1. Biomechanics
of the
Knee Complex : 7
DR. DIBYENDUNARAYAN BID [PT]
SENIOR LECTURER
THE SARVAJANIK COLLEGE OF PHYSIOTHERAPY,
RAMPURA, SURAT

2. Muscles
 The muscles that cross the knee are typically thought
of as either flexors or extensors, because flexion and
extension are the primary motions occurring at the
tibiofemoral joint.
 Each of the muscles that flex and extend the knee
has a moment arm (MA) that is capable of
generating both frontal and transverse plane
motions, although the MAs for these latter motions
are generally small.

3.  Therefore, each of the muscles, although grouped as
flexors and extensors, will also be discussed with
regard to its role in controlling frontal and transverse
plane motions.

4. Knee Flexor Group
 There are seven muscles that flex the knee. These are
the semimembranosus, semitendinosus, biceps
femoris (long and short heads), sartorius, gracilis,
popliteus, and gastrocnemius muscles.
 The plantaris muscle may be considered an eighth
knee flexor, but it is commonly absent.

5.  With the exception of the short head of the biceps
femoris and the popliteus, all of the knee flexors are
two-joint muscles.
 As two-joint muscles, the ability to produce effective
force at the knee is influenced by the relative position
of the other joint over which that muscle crosses.

6.  Five of the flexors (the popliteus, gracilis, sartorius,
semimembranosus, and semitendinosus muscles)
have the potential to medially rotate the tibia on a
fixed femur, whereas the biceps femoris has a MA
capable of laterally rotating the tibia.

7.  The lateral muscles (biceps femoris, lateral head of
the gastrocnemius, and the popliteus) are capable of
producing valgus moments at the knee, whereas
those on the medial side of the joint
(semimembranosus, semitendinosus, medial head of
the gastrocnemius, sartorius, and gracilis) can
generate varus moments.

8.  The semitendinosus, semimembranosus, and the
long and short heads of the biceps femoris muscles
are collectively known as the hamstrings.
 These muscles each attach proximally to the ischial
tuberosity of the pelvis, except the short head of the
biceps, which has a proximal attachment on the
posterior femur.

9.  The semitendinosus muscle attaches distally to the
anteromedial aspect of the tibia by way of a common
tendon with the sartorius and the gracilis muscles.
 The common tendon is called the pes anserinus
because of its shape (pes anserinus means “goose’s
foot”) (Fig. 11-32).

10.  The semimembranosus muscle inserts
posteromedially on the tibia (and, as noted
earlier, has fibers that attach to the medial meniscus
that can facilitate posterior distortion of the medial
meniscus during knee flexion).
 Both heads of the biceps femoris muscle attach
distally to the head of the fibula, with a slip to the
lateral tibia.

11.  The short head of the biceps femoris muscle does not
cross the hip joint and, therefore, acts uniquely at the
knee joint.
 The rest of the hamstring muscles cross both the hip (as
extensors) and the knee (as flexors); therefore, their
efficacy in producing force at the knee is dictated by the
angle of the hip joint.
 Greater hamstring force is produced with the hip in
flexion when the hamstrings are lengthened over that
joint, regardless of knee position.

12.  When the two-joint hamstrings are required to
contract with the hip extended and the knee flexed to
90° or more, the hamstrings must shorten over both
the hip and over the knee.
 The hamstrings will weaken as knee flexion proceeds
because not only are they approaching maximal
shortening capability, but also the muscle group
must overcome the increasing tension in the rectus
femoris muscle that is approaching passive
insufficiency.

13.  In non-weight-bearing activities, the hamstrings
generate a posterior shearing force of the tibia on the
femur that increases as knee flexion increases,98
peaking between 75° and 90° of knee flexion.
 This posterior shear or posterior translational force
can reduce strain on the ACL, although conceivably
increasing strain on the PCL.

15.  The gastrocnemius muscle originates by two heads
from the posterior aspects of the medial and lateral
condyles of the femur and attaches distally to the
calcaneal (or Achilles) tendon.
 Except for the small and often absent plantaris
muscle, the gastrocnemius muscle is the only muscle
that crosses both the knee joint and the ankle joint.

16.  Much like the hamstrings’ interaction with the hip
joint, the gastrocnemius muscle quickly weakens as a
knee flexor as it loses tension with the ankle in
simultaneous plantarflexion.
 The gastrocnemius muscle (capable of generating a
large plantarflexor torque at the ankle) makes a
relatively small contribution to knee flexion,
producing the most knee flexion torque when the
knee is in full extension.

