skeletal sYstem 49
useful instruction for people whose shoulders have slid forward around their rib cage. If,
however, there is an issue in the spine, pulling the shoulders back might increase neck
and upper back effort without addressing the underlying spinal issue. Also, it might be an
effective instruction once or twice, but if someone continues to pull the shoulders back
for an extended period of time, that person will end up pulled so far back that he is out of
balance in the other direction.
Joint actions
It is a fundamental fallacy to think that our human bodies work like the structures that
humans have built. Human joints are frequently compared to devices used in construc-
tion to create joints, such as a hinge or a ball and socket. The mechanics of a human joint,
however, are not the same as those of a joint between pieces of wood or metal or ceramic
or plastic, in part because of the nature of the materials.^2
Useful as it might be on a superficial level to compare the workings of the elbow joint to
a hinge, drawing this parallel limits our ideas about how movement happens at the joint.
Nothing in the body is perfectly flat or straight or less than three-dimensional, including
the articulating surfaces of the bones. Because these articular surfaces always have volume
and contour, movement in the joints is always three-dimensional.
The conventional terms used to describe movement at the joints, joint actions, describe
fairly simple movements that are flat and two-dimensional and happen in a single plane. No
single joint action takes into account the volume of the movement possibilities at every joint.
The implication of using two-dimensional language to describe movement at our joints is
that we simplify our concept of what movements are possible and then simplify the move-
ments we do. The danger is that we deprive ourselves of movement choices and overuse
the few options we think are available to us.
Because all the articulating surfaces in our joints are three-dimensional, every joint is
capable of more than one joint action, if not three or four. Equal amounts of movement
are not possible in each action, but even if it is a tiny movement, the joint has movement
in every dimension. That tiny movement could have huge repercussions on two or three
joints or in 5 to 10 years down the line.
Conventional definitions of Joint actions
The basic terms that describe joint actions apply to a majority of the joints in the body.
Several terms have specific meanings in particular joints, and some terms are used in more
than one joint but mean different things in different joints.
Anatomical definitions of joint actions often use planes to describe the movement. A
plane is a two-dimensional surface, and the three basic planes intersect at right angles to
each other. When the planes are oriented so that they intersect in the center of the body,
they can be used to describe relationships within the body (anterior and posterior describe
a sagittal relationship of body parts) or movements (flexion and extension describe sagittal
movement of the spine). The vertical plane (also called the coronal or door plane) divides
the body into front and back. The horizontal plane (also called the transverse or table plane)
divides the body into top and bottom. The sagittal plane (also called the median or wheel
plane) divides the body into right and left sides.
(^2) If you are interested in reading more about these differences, Steven Vogel has written a fascinating book called Cats’
Paws and Catapults: Mechanical Worlds of Nature and People (W. W. Norton & Company, 1998).