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UNIT
VII – The Knee
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Objectives:
At the completion of this unit the student will be
able to:
1.
Identify the key bony structures of the knee joint.
2.
Identify the articulations that comprise the knee joint.
3.
Identify the key ligaments, capsule, and menisci about the knee.
4.
Name the anatomical motions of the knee.
5.
Name the key muscles that perform the motions about the knee,
including the anatomical attachments.
6.
Describe the role of the patella to knee function.
7.
Describe the interrelationship of the static and dynamic knee
stabilizers and their roles in stability of the knee joint.
8.
Integrate knowledge of the knee to important lower extremity issues
for the dancer.
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The knee joint is the center joint that makes up the lower
extremity kinetic chain. It is midway between the pelvis/hip complex and the
foot/ankle. |
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The knee joint is comprised of three bones: the
femur, the tibia, and the patella. The distal end of
the femur articulates with the tibia in the lower leg and the patella or
knee cap. The tibia below does not articulate with the patella. The
distal end of the femur and the proximal aspect of the tibia splay out to
provide a more stable base, like the column of a building.
The fibula, though not part of the knee joint proper, also plays a
role in muscles attachment and overall range of motion at the ankle/knee
complex. |
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The Distal Femur |
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As you recall from the previous unit on the hip, the main
length of the femur is called the shaft. The distal end that forms part of
the knee articulation consists of several important bony landmarks,
including the lateral and medial epicondyles.
These are the more proximal aspects of the bony protrusions. |
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Just distal
to the epicondyles are the lateral and medial condyles of the femur. These
condyles articulate with the tibia below.
Between the condyles is the
patellar surface that articulates with the patella or knee cap. These
landmarks are important attachments for ligaments and muscles which will
be discussed later in this unit. |
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On the posterior aspect of the distal femur
is the intercondylar fossa. This
is a large posterior opening between the two condyles. |
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The Tibia and Fibula |
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The tibia and fibula comprise the two bones of the lower
leg. The tibia is the larger of the two bones and articulates with
the femur to form the primary component of the knee joint. The tibia is
wide at the proximal and distal ends to increase stability of the knee and
ankle. |
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The proximal tibia has
lateral and medial condyles that fit against the structures of the same
name on the distal femur above. The medial condyle is larger than the
lateral condyle. The intercondylar eminence is a bony ridge that separates
the two condyles and fits into the intercondylar fossa of the femur during
flexion.(See picture below) |
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There is a large tibial tuberosity on the anterior surface of the
proximal tibia. This tuberosity is an important attachment for muscle. Also on the lateral surface is the interosseus border for
attachment with the interosseus membrane that connects the tibia and
fibula along the length of their shaft. The anterior shaft of the tibia is
also called the “shin” and has a sharp ridge.
The distal end of the tibia and fibula articulate to form the ankle
joints and will be discussed in Unit VIII. |
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The Patella |
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The patella is a sesamoid bone that develops within the
tendon of the quadriceps muscle. Sesamoid bones are round or oval nodules
that develop in certain tendons. They were originally called sesamoid
bones because of their resemblance to sesame seeds. They are commonly
found where tendons cross the end of long bones (as in the knee) and serve
to protect the tendon from excessive wear. |
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The sesamoid bone, the patella, also changes
the angle of the tendon as it passes to its attachment, thus providing
better mechanical advantage for the muscle.
