Biomechanics of the Knee Joint PPT for the teaching purposes.

1. LOWER LIMB BIOMECHANICS

2. CONTENTS
• Introduction
• Normal Joint Ranges of Motion
• Muscle Action
• Joint moments in the lower limb

3. Introduction
• The human knee is important in maintaining
the body centre of gravity on a smooth
sinusoidal pathway during walking, as seen in
the sagittal plane
• Knee flexion during early stance coupled with
ankle planter flexion to provide shock
absorption) is followed by controlled
extension during mid stance and flexion
during late stance

4. Introduction Cont…
• Also foot clearance during the swing phase is
achieved by knee flexion
• The wearing of a knee Orthosis (KO) or KAFO
may inhibit knee flexion and the patient is
committed to an abnormal gait because the
loss of knee joint has a considerable impact on
the function and appearance of gait.

5. Introduction Cont…
• The patient with a fixed knee and ankle has to
perform the whole exercise of initiating and
controlling the swing of their Orthosis with
their remaining hip flexors
• Where an Orthosis is required to stabilize the
knee in extension, an energy penalty must be
accepted

6. Normal joint range of motion
The motion of the knee joint may be
considered to have two distinct elements
rotational about a transverse (medio-lateral)
axis which, because of the geometry of the
articulating surfaces, moves as the joint flexes
antero-posterior sliding of one limb segment
relative to the other

7. Normal joint range of motion Cont..
• It can be seen that, as
the joint flexes, the
radius of the curvature
of the approximately
circular surface is ever
changing
• the changing curvature
means a constantly
changing joint centre of
rotation.

8. Normal joint range of motion Cont..
• The contributions of translation and rotation
to the motion of a normal knee joint are
determined by the configuration of its
ligamentous structures.
• There are two major pairs of ligaments which
control the knee
cruciate ligaments
collateral ligaments

9. Ligaments
• The cruciate ligaments
are arranged within the
joint in such a manner
that they control
antero-posterior joint
motion (towards genu
recurvatum,or knee
hyperextension

10. Ligaments
• The collateral ligaments
are aligned to give good
resistance to lateral and
medial deviations of the
knee (towards varus
and valgus)

11. Joint range of motion
The cruciate and collateral ligaments together
control axial rotation.
• The normal full ranges of sagittal knee joint
motion are;
Flexion 1300
Extension 00
The ranges required for normal gait are;
Flexion 650
Extension 00

12. Joint range of motion
• If the knee attitude is incorrect then the
alignment of the ground reaction force/weight
line, relative to the knee joint will be grossly
affected.
• Thus if the knee is in a hyperextended, varus
or valgus attitude at mid stance the joint will
be subjected to an abnormal and potentially
damaging moment, Fig 1 depicts the case of a
valgus knee

13. Joint range of motion
• Thus if the knee is in a
hyperextended, varus
or valgus attitude at
mid stance the joint will
be subjected to an
abnormal and
potentially damaging
moment, Fig 1 depicts
the case of a valgus
knee
Fig 1

14. Muscle Action
• If there is a flexion contracture which results
in excessive flexion at mid stance then the
load alignment will not be directly damaging
to the ligamentous structures but will create
an abnormally large flexion moment, with a
consequent increase in joint loading.
• As we shall see, this will require extra knee
extensor muscle activity to maintain stability.

15. Muscle Action
• it is usually sufficient to consider the muscles,
which cross the knee joint, as either extensors or
flexors
• The major muscle groups which directly control
the knee joint are the quadriceps (extensors) and
hamstrings (flexors) although the gastrocnemius
also acts as a knee flexor
• The action of these muscle groups is complicated
by the fact that several of them are shared with
hip or ankle

16. Muscle Action
• In normal standing the knee extensors do not
need to act because the load line does not
pass behind the knee joint centre.
• However if for example flexion contractures
prevented full extension of both knees, then
the individual would collapse if the quadriceps
did not exert a controlling extension moment

17. Muscle Action
• The relationship between the required
stabilizing knee extensor muscle force and the
flexion angle of the knees in standing is easily
determined. Fig 1 shows the knees flexed at
angle of ф

18. Muscle Action
• The angle ф is the sum of
the angles of inclination
of the thigh and lower
limb.
• If we assume that the hip
joint is vertically above
the ankle joint, and that
the thigh and lower leg
are of equal length, then
the inclination of the
lower leg to the vertical
axis will be ф/2. Fig 1

19. Muscle Action
• During normal standing the ground reaction
force, F, on each leg is vertical and equal to
half the body weight. The flexion moment MKF
due to F , which acts at a perpendicular
distance a from the knee joint is;
• MKF = F*a

20. Muscle Action
• The required stabilization moment, provided
by the quadriceps force Q acting at a distance
b from the joint centre is;
• MKQ = Q*b
• Since the stabilization moment balances the
flexion moment
• Q*b = F*a

21. Muscle Action
• Let us assume that the ground reaction which
acts vertically is a distance, c, anterior to the
ankle joint
• But the length l of the lower leg between the
knee and the ankle joints is related to
distances a and c and angle ф as follows (see
Fig 1)
• (a+c)/l = sin(ф/2)

22. Muscle Action
Which rearranges to;
a = ((l*sin(ф/2) )-c)
• Assuming as stated above that the flexion at the
hip and ankle are about half that at the knee
(assuming in turn that upper and lower leg
segments are approximately equal in length)
This gives;
Q*b = F*((l*sin(ф/2) )-c)
Q = F*((l*sin(ф/2) )-c)
b

23. Muscle Action
Assignment;
• A person weighing 74kg
stands with both knees
flexed at 300. The line of
action of the quadriceps is
assessed to be 52mm from
the knee joint centre and
the length of the lower leg
is 312mm. The ground
reaction force acts 19mm
anterior to the ankle joint.
Estimate the quadriceps
muscle force Q.