17.  As the knee is flexed, the ability of the gastrocnemius
muscle to produce a knee flexion torque is significantly
diminished.
 The gastrocnemius muscle does, how-ever, work
synergistically with the quadriceps and, during gait, may
be capable of increasing the stiffness of the knee joint.
 At the knee, therefore, the gastrocnemius muscle appears
to be less of a mobility muscle than a dynamic stabilizer.

18.  The sartorius muscle arises anteriorly from the
anterosuperior iliac spine (ASIS) and crosses the femur to
insert into the anteromedial surface of the tibial shaft
(most often as part of the common pes anserinus tendon).
 Variations in the distal attachment of the sartorius muscle
are not uncommon and may be functionally relevant.
 When attached just anterior to its typical location, the
sartorius muscle may fall anterior to the knee joint axis,
serving as a mild knee joint extensor rather than as a knee
flexor.

19.  Typically, however, the sartorius muscle functions as
a flexor and medial rotator of the tibia.
 Despite its potential actions at the knee, activity in
the sartorius muscle is more common with hip
motion rather than with knee motion.
 During gait, the sartorius muscle is typically active
only during the swing phase.

20.  The gracilis muscle arises from the symphysis pubis and
attaches distally to the common pes anserinus tendon.
 The gracilis muscle functions primarily as a hip joint
flexor and adductor, as well as having the capability to
flex the knee joint and produce slight medial rotation of
the tibia.
 The three muscles of the pes anserinus appear to
function effectively as a group to resist valgus forces and
provide dynamic stability to the anteromedial aspect of
the knee joint.

21.  The popliteus muscle is a relatively small single-joint
muscle that attaches to the posterolateral lateral
femoral condyle and courses inferiorly and medially
to attach to the posteromedial surface of the
proximal tibia.
 The primary function of the popliteus muscle is as a
medial rotator of the tibia on the femur.

22.  Because medial rotation of the tibia is required to unlock
the knee, the role of unlocking the knee has been
attributed to the popliteus muscle.
 However, it should be noted that unlocking is part of
automatic rotation and is due in part to the obliquity of
the joint axis and the anatomy of the articular surfaces.
 The obligatory medial rotation of the knee joint during
early flexion is a coupled motion that would likely occur
even with paralysis of the popliteus muscle.

23.  The popliteus muscle does, however, play a role in
deforming the lateral meniscus posteriorly9 during
active knee flexion, given its attachment to the
lateral meniscus.
 Activity of both the semimembranosus and the
popliteus muscles will generate a flexion torque at
the knee, as well as contribute to the posterior
movement and deformation of their respective
menisci on the tibial plateau.

24.  The menisci will move posteriorly on the tibial
condyle even during passive flexion.
 However, active assistance of the semimembranosus
and popliteus muscles ensures that tibiofemoral
congruence is maximized throughout the range of
knee flexion as the menisci remain beneath the
femoral condyles, while also minimizing the chance
that the menisci will become entrapped, thus
limiting knee flexion and risking meniscal injury.

25.  The soleus and gluteus maximus muscles do not
cross the knee joint. However, we would be remiss if
we did not mention their function at the knee during
weight-bearing activities.
 The soleus muscle attaches proximally to the
proximal posterior aspect of the tibia and fibula and
attaches distally to the calcaneal tendon.

26.  With the foot fixed on the ground by weight-bearing,
a soleus muscle contraction can assist with knee
extension by pulling the tibia posteriorly (Fig. 11-33).
 As noted earlier, the posterior pull of the soleus on
the weight-bearing leg can also assist the hamstrings
in restraining excessive anterior displacement of the
tibia.

27.  The gluteus maximus muscle, like the soleus muscle,
is capable of assisting with knee extension in a
weight-bearing position. It is well known that the
large muscle mass of the gluteus maximus functions
well as a hip extensor.
 With the foot flat on the ground and the knee bent, a
contraction of the gluteus maximus must influence
each of the joints below it. In this case, the
contraction generates knee extension and ankle
plantarflexion (see Fig. 11-33).

28.  The gluteus maximus, however, would produce, if
anything, a posterior shear of the femur on the tibia
(or a relative anterior shear of the tibia on the femur)
that would increase tension in the ACL without
offsetting co-contraction of other muscles.

