk, you stop cold and lift your toes off the ground, with your heel still on the floor. That disgusting scenario is a perfect example of dorsiflexion, and it is a scenario that my daughter's favorite cat, Jimmy, has left for me far too many a morning!
At least plantar flexion is a little less gross. Stand up, and then stand on tiptoe. That's it, plantar flexion. Presumably they are both called flexion because anatomical position starts out with the foot at a 90-degree angle. If you stand up on tiptoe you can also feel very clearly which muscle is being used: the gastrocnemius. Figure 7.6 compares these different movements.
A little higher up and you can focus on some movements that involve the jaw. Have you ever been so surprised that your jaw drops? That movement is called depression, because to “depress” is to move down. If that's hard to swallow, the opposite makes perfect sense: To raise the jaw is called elevation (see me in Figure 7.7).
The other two movements are kind of funny looking, and Jim Carey has made a good career out of using these two. Protraction is when you stick your jaw forward, as if you are trying to impersonate a cave man! Now imagine you are just offered a strawberry and used kitty litter pie … I know, that's gross, but that's the point! Don't you pull your jaw back in disgust? At least I hope you do! That movement is called retraction (see me in Figure 7.7).
There are just two movements left in the dancer's vocabulary. One involves the head. If you tilt your head from side to side, that is another form of flexion, and since it's to the side, it makes sense that it be called lateral flexion (see Figure 7.8). Just as you had to specify with rotation of the head, you need to specify right and left.
Saving the best for last, I have just one more movement. This movement is only possible with the special gift called an opposable thumb, which is why it's called opposition. Look at your hands, and, with one hand, touch the tip of your thumb, in quick succession, with the tip of each of your other fingers. That little movement makes something called the precision grip possible.
When my daughter was very young (I'm talking a few months old) she used to pick up Cheerios off her high chair by the handful. When her hand got sticky enough she would just put her wet hand on the pile of Cheerios and eat them off her hand. As she grew older, and her brain trained her thumb and forefinger to pick up Cheerios one by one, she would do it that way. It is a far less efficient way to eat cereal, but that is a crucial step in survival, that precision grip, which led humans to excel in terms of using tools. Even in the details of eating Cheerios, there is evidence of evolution!
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.