BIOM9510

Introductory Biomechanics

Lecture 4

Anne Simmons
a.simmons@unsw.edu.au
Free body diagrams

 The system is carefully defined and drawn

in a simplified form often as a stick figure
or an outline of the system
 Tail of the vector is placed at the point of
application
 Magnitude and direction of the vectors are
shown accurately
Free body diagrams
Joint loads in the static case - elbow exercise
 Redo the calculations for holding the weight in
the hand assuming the forearm/hand weighs
3kg and its CG is 10cm from the axis of rotation
 Then add the effects of the brachialis and
brachioradialis muscles assuming the following
 Brachialis moment arm is 5cm and its force is
50% of the force of the biceps in the y
direction only
 Brachioradialis moment arm is 15cm and its
force is 30% of the force of the biceps at 30°
to the horizontal axis
 Compare with results from other analyses
Biomechanics of musculoskeletal systems
 Analysis and knowledge of the mechanisms
involved in producing segmental
movements
 Upper limb – elbow
 Upper limb – shoulder
 Lower limb examples
Shoulder
 Articulation of the glenoid fossa of the scapula
and the head of the humerus
Shoulder

 Stabilising muscles
Shoulder
 Muscles as movers
Deltoid
 Origin - Lateral third of
clavicle, acromion, and spine
of scapula
 Insertion - Deltoid tuberosity
of humerus
 Action –
 Anterior part: flexes and
medially rotates arm
 Middle part: abducts arm
 Posterior part: extends
and laterally rotates arm
Shoulder exercise
 An athlete is holding a 10kg weight in his hand.
His shoulder is abducted to the horizontal and his
elbow is fully extended.
 If only the deltoid is in play, calculate the force that
the deltoid muscle is exerting to maintain
equilibrium in this position
 Calculate the force on the shoulder joint due to the
weight and action of the deltoid muscle.
 Detail all assumptions
 Expand the complexity of the situation with other
muscular involvement
Biomechanics of musculoskeletal systems

Hip joint
 Articulation of the pelvis and
the femur
 Femoral head
 Acetabulum of the pelvis
Biomechanics of musculoskeletal systems
Hip joint

 Femur
Biomechanics of musculoskeletal systems

Hip joint
 Pelvis
Biomechanics of musculoskeletal systems

Hip joint
 Articulation of the
pelvis and the
femur
 Femoral head
 Acetabulum
 Moderately strong
 More stable in
posterior
displacement
Hip

 The bony and

ligamentous
arrangement of the
hip joint - anterior
 Iliofemoral Y
ligament – strongest
in the body
 Pubofemoral
ligament
 Anterior stability
Hip

 The bony and

ligamentous
arrangement of
the hip joint -
posterior
 Ischiofemoral
ligament
 Ligamentum teres
 Posterior stability
Hip

 Ligamentum teres
Knee joint

 Articulation of
the distal end of
the femur and
proximal end of
the tibia
 Semicircular
femoral condyles
 Shallow convex
tibial surfaces
 Weak stability
Knee joint

Shallow convex tibial surfaces

Semicircular femoral
condyles
Knee joint

 Ligamentous and cartilaginous support

 Menisci
 Cruciates
 Collaterals
 Patellar ligament
Knee joint
Knee joint
Knee joint
Knee joint
Knee joint
Hip and knee joint - muscles