29. Muscles of the Thigh Part 3 - Posterior Compartment video

30. Knee Extensor Group
 The four extensors of the knee are known collectively as
the quadriceps femoris muscle.
 The only portion of the quadriceps that crosses two joints
is the rectus femoris muscle, which crosses the hip and
knee from its attachment on the anterior inferior iliac
spine.
 The vastus intermedius, vastus lateralis, and vastus
medialis muscles originate on the femur and merge with
the rectus femoris muscle into a common tendon, called
the quadriceps tendon.

31.  The quadriceps tendon inserts into the proximal aspect
of the patella and then continues distally past the
patella, where it is known as the patellar tendon (or
patellar ligament).
 The patellar tendon runs from the apex of the patella into
the proximal portion of the tibial tuberosity.
 The vastus medialis and vastus lateralis also insert
directly into the medial and lateral aspects of the patella
by way of the retinacular fibers of the joint capsule (see
Fig. 11-14).

33.  Together, the four components of the quadriceps
femoris muscle function to extend the knee.
 In 1968, Lieb and Perry examined the direction of
pull of each of the components of the quadriceps.
 The pull of the vastus lateralis muscle alone was
found to be 12° to 15° lateral to the long axis of the
femur, with the distal fibers the most angled.

34.  The pull of the vastus inter-medius muscle was
parallel to the shaft of the femur, making it the
purest knee extensor of the group.
 The angulation of the pull of the vastus medialis
muscle depended on which segment of the muscle
was assessed.

35.  The upper fibers were angled 15° to 18° medially to
the femoral shaft, whereas the distal fibers were
angled as much as 50° to 55° medially.
 Powers et al., using more current technology,
reported that the resultant pull of vastus lateralis
muscle was 35° laterally, whereas the resultant pull
of the vastus medialis muscle was 40° medially
(Fig. 11-34A).

36.  Because of the drastically different orientation of the upper
and lower fibers of the vastus medialis muscle, the upper
fibers are commonly referred to as the vastus medialis longus
(VML), and the lower fibers are referred to as the vastus
medialis oblique (VMO).

37.  The obliquity of the distal portion of the vastus
medialis muscle has become the focus of attention
in patients with patellofemoral pain as clinicians
and researchers have attempted to try to
preferentially recruit the VMO to maximize its
medial pull on the patella.

38.  It should be noted, however, that despite the
different orientation of the fibers of the VMO and
VML, these fibers are simply portions of the same
muscle.

39.  Lieb and Perry found the resultant pull of the four
portions of the quadriceps muscle to be 7° to 10° in
the lateral direction and 3° to 5° anteriorly in relation
to the long axis of the femur.

40.  Powers et al., however, used a multiplane analysis
and noted that the relatively large vastus lateralis
and vastus medialis muscles have a posterior
attachment site,
which results in a net posterior or compressive force
that averages 55 ° in the extended knee (see Fig 11-
34B).

41.  The compressive force from these muscles is present
throughout the ROM but is minimized at full
extension and increases as knee flexion continues.

42. Patellar Influence on Quadriceps Muscle Function
 Function of the quadriceps muscle is strongly
influenced by the patella (which, in turn, is strongly
influenced by the quadriceps, as we shall see
shortly).
 From the perspective of mechanical efficiency, the
patella lengthens the MA of the quadriceps by
increasing the distance of the quadriceps tendon and
patellar tendon from the axis of the knee joint.

43.  The patella, as an anatomic pulley, deflects the action
line of the quadriceps femoris muscle away from the joint
center, increasing the angle of pull and the ability of the
muscle to generate an extension torque.
 The patella does not, however, function as a simple
pulley because in a simple pulley the tension is equal on
either side of the pulley.
 In contrast, the tension in the patellar tendon on the
inferior aspect of the patella is less than the tension in
the quadriceps tendon at the superior aspect of the
patella.

44.  The knee joint’s geometry and the patella together
dictate the quadriceps angle of pull on the tibia as
the knee flexes and extends.
 During early flexion, the patella is primarily
responsible for increasing the quadriceps angle of
pull. In full knee flexion, however, the patella is fixed
firmly inside the intercondylar notch of the femur,
which effectively eliminates the patella as a pulley.

45.  Despite this, the quadriceps maintains a fairly large
MA because the rounded contour of the femoral
condyles deflects the muscle’s action line and
because the axis of rotation has shifted posteriorly
into the femoral condyle.