24. Joint moments in the lower limb

25. Joint moments during a squat
exercise
• The squat exercise aims to
work the quadriceps;
however we can determine
any additional effects this
may have about the ankle
knee and hip joints
• To determine the effect we
need to know the point of
application of the ground
reaction force and the
position of the ankle, knee
and hip joints

26. Joint moments during a squat
exercise
• In the diagram we can see that the ground
reaction force falls in front the ankle joint ,
behind the knee joint and through the hip
joint.
• Before we consider the effects on the muscles
around the ankle, knee and hip joints we are
going to consider the moments at the ankle,
knee and hip joints created by the ground
reaction force

27. Moments about the ankle, Knee
and Hip joints
• If we know the magnitude of the ground
reaction force and the horizontal distances
from the ground reaction force to the ankle,
knee and hip joints, we can consider the effect
of the ground reaction force for each joint
separately.
• So for each joint we focus only on the position
of the joint and the force acting.

28. Moments about the ankle joint
• If the ankle joint is a horizontal distance of about 0.08m away
from the ground reaction force of 800N all we have to do is to
consider the force and the joint and whether the force will try
and turn clockwise or anticlockwise.
• So all we are considering in fact is what effect an 800N force
will have about a pivot.
• If we consider the pivot to be fixed in space then the 800N
force will try and push up , which will try and turn the distal
segment of the joint in an anticlockwise direction.
• Therefore the moment will be considered as negative
moment

29. Moments about the ankle joint
So moments = Force x distance
• Moment about ankle joint = -(800N x 0.08m)
= -64Nm
• The negative sign indicates that the effect is to try and turn the distal
(foot) segment anticlockwise in relation to the proximal (tibia)
segment of the joint.

30. Moments about the knee joint
So moment = Force x distance
Moment about the knee = + (800x 0.16)
= 128Nm
The positive sign indicates that the effect is to try and turn the distal (tibia)
segment clockwise in relation to the proximal (femoral) segment of the joint

31. Moment about the Hip joint
So moment = Force x distance
Moment about hip joint = 800 x 0

32. Ground reaction forces during
walking
• During walking external ground reaction force
acts on the lower limb.
• These are due to the foot hitting or pushing
off from the ground and the acceleration and
deceleration of the body.
• Therefore the ground reaction force during
walking are more complicated than those
during the squat as are not only going to
simply move upwards

33. Ground reaction forces during
walking
• For a more complete analysis, the acceleration
and deceleration of the body segments should
be considered as these will have an effect on
the moments about joints.
• However at first we will consider the effect of
the ground reaction force

34. Ground reaction forces during
walking
• If the point of application and
angle of the ground reaction
force and the position of the
ankle, knee and hip joints are
known, we can calculate the
turning moments produced by
the ground reaction force about
the ankle joint, knee joint and hip
joints.
• From these we can determine
what muscle groups must act to
support these moments

35. Ground reaction forces during
walking
A ground reaction force of 950N is acting at an angle
of 820 to the horizontal
• Horizontal ground reaction force = 950 x Cos 82
= 132.21N
• Vertical ground reaction force = 950 x Sin 82
= 940.75N

36. Calculating the moments about
the joints during walking
• Now that the vertical and horizontal force
components have been found, we can
consider the action of each component about
each joint separately

37. Moments about the ankle joint
• The vertical component of the ground
reaction force acts straight through the ankle
joint. Therefore this will not produce a
moment.
• The horizontal component acts to the left and
below the ankle joint
Moment about the ankle joint = 940.75 x 0 + 132.21x 0.1
= 13.22Nm

38. Moments about the Knee joint
• The vertical component of the ground
reaction force acts in front of the knee joint
• The horizontal component acts to the left and
below the knee
Moment about the knee joint = -940.75 x 0.08 + 132.21 x 0.04
= - 75.26 + 5.28
= -69.98Nm

39. Moments about the hip joint
• The vertical component of the ground
reaction force acts in front of the hip joint
• The horizontal component acts to the left and
below the hip joint
Moment about the hip joint = -940.75 x 0.25 + 132.21 x 0.85
= -235.18 + 112.37
= -122.81Nm

40. So what are the effects of these
moments on the muscles?
• The moment about the ankle joint is a planter flexing
moment; therefore the muscles in the anterior
compartment of the ankle joint the dorsiflexor must
be active.
• The moment about the knee joint is a flexing
moment; therefore the muscles in the anterior
compartment of the knee joint, knee extensors must
be active.
• The moment about the hip joint is a flexing moment;
therefore the muscles in the posterior compartment
of the hip joint, hip extensors must be active.