The Knee and Patellofemoral Joints 261of quadriceps contraction and the angle of knee
flexion. That is, the harder the quadriceps contracts
the greater the compression force, and the farther
the knee bends the greater the compression force.
However, often these two factors are linked together.
For example, when one bends the knee from a stand-
ing position such as in a grand plié, the center of
gravity of the body falls increasingly posterior to the
axis of rotation of the knee joint as the plié proceeds.
This increases the effect that gravity has to make the
knees bend further (increased moment arm of the
resistance), requiring greater quadriceps force to
counter this effect of gravity and a resultant higher
compression force.
At the same time, as the knee bends, the effect of
the changing angle between the quadriceps tendon
and patellar tendon also contributes to the rising com-
pression force. When the knee is in full extension or
very slight flexion, the patella is being pulled upward
almost parallel to the femur, and the quadriceps tendon
and patellar tendon are almost in line. Hence, this
force is acting primarily in a vertical direction, and
there is little or no patellofemoral compressive force
(C 1) irrespective of the magnitude of quadriceps
tension (Fm 1) as seen in figure 5.18A. However, as
knee flexion proceeds, the angle between the quad-
riceps tendon and patellar tendon changes; now the
force does not just act vertically, but rather there is a
larger component of the quadriceps force that acts
in a direction to create compression of the patella
against the underlying femur (C 2) as seen in figure
5.18B. Thus, with increasing knee flexion, an increas-
ing quadriceps force (Fm 3) and an increase in the
percentage of this force that is being directed toward
the patellofemoral joint act together to increase com-
pression forces (C 3) as seen in figure 5.18C.
Drawing from activities of daily living, as the
angle of knee flexion and magnitude of quadriceps
contraction increase from walking to stair climbing
to deep knee bends, associated compression forces
have been calculated to rise from approximately .5
to 1.2 times body weight to 3.3 times body weight to
7.6 times body weight (Reilly and Martens, 1972). For
a dancer weighing 120 pounds (54 kilograms) these
activities would be associated with approximately 60
to 144 pounds (27-65 kilograms), 396 pounds (180
kilograms), and 912 pounds (414 kilograms) of com-
pression force, respectively. Examples of movements
from dance that would have high patellofemoral
compression forces include the grand plié, fondu,
lunge, movements used to get up from and down
to the floor, and the jump. Large jumps have been
estimated to be associated with forces of about 20
times body weight (Dowson and Wright, 1981), that
is, about 2,400 pounds (1,089 kilograms) of compres-
sion force for a 120-pound dancer.Muscular Analysis of Fundamental Knee Movements.
As previously described, the knee is primarily capable
of flexion and extension, with some transverse rota-
tion. The knee joint functions to help support the
weight of the body as well as to change the length
of the lower limb in accordance with movement
requirements. For example, in walking, through
appropriate timing of flexion and extension of
the knee, vertical movement of the whole body’s
center of gravity is minimized and energy economy
maximized. A summary of the key muscles capable
of producing the fundamental movements of the
knee is provided in table 5.2.Knee Flexion
Remember that knee flexion involves bringing the
posterior surfaces of the upper leg and lower legFIGURE 5.17 Law of valgus. Orientation of quadriceps
in the frontal plane, Q angle, and consequent valgus
vector (right knee, anterior view).