46.  Consequently, the quadriceps maintains a reasonable
ability to produce torque in full knee flexion, although
the patella is not contributing to its MA.
 During knee extension from full flexion, the MA of the
quadriceps muscle lengthens as the patella leaves the
intercondylar notch and begins to travel up and over the
rounded femoral condyles.
 At about 50 of knee flexion, the femoral condyles have
pushed the patella as far as it will go from the axis of
rotation.

47.  The influence of the changing MA on quadriceps
torque production is readily apparent when knee
extension strength is measured throughout the
ROM.
 Peak torques are often observed at approximately
45° to 60° of knee flexion, a region in which both the
MA and the length-tension relationship of the
muscle are maximized.
 Finally, with continued extension, the MA will once
again diminish.

48.  Although the patella’s effect on the quadriceps’ MA is
diminished in the final stages of knee extension, the
small improvement in joint torque provided by the
patella may be most important here.
 Near end range extension, the quadriceps is in a
shortened position, which reduces its ability to
generate active tension.

49.  The decreased ability of the quadriceps to produce
active force makes the relative size of the MA critical
to torque production in the last 15° of knee
extension.
 In this range, the quadriceps must increase motor
unit activity to offset the loss in active tension-
generating ability and the decrease in MA.

51. Continuing Exploration: Quadriceps Lag
 If there is substantial quadriceps weakness or if the
patella has been removed because of trauma (a
procedure known as a patellectomy), the quadriceps may
not be able to produce adequate torque to complete the
last 15° of non-weight-bearing knee extension.
 This can be seen clinically in a patient who demonstrates
a “quad lag” or “extension lag.” For example, the patient
may have difficulty maintaining full knee extension while
performing a straight leg raise (Fig. 11-35).

52.  With the tibiofemoral joint in greater flexion, removal of
the patella or quadriceps weakness will have less effect
on the ability of the quadriceps to generate extension
torque because the femoral condyles also serve as a
pulley, and the total muscle tension of the quadriceps will
be greater than in the muscle’s shortened state.
 The patient will not have a “quad lag” in weight-bearing
because the soleus and gluteus maximus muscles can
assist the quadriceps with knee extension once the foot is
fixed.

53.  The patella’s role in increasing the angle of pull of
the quadriceps enhances the quadriceps’ torque
production but at a cost.
 Increasing the quadriceps’ MA also, by definition,
increases the rotatory (Fy) component of the pull of
the quadriceps on the tibia.
 The Fy component not only produces extension
torque but also creates an anterior shear of the tibia
on the femur (Fig. 11-36A).

54.  This anterior translational force must be resisted by
active or passive forces capable of either producing a
posterior tibial translation or passively resisting the
anterior tibial translation imposed by the quadriceps.
 The ACL represents the most prominent passive restraint
to the imposed anterior tibial translation of the
quadriceps.
 Increases and decreases in the angle of pull of the
quadriceps are accompanied by concomitant increases
and decreases in stress in the ACL.

55.  The strain on both bands of the ACL ordinarily
increases as the knee joint approaches full extension.
 In the absence of passive stabilizers such as the ACL,
a quadriceps contraction near full extension has the
potential (even with a relatively small Fy component)
to generate a large anterior tibial translation, which
the patient may describe as “giving way.”

56.  The strain on the ACL evoked by a quadriceps
contraction is substantially diminished as the knee is
flexed beyond 60° and as the Fy component of the
quadriceps diminishes from its maximum value (see
Fig. 11-36B).

58.  During weight-bearing activities, the quadriceps’
activity in knee extension is influenced by a number
of other factors.
 Muscles such as the soleus and gluteus maximus
muscles are capable of assisting with knee joint
extension.

59.  When an erect posture is attained, activity of the
quadriceps is minimal because the line of gravity
passes just anterior to the knee axis for
flexion/extension, which results in a gravitational
extension torque that maintains the joint in
extension.

60.  The posterior joint capsule, ligaments, and largely
passive posterior muscles maintain equilibrium by
offsetting the gravitational torque and preventing
hyperextension.
 In weight-bearing with the knee somewhat flexed, as
during a squat or when someone cannot fully extend
the knee (as in the case of a flexion contraction), the
line of gravity will pass posterior to the knee joint
axis.