The broad superior surface of the patella is called the base and
the more pointed inferior edge is called the apex. The patella is shaped
in a triangular pattern if viewed from the side. The posterior surfaces
that contact the patellar surface of the femur are the bottom two angles
of the triangle and are called the articular facets. These facets are
covered with cartilage. |
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Other Important Anatomical Knee Structures
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Menisci of the Knee
Another important anatomical component of the knee are the menisci. These structures are also called knee “cartilages”. They are
‘C’ shaped intra-articular discs that are made of fibrocartilage. The
menisci are thin pads made of strong, fibrous tissue that rest between the
condyles of the femur and the condyles of the tibia. They act as shock
absorbers between the bony surfaces. |
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The menisci are slightly mobile and
aid in spreading synovial fluid during movements of the knee. There is a
lateral and a medial meniscus in each knee. These menisci also attach to
the intercondylar eminence of the tibia, as well as to portions of
ligament and muscle around the knee joint. The menisci provide a greater
surface for articulation with the femoral condyles and promote
better distribution of forces at the knee and provide knee stability. |
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Without the menisci for shock absorption there is difficulty with
activities such as running due to the wear and tear of bone-on-bone as the
femur contacts the tibia. Also, there can be some loss of knee stability
if a menisci is removed completely.
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The Knee Joint Capsule
The capsule of the knee joint attaches just outside
the articular surfaces of the three bones that make up the knee joint. The
patella is contained within the anterior capsule.
There is a single synovial cavity that encapsulates the femur,
patella and tibia and synovial fluid circulates within the capsule.
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The capsule has folds, especially in the anterior aspect and thus
allow full knee flexion motion. The
posterior aspect of the capsule is thicker and stronger and provides for
stability in the standing position. The knee joint has two fairly flat surface
bones that rest on one another as the primary articulating surfaces.
Therefore, the joint is unstable in its bony configuration. The knee
relies heavily on other static stabilizers such as the capsule and the
ligaments, as well as the dynamic stabilizers, the muscles, for stability
at the joint.
Let's take a look at the static stabilizers, the ligaments, now. There are 4 important ligaments in the knee area. |
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The Cruciate Ligaments |
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There are two cruciate ligaments and two collateral
ligaments of the knee. The cruciates or “crossed” ligaments are
located in the intercondylar fossa of the femur. They are named according
to where they attach below on the tibia. The anterior cruciate ligament is
attached to the anterior intercondylar area of the tibia. It runs
posteriorly and superiorly and laterally to attach to the medial aspect of
the lateral femoral condyle. |
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The posterior cruciate ligament, which is
stronger than its anterior counterpart, attaches to the posterior
intercondylar area of the tibia. It runs anteriorly, superiorly, and
medially to attach to the lateral surface of the medial femoral condyle.
The anterior cruciate resists anterior displacement or shifting of the tibia on the
femur. The posterior cruciate resists posterior displacement or shifting of the tibia
on the femur. The two ligaments together form an “X” inside the knee
joint capsule.
If a cruciate
is damaged the knee can become quite sloppy and unstable. The cruciates,
especially the anterior, are often injured when the foot is planted and the
individual pivots above, thus stressing the ligaments too far. Once torn,
the individual will usually have the sensation that the knee is slipping
or going out from under them.
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The Collateral Ligaments |
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The collateral ligaments are located on the sides of the
knee joint and serve to reinforce the knee joint capsule. The medial
collateral ligament runs from the medial epicondyle of the femur to the
medial condyle and upper medial shaft of the tibia. The medial collateral
ligament functions to stabilize the joint and prevent it from opening on
the medial (inside aspect of the knee). The lateral collateral ligament
runs from the lateral epicondyle of the femur to the head of the fibula.
This ligament prevents the joint from opening on the lateral (outer) side
of the knee. |
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If either one of these ligaments is damaged or ruptured the
knee will be unstable side-to-side, depending on which side was damaged.
The medial collateral ligament is thicker than the lateral collateral
ligament. This is due to the typical angle of the knee from the hip. This
angle places more of a stress on the medial aspect of the knee joint, thus
the need for further stabilization on that side of the joint. The
collateral ligaments also serve to resist hyperextension of the knee and
provide some stabilization for medial and lateral rotation of the tibia on
the femur.
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Motions of the Knee Joint
The knee joint is primarily a hinge joint. It has two major
motions – flexion and extension. There is, however, some medial and
lateral rotation of the knee, especially when it is in a flexed position. |
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Extension - This occurs when the leg is straightened.