 Quadriceps
 Hamstrings
 Iliopsoas
Hip and knee joint - muscles

 Quadriceps muscles
 4 separate muscles
 Vastus x 3
 Rectus femoris
Hip and knee joint - muscles
 Rectus Femoris
 Origin
 Straight head from anterior inferior iliac
spine; reflected head from groove just
above acetabulum
 Insertion
 Base of patella to form the more central
portion of the quadriceps femoris
tendon
 Action: hip flexion
knee extension
Hip and knee joint - muscles
Quadriceps muscles
 Vastus Intermedius
 Origin
 Superior 2/3 of anterior and lateral
surfaces of femur; also from lateral
intermuscular septum of thigh
 Insertion
 Lateral border of patella; also forms
the deep portion of the quadriceps
tendon
 Action: extends the knee
Hip and knee joint - muscles
Quadriceps muscles
 Vastus Lateralis
 Origin
 Superior portion of intertrochanteric line,
anterior and inferior borders of greater
trochanter, superior portion of lateral lip of
linea aspera, and lateral portion of gluteal
tuberosity of femur
 Insertion
 Lateral base and border of patella; also forms
the lateral patellar retinaculum and lateral
side of quadriceps femoris tendon
 Action: extends the knee
Hip and knee joint - muscles
Quadriceps muscles
 Vastus Medialis
 Origin
 Inferior portion of intertrochanteric line,
spiral line, medial lip of linea aspera,
superior part of medial supracondylar ridge
of femur, and medial intermuscular septum
 Insertion
 Medial base and border of patella; also forms
the medial patellar retinaculum and medial
side of quadriceps femoris tendon
 Action: extends the knee
Hip and knee joint - muscles

Quadriceps muscles
 Small angle of attachment at the
tibia
 Large stabilising component acting
at the knee joint
Hip and knee joint - muscles

 Hamstrings
 Comprises three muscles
 Semimembranous
 Semitendinous
 Biceps femoris
Hip and knee joint - muscles
 Semimembranous
 Origin
 Superior lateral quadrant of the
ischial tuberosity
 Insertion
 Posterior surface of the medial
tibial condyle
 Action
 Extends the thigh, flexes the knee,
and also rotates the tibia medially,
especially when the knee is flexed
Hip and knee joint - muscles
 Semitendinosus
 Origin
 From common tendon with long head
of biceps femoris from superior
medial quadrant of the posterior
portion of the ischial tuberosity
 Insertion
 Superior aspect of medial portion of
tibial shaft
 Action
 Extends the thigh and flexes the knee,
and also rotates the tibia medially,
especially when the knee is flexed
Hip and knee joint - muscles
 Biceps femoris – long head
 Origin
 Long head from superior medial quadrant
of the posterior portion of the ischial
tuberosity; short head from lateral lip of
linea aspera, lateral supracondylar ridge of
femur, and lateral intermuscular septum of
thigh
 Insertion
 Fibular head and lateral collateral ligament
and lateral tibial condyle
 Action
 Flexes the knee, rotates the tibia laterally;
long head also extends the hip joint
Hip and knee joint - muscles
Iliopsoas
 Origin
 Psoas from anterior surfaces and lower
borders of transverse processes of L1 - L5
and bodies and discs of T12 - L5;
 Iliacus from upper 2/3 of iliac fossa of
ilium, internal lip of iliac crest, lateral
aspect of sacrum, ventral sacroiliac
ligament, and lower portion of iliolumbar
ligament
 Insertion
 Lesser trochanter
 Action
 Flexion of the torso and thigh with respect
to each other
Hip and knee joint - muscles
Gastrocnemius
 Origin
 Medial head from posterior nonarticular surface
of medial femoral condyle; Lateral head from
lateral surface of femoral lateral condyle
 Insertion
 The two heads unite into a broad aponeurosis
which eventually unites with the deep tendon of
the soleus to form the Achilles tendon, inserting
on the middle 1/3 of the posterior calcaneal
surface
 Action
 plantar flexion of ankle
 Knee flexion
Hip joint – muscles as movers

 Movements are
 flexion, extension
 abduction, adduction
 medial and lateral rotation
 transverse abduction (extension) and
adduction (flexion)
 Mainly longitudinal muscles
 Mainly two joint muscles
Hip joint - muscles

 Stabilisers
 Six external rotators
- posterior
 Three gluteals –
posterior
 Adductors – medial
 Tensor fascia lata -
lateral
Hip joint - muscles

 Stability during one

leg weight bearing
is very important
Hip joint - muscles

 Stability during one leg weight bearing is

important
 Anterior posterior axis through hip joint
 Pelvis abducts and adducts relative to the thigh
 Resistive force is the weight of the unsupported
body through the CG
 Motive force is supplied by the muscles that
work to hold the pelvis level ie hip abductors
on the supported side
Hip joint - muscles