61.  The gravitational torque will now tend to promote
knee flexion, and activity of the quadriceps is
necessary to counterbalance the gravitational torque
and maintain the knee joint in equilibrium.

62.  Because the quadriceps femoris muscle has the
responsibility of supporting the body weight and
resisting the force of gravity, it is about twice as
strong as the hamstring muscles.
 Although the hamstrings perform a similar function
in supporting the body weight when there is a
gravitational flexion moment at the hip, the
hamstrings are assisted in this function by the large
gluteus maximus while the quadriceps are the
primary knee joint extensor.

63.  Clearly, the quadriceps functions differently, depending
on the activity or the exercise condition.
 In non–weight-bearing knee extension, the MA of the
resistance (i.e., weight of the leg plus external resistance)
is minimal when the knee is flexed to 90° but increases
as knee extension progresses (Fig. 11-37).
 Therefore, greater quadriceps force is required as the
knee approaches full extension. The opposite happens
during weight-bearing activities.

64.  In a standing squat, the MA of the resistance (i.e.,
the superimposed body weight) is minimal when the
knee is extended and yet increases with increasing
knee flexion (Fig. 11-38).
 Therefore, during weight-bearing activities such as a
squat, the quadriceps muscle must produce more
force with greater knee flexion.

65. Quadriceps Strengthening:
Continuing Exploration:
Weight-Bearing versus Non–Weight-Bearing
 Wilk and coworkers110 investigated anteroposterior
shear force, compression force, and extensor torque
at the knee in weight-bearing versus non-weight-
bearing exercises that are used for quadriceps muscle
strengthening.
 These authors found that the weight-bearing
quadriceps exercises of a squat and leg press resulted
in a posterior shear force at the knee throughout the
entire ROM, peaking between 83° and 105 ° of knee
flexion.

66.  The posterior shear would presumably stress the
PCL. There was no anterior shear anywhere in the
ROM.
 In contrast, there was an anterior shear force in a
non–weight-bearing knee extension exercise when
the quadriceps actively extended the knee from 40°
to 10°, with the maximal anterior shear occurring
between 20° and 10°.

67.  One might assume that the ACL was a key element in
resisting the anterior shear that was found.
 A posterior shear force was also found during non–
weight-bearing exercise, but this force was present
only between 60° and 101° of flexion.

68.  Weight-bearing exercises are often prescribed after
ACL or PCL injury on the premise that they are less
stressful, more like functional movements, and safer
than non–weight-bearing exercises.

69.  This study demonstrated that the stress on the PCL
that is present during some types of weight-bearing
exercises may actually be detrimental to the healing
process if this ligament is damaged.

72. Muscles of the Thigh Part 1 - Anterior Compartment video

73. Stabilizers of the Knee
 Since the beginning of this chapter, we have
identified the role of both passive
(capsuloligamentous) and active (muscular) forces in
contributing to stability of the tibiofemoral joint.
 However, attempting to credit structures with
contributing primarily to one type of stabilization is
extremely difficult and generally requires
oversimplification.

74.  The contribution of both muscles and
capsuloligamentous structures to maintaining
appropriate joint stability are dependent on the
position not only of the knee joint but also of the
surrounding joints, the magnitude and direction of
the applied force, and the availability of secondary
restraints.

75.  There can also be considerable variation among
individuals (as well as between knees in the same
individual) that contributes to the diversity of
findings observed by both clinicians and researchers.
 Although admittedly an oversimplification,
Table 11-2 summarizes the potential contribution of
the different structures that limit:
anteroposterior translation or knee joint
hyperextension, varus/valgus rotation, and
medial/lateral rotation of the knee joint.

78.  Table 11-2 describes stability in terms of straight
plane movements. In reality, there are more
complicated motions that are possible.
 Therefore, stability is often described as coupled
stability, or as rotatory stability (a combination of
uniplanar motions) (Table 11-3).

79.  For example,
injury to the posterolateral corner (i.e.,
posterolateral joint capsule, popliteus muscle,
arcuate ligament) can yield posterior instability and
excessive lateral tibial rotation.
This is termed posterolateral instability.

80.  In contrast, damage to the POL, medial
hamstrings, MCL, and posteromedial joint capsule
contribute to posteromedial instability.
 The extensor retinaculum, which is composed of
fibers from the quadriceps femoris muscle, fuses
with fibers of the joint capsule to provide dynamic
support for the anteromedial and anterolateral
aspects of the knee.

81. End of Part - 7