There is 0 degrees of bend in the knee between the lower leg and the
thigh. In this position all of the ligaments are taut and the joint can be stabilized with out a lot of muscular
activity, i.e. balancing on one foot. |
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Knee extension can be limited if the
hip is flexed and the hamstrings are tight. Due to muscle shortening in
the back of the leg, the person is not able to get the knee straight when
the hamstring is stretched. |
It is also easy to produce hyperextension of
the knee if the individual has a lot of flexibility at the joint.(See
picture of dancer above) |
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Relevance to the Dancer- Because dancers tend to be
very flexible, knee hyperextension is very common. In this
situation the dancer lets the knee bow back beyond the normal extension
range. The proximal aspect of the tibia is actually behind the distal end
of the femur. This position puts a great deal of stress on the posterior
capsule and the ligaments of the knee. It also moves the good axis of
alignment so that the knee joint rests behind that line for the leg. It is
not functional, or desirable, and can result in injury or chronic pain in the
knee. |
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Flexion – This occurs when the tibia swing
into a bent position and moves toward the back of the thigh. Normal active knee flexion is about 135 degrees. It is
limited by contact of the muscles in the back of the calf and the leg.
When the knee is bent passively there is additional available motion.
There is also less knee flexion when the hip is not bent, due to the
muscle attachment of the rectus femoris that crosses both joints. |
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Medial and Lateral Rotation of the Knee – When the
knee is extended there is not much rotation at the knee joint because the
ligaments are taut and hold the femur stable on the tibia.
When the knee is flexed the ligaments are relaxed. There is some
normally occurring knee medial and lateral rotation of the tibia below the
femur during normal walking. This is due to the shape of the knee joint
condyles. Medial rotation is when the tibial tuberosity moves medially (or
inwardly) while lateral rotation is when the tibial tuberosity moves
laterally (or outwardly). |
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Relevance to the Dancer - Remember this is the tibia below the femur that is
rotating. It is important to
distinguish this motion from hip medial and lateral rotation that occurs
above at the hip joint. It is
very important that dancers maintain outward rotation by controlling it
from the top of the femur with the lateral rotators, not the knee joint. “Twisting the tibia” by forcing outward rotation from the
feet will cause injury to the knee joint.
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The muscles that move the knee joint will be
discussed relative to the motions they perform at the knee. |
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Knee Extensors
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The primary knee extensor is the quadriceps femoris. It is a
very large muscle group comprised of 4 muscles. This muscle group as a
whole inserts into a single
quadriceps tendon at the knee. The tendon inserts on and surrounds the
patella bone, then continues as the patellar tendon to insert on the
tibial tubersosity on the tibia bone. Collectively, the four muscles
acting together extend or straighten the knee.
The quadriceps femoris is
the strongest muscle in the body. The four component muscles of the
quadriceps femoris are the: vastus lateralis, vastus medialis, vastus
intermedius (beneath the central rectus femoris), and the rectus
femoris. |
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Vastus Intermedius – This is the deepest
muscle of the four. It arises from the upper two thirds of the anterior
femoral shaft. It passes along the anterior aspect of the femur in line
with the bone and joins the patellar tendon below. |
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Vastus Lateralis and Medialis – These two muscles
arise from either side of the linea aspera of the femur, just inferior to
the borders of the greater trochanter on the posterior femoral shaft. They
wrap around the sides of the femur and meet anteriorly, superificial to the
vastus intermedius. They essentially encircle the intermedius from both
sides and then join to pass below to insert into the patellar tendon as
well. |
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Rectus Femoris – This is an important two
joint muscle in the lower leg. It arises from the anterior inferior iliac
spine and the groove just above the rim of the acetabulum on the ilium
above. This muscle, therefore, crosses the hip joint and acts on the hip
joint for motion. The vastus muscles
just discussed do not cross the hip, only the knee joint, and thus have
no function at the hip above. The rectus femoris passes superficial to
the vasti muscles to insert on the common patellar tendon that attaches at
the tibial tuberosity below.
