Dance Anatomy & Kinesiology

256 Dance Anatomy and Kinesiology

When the knee is not weight bearing, such as

when the knee is extended in the air in a kick or

développé, the tibia is no longer fixed and is now

the most easily moved segment. Hence, the rota-

tion occurs primarily in the tibia relative to the

femur rather than in the femur relative to the

tibia. In this condition the close-packed position

of the condyles is achieved through external rota-

tion of the tibia relative to the femur as shown in

figure 5.13A, rather than the internal rotation of

the femur seen in weight-bearing conditions. This

external rotation of the tibia generally occurs during

the last 20° to 30° of knee extension. This natural

mechanics of the knee is sometimes emphasized

when dancers straighten the knee to enhance the

look of turnout.

Structure and Movements of the Patellofemoral Joint

As described in chapter 1, the patella is formed

within the tendon of the quadriceps femoris muscle

group and is the largest sesamoid bone found in

the human body. Recall that the quadriceps muscle

complex inserts into the superior border of the

patella, and the inferior pole of the patella is joined

to the tibial tuberosity via the patellar ligament or

patellar tendon as seen in figure 5.14A. Some texts

refer to this band as the patellar ligament since it

attaches bone to bone, while others term it a tendon

because functionally it is a tendon, being composed

of fibers continuous with those of the quadriceps

tendon. The patella is located slightly above and

in front of the knee joint proper as seen in figure

5.14B, and the facets located on its posterior surface

articulate with the slightly concave femoral groove,

or patellar groove, as shown in figure 5.14, C and

D; this articulation is termed the patellofemoral

joint.

Functions of the Patella

As with other sesamoid bones, the patella increases

the ability of the muscle within which it is located to

produce effective force or torque. The patella serves

to increase the moment arm, that is, the perpen-

dicular distance of the line of action of the quadri-

ceps femoris from the axis of rotation of the knee

joint (figure 5.15B). Since torque is determined

by the force generated by the muscle times the

perpendicular distance from the line of pull of the

muscle to the axis of rotation (chapter 2), the same

force of contraction of the quadriceps will result

in greater torque than if a patella were not present

and the quadriceps ran closer to the middle of the

knee joint (figure 5.15C). Decreases in quadriceps

torque of up to 49% have been found when the

patella has been surgically removed (Levangie and

Norkin, 2001). The patella also serves to central-

ize the divergent pulls of the four muscles of the

quadriceps femoris complex, serves as a retainer

to help prevent the femur from sliding off the tibia

anteriorly, and allows for a better distribution of

compression stresses on the femur by increasing

the surface area of contact. A reduction of compres-

sion stresses is further facilitated by the fact that

the undersurface of the patella is lined with thick

articular cartilage that deforms under load in such

a way as to distribute forces over an even greater

contact area. The smooth properties of this cartilage

also allow transmission of quadriceps force around

an angle during knee flexion, minimizing the

losses due to friction. These latter functions are

critical for preventing injury to the quadriceps

tendon, since tendons are not designed to with-

stand either large compressive forces or high fric-

tion.

FIGURE 5.13 Locking mechanism of the knee with (A)

closed kinematic chain and (B) open kinematic chain

movements.