The Upper Extremity 415between individuals; and many dancers, particularly
female dancers, have the ability to extend the arm
well beyond straight, that is, to hyperextend the elbow
(figure 7.40). Whether due to ligamental laxity or a
short olecranon process, dancers with marked elbow
hyperextension often have to utilize muscular contrac-
tion of the elbow flexors in isolation or in coordina-
tion with the elbow extensors to avoid this undesired
aesthetic, particularly in movements requiring arm
support. As with knee hyperextension, it often takes a
retraining of the kinesthetic sense with use of outside
feedback, such as looking in the mirror, to relearn
the position of straight versus hyperextended.
Description and Functions of Selected Individual Muscles of the Elbow.
The muscles of the elbow are arranged so that those
that cross anteriorly are in a position to cause elbow
flexion and those that cross posteriorly are in a posi-
tion to cause elbow extension. This arrangement is
similar to that at the shoulder or hip, but opposite
to that at the knee. (See Individual Muscles of the
Elbow, pp. 416-422.)
Structure and Movements of the Radioulnar Joints
The radius and ulna are connected via two synovial
joints—the proximal radioulnar joint and the distal
radioulnar joint (figure 7.45). They are also connected
via a ligamentous sheet (middle radioulnar joint).
The proximal radioulnar joint is a uniaxial, pivot
joint lying within the capsule of the elbow joint. It is
formed by the articulation between the side of the
head of the radius and a notch on the ulna (radial
notch) as seen in figure 7.45A. A strong ligament, the
annular ligament, forms a three-quarters ring around
the head of the radius, keeping it close to the ulna
so that the desired rotation can occur without other
undesired motions. You can feel the head of the radius
moving under the skin by placing a finger about an
inch below the lateral epicondyle of the right humerus
and then slowly and repetitively bringing the palm
of the right hand to face downward (forearm prona-
tion) and then upward (supination).
The middle radioulnar joint is not a synovial
joint, but rather a fibrous joint involving connec-
tion via a ligamentous sheet called the interosseous
membrane. This joint functions to keep the radius
and ulna from excessively separating or sliding apart
from each other in a longitudinal direction as seen
in figure 7.45, A and B.
The distal radioulnar joint is a pivot joint formed
between the head of the ulna and a concave surface
of the radius (ulnar notch). Note that while the
radius is the smaller bone proximally and terminates
in a “head,” the ulna is the smaller bone distally and
terminates in a “head.” This distal radioulnar joint
allows that radius (with wrist and hand) to pivot
around the head of the ulna (figure 7.45B). A trian-
gular articular disc connects the radius and ulna and
provides stability to the distal radioulnar joint.
Together, the radioulnar joints allow the radius
to rotate relative to the ulna so that the palm is
facing downward with the thumb positioned medi-
ally, termed pronation, or the palm is facing upward
with the thumb positioned laterally, termed supina-
tion (figure 7.45C). The axis of this motion can be
pictured as running between the head of the radius
proximally and the head of the ulna distally (figure
7.45A). This motion is more complex than pure rota-
tion about one bone, in that in a position of supination
(e.g., anatomical position) the radius and ulna lie
parallel to one another with the radius located lateral
to the ulna. However, with pronation, although the
proximal radius remains on the same side of the ulna
as with supination, the distal radius crosses over to
the medial side of the ulna. (See Individual Muscles
of the Radioulnar Joints, pp. 423-424.)FIGURE 7.40 Elbow hyperextension.(Text continues on p. 424.)