Since the rectus femoris crosses both joints it has actions at
both the hip and knee joints. When the pelvis is fixed, the rectus femoris
flexes the hip and extends the knee (example is walking). It can also act
to pull the pelvis forward from its superior aspect (anteversion) if the femur below is
fixed. |
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The Role of the Patella in Knee
Function
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The patella bone is imbedded in the patellar tendon of the
quadriceps femoris. It serves 2 primary purposes.
- It serves as a shock
absorber to protect the knee when falling forward onto the knee.
- It also
increases the angle for pull of the quadriceps tendon since it sticks out
anteriorly. This increased angle increases the force in which the
quadriceps can contract.
The patella tracks in the groove between the
femoral condyles like a rope in a pully.(See picture bottom left)
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The patella is not stable laterally
because it follows the shaft of the femur bone. Since there is an angular
pull on the patella it is at more risk to dislocate laterally. There are a
lot of stresses and strains on the patella and it is at risk for arthritis
and inflammatory conditions. More information on its alignment in the
lower leg will be discussed at the end of this unit. |
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Relevance to the Dancer-
The rectus femoris is a very strong, superficial muscle and can often be
substituted as a hip flexor, due to its dual function at the hip and knee.
Dancers are often challenged to utilize the iliopsoas muscle group more
effectively for hip flexion, rather than the rectus femoris. The iliopsoas
provides more stability for the center of the trunk and pelvis. Use of the
rectus as a primary hip flexor results in strong contraction of the
quadriceps muscle in the front of the leg. This is not efficient nor
functional for the dancer.
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Other Secondary Knee Extensors
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Although the quadriceps femoris is the primary knee
extensor, there is some action of knee extension provided by the
superficial component of the gluteus maximus and from the tensor fascia
latae which have already been discussed in the previous unit.
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Knee Flexors
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Semimembranosus – (far left) This is one of the hamstring muscles located on
the medial aspect of the posterior thigh. It arises from the ischial
tuberosity and passes down the back of the thigh to insert on the
posteromedial aspect of the tibial condyle. It is a wider muscle belly
than its companion muscle, the semitendinosus.
Semitendinosus – (immediate left) This muscle is the other hamstring
muscle located on the medial aspect of the thigh. It also arises from the
ischial tuberosity. It inserts to the superomedial tibial shaft on the
posterior knee below. Its insertion is distal and medial
to the insertion of the semimembranosus. |
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This picture to the left is a posterior view of
both the semimembranosus and the semitendinosus. They are powerful
knee flexors. They also help to maintain the position of the pelvis (they
attach to the "sitz bones) on a standing straight
leg.
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| The hamstrings play a role in extension of the leg
(especially from a flexed position). They also serve as medial and lateral
rotators at the knee that will be discussed later in this unit. |
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Biceps Femoris – This is the hamstring
muscle group located laterally on the posterior thigh. It is comprised of
two heads. The long head
arises from the ischial tuberosity. The short head arises from the
posterior femoral shaft more distally. The two heads merge inferiorly in
the posterior thigh area and insert via a common tendon to the head of the
fibula. These hamstring
muscles, like the rectus femoris in the front of the thigh, cross both the
hip and the knee joints, and thus act at both joints. |
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Relevance to the dancer: Tight
hamstrings can be stretched effectively by hanging over, hands to the
floor. Allow gravity to lengthen the spine and elongate the back of the
legs. If it is difficult to maintain extended knees, then soften or
bend the knees until the stretching process allows the muscle to lengthen.
Maintain the alignment of hip to knee to ankle as seen to the left. |
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Secondary Knee Flexors
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There are two other muscle groups that contribute to knee
flexion: the popliteus and the gastrocnemius. |
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Popliteus – The popliteus is a small muscle that
originates from the lateral aspect of the lateral femoral condyle and
passes diagonally to insert on a triangular area of the posteromedial
tibial shaft below. It flexes and medially rotates the knee. |
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Gastrocnemius – This muscle originates from two heads
on the posterior/inferior femur, just above the condyles. It crosses the
knee joint posteriorly and attaches on the calcaneus (heel bone) below.