 One leg weight bearing

 Weight of body – weight of leg is
a force vector through CG
 Resistive torque is high
 Body is drawn over supporting
limb by the adductors
 Body shifts to the supporting side
Hip joint - muscles

 One leg weight bearing

 Hip abductors prevent
the pelvis dropping
Hip joint - muscles

 Stabilising
components are
large
 ∴ compression force
on the hip joint is
much larger than
the weight of the
unsupported body
Hip joint

 Peak loads on the hip joint occur at

every step in the heel on and toe off
phases of walking
 Healthy subjects perform >1 million
steps per year
 High repetitive loading
Hip joint - muscles
 EXERCISE : estimate the
ratio of the compressive
force on the hip and the
weight of the
unsupported body
based on the action of
the hip abductors alone
during one leg weight
bearing
 Draw FBD
Hip joint - muscles

 Compression force on the hip joint is

much larger than the weight of the
unsupported body
 In normal walking, healthy body copes
 But loads can cause arthritis, fatigue
fractures of the femoral neck, overuse
syndrome of the soft tissues in long
distance runners
Hip joint – estimate of loads

 The load on the hip can be influenced by

changing the moment arm of the
gravitational force, changing the
moment arm of the abductors muscles
group or by using walking aids
Hip joint – influence of gait

 Duchenne limp
 CG of the body is shifted to
the side of the supporting
leg
 Moment arm D is reduced
 Force of abductors is
reduced
 Decreased load on hip joint
 Bending of lumbar spine is
unsustainable long term
Hip joint – orthosis

 Use of orthosis
 Support at ischial tuberosity
 Centre of rotation shifted to
ischial tuberosity from femoral
head
 Therefore reduced load on hip
joint
Hip joint – cane
 Use of cane
contralaterally
 Consider forces in y
direction only in stance
phase of gait
 S is force from the cane
to the hand
 W = 0.8 * m * g
 D = 2 * L1
 E = 4 * L1
 Calculate the force on the
hip joint in the y
direction
Hip joint – surgical intervention
 Varization osteotomy
where a bone wedge is
removed from the femur
 Results in a decrease in
angle between the femoral
neck and the femur
 The moment arm of the
abductor muscle is 15%
larger
Knee joint

 Articulation of
the distal end of
the femur and
proximal end of
the tibia
 Semicircular
femoral condyles
 Shallow convex
tibial surfaces
 Weak stability
Knee joint

Shallow convex tibial surfaces

Semicircular femoral
condyles
Knee joint

 Ligamentous and cartilaginous support

 Menisci
 Cruciates
 Collaterals
 Patellar ligament
Knee joint

 Good stability when knee is extended due

to taut ligaments around and within
 Less stability when flexed to accommodate
the need to change direction during
locomotion
 During flexion, ACL and collateral ligaments
are less taut allowing more mobility
Knee joint – muscular stability
Knee joint – muscular stability

 Knee must provide mobility and

stability
 Movements are flexion and extension
with some lateral and medial rotation
during flexion
 The angle of attachment of the quads
to the tibia is small and the
stabilising force exerted on the knee
by this muscle is large
Knee joint – muscular stability

 Particularly relevant when the

hamstrings are flexing the knee past
90°
 Hamstrings have a backward
dislocating component
Knee joint – muscular stability

 In this situation the stabilising

component of the quads is equal in
magnitude to the dislocating
component of the hamstrings
 Particularly relevant when the
hamstrings are flexing the knee past
90°
 Hamstrings have a backward
dislocating component
Knee joint – muscular stability
Knee joint – instrumentation

 In early 2007, the

first instrumented
tibial tray was
implanted
successfully in a 63
years old male
patient
Knee joint – instrumentation

 One week later, the

first in vivo
measurements during
weight bearing and
during physiotherapy
were captured.
Knee joint – instrumentation
Knee joint – instrumentation
Knee joint – instrumentation
Exercise

 Consider the static one leg weight

bearing situation described
previously
 Estimate the ground force acting
vertically through the foot of a 70kg
man in this position
 Estimate the joint force at the knee