The gastrocnemius is primarily a muscle that functions at the ankle joint.
However, due to its attachment on the femur, it also serves as a flexor
and medial rotator of the
knee. |
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As previously noted, the primary motions at the knee are
flexion and extension. During knee flexion, however, there is some medial
and lateral rotation of the knee. Several muscle group contribute to these
motions.
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Medial and Lateral Rotators of the Knee
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The sartorius, the medial hamstrings –
semimembranosus and semitendinosus, the gracilis (an adductor of the hip),
and the popliteus all contribute to medial rotation at the knee.
The tensor fascia latae of the lateral leg, the
superficial component of the gluteus maximus, and the lateral hamstring
– the biceps femoris, contribute to lateral rotation of the knee.
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Important Biomechanics of the Knee and Leg
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- As you can see from the discussion of muscles both at the
hip and at the knee, there are many muscles that cross both joints and
thus have multiple functions for lower body movement. This is also true
for the ankle that will be discussed in Unit VIII. Since the primary
function of the lower body is weight bearing for walking and locomotion
through space, there is integration of all the structures in order for
this to happen efficiently.
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- There are several important considerations at the knee that relate to
this issue. The first is the relationship of the static and dynamic
stabilizers around the knee joint. As has been mentioned, the knee joint is
not inherently stable from its bony structures. It must rely on the other
structures to provide that stability. The ligaments, capsule, and menisci
are considered the “static” stabilizers that provide holding
stability at the joint. The muscles that function about the knee are the “dynamic”
stabilizers that actively contract to provide both motion and muscular
stability at the knee.
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- For optimal knee function it is important for the individual to have
good integrity of the static stabilizers and good strength in the dynamic
stabilizers. Unfortunately, there is a high risk of injury to the static
stabilizers. These are usually injured when the foot is planted and the
individual twists or pivots the body above. When this occurs there can be
damage to the menisci, ligaments, or the capsule. When this occurs,
significant instability in the knee results. It then becomes the job of
the dynamic stabilizers (the muscles) to take up the slack to stabilize
the knee.
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Relevance to the Dancer
It is very important for dancers and athletes to have strong lower
extremity muscles to protect the static stabilizers. Maintaining good
alignment of the knee under the trunk and over the foot with activities
will also minimize the risk of knee injury.
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The Q Angle of the Knee |
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Another important biomechanical consideration at the knee is
the Q angle. The Q angle is the angular measurement between the line of
the thigh as is passes down from the hip to the knee and the angle of the
lower leg passing down through the midline of the tibia.
As you recall from the discussion on the hip joint, the head of the
femur is angulated upwardly to articulate with the acetabulum. This angle
places the angle of the thigh as it passes to the knee joint at an inward
angle, not a straight line toward the floor. |
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This angle is more marked in
females because they have a wider pelvis for childbirth. The normal angle
for men is 10-13 degrees and for women 13-18 degrees. If this angle is
larger it can place the individual at risk for patellar instability and
inflammatory conditions around the knee. This can be important for the
female dancer if she has a large Q angle. It is a bony configuration that
cannot be changed. It is important to recognize that this may place her at
risk for knee conditions, and she should be especially careful with good lower leg
mechanics when dancing. Maintaining strong leg muscles will also be
helpful. |
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The knee is an important component for walking. This aspect
will be discussed in detail after the next unit on the ankle. It is
important to understand the anatomy structures for the hip, knee and ankle
before tackling the biomechanics of walking and locomotion.
This concludes Unit VII. Return to Blackboard to complete Quiz #3
for Unit VI & VII.
After completing Quiz #3 you will take the Mid-Term Exam, covering
Units I-VII, before proceeding to Unit VIII. See the syllabus schedule.
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