Detailed lecture regarding tendon mechanics

1. TendonMechanics

2. LectureAims
32
• T
o remind you of the mechanical concepts of
strain, stiffness andcompliance
• T
o examine the rheological properties of
materials & illustrate mechanical responses to
loading
• T
o discuss the influence of tendon mechanical
properties on musclefunction
• T
o introduce the affect of stretching & training on
tendon mechanicalproperties
• T
o understand in vivo measurement of tendon
stiffness during dynamicexercises

3. TheMuscle-Tendon-Unit (MTU)
Vastus Lateralis
24
Gastrocnemius Hill’sMuscleModel

4. Strain
47
• Theloading of amaterial will causeadeformation, which is
known asstrain
• There are 3main types of load and thereforestrain:
• Tension =pulling force - makesobject longer andthinner
• Compression =pushing force - makesobject shorter and
thicker
• Shear = a load comprised of 2 equal, opposite and parallel
forces that tend to displace one part of an object with
respect to an adjacent part along a plane parallel and
between the line of theforces

5. Load Characteristics – Principle Strain
L
∆L
T
ensile Compressive Shear
61

6. Loading characteristics
Tension Com pression
Combination of tensileand
compressive loading forces
Combination of compressive,
tensile and shear loading forces
11
Bending T
orsion

7. CalculatingStrain
• If the loading is longitudinal i.e. in tension the material
will tend toelongate
• Thestrain canbe defined aschange in length/original
length
• Strain, =Changein length
Original length
7
r
r
• Thisis unit lessi.e. cm/cm and is usually expressed asa
percentage - Strain =∆L/L*100

8. Stress
• When amaterial undergoes adeformation asaresult of
applied forces it reacts tothis change
Stress:
=the resistance of the intermolecular bonds ofan object to
the strain caused by aload
=the measure of amaterial’s ability to resist an appliedforce
• Canbe defined as“the internal force per unit area upona
cross section of that apart ofabody”
Stress, =Force
Cross-SectionalArea
28
F (Pa)
A
• Stresscan be longitudinal (normal) or transverse to the
cross section

9. Rheological Properties ofMaterials
49
Rheology - the study of the deformation and flow of matter
• Elasticity – relates to the ability of the material to return to
its original dimension after loading – for a purely elastic
material the relationship between loading and deformation
will be astraight line – i.e. energy isstored
• Viscosity – here the material will deform with loading but
will have a lag between developing stress and the resultant
strain – the greater the rate of loading, the greater the
stress developed – the material will retain its new
shape/size – i.e. energy is absorbed

10. Rheological Properties ofMaterials
56
• Material can possess properties of both viscosity
and elasticity and hence be viscoelastic. Here the
material will tend to deform and return to its
original shapein anon linearfashion
• Plasticity – here when the material is deformed it
tends to retain its new shape/size. Deformation
tends to be without alag and energy isabsorbed

11. Stress – StrainRelationship
Stress
Strain
HR =HookeanRange
(Linear)
HR
ER
ER =ElasticRange
PR PR =PlasticRange
A
B
C
A
A
E=
Young’sModulus
(ElasticModulus)
A
A
14

12. Stress – Strain Relationship
(Stiffness andCompliance)
• It allows the description of the material in terms of the
rheological properties previouslydefined
• It relates to Hooke’slaw and allows the determination of
the material ‘stiffness’ –Young’smodulus (E =∆σ/∆ε)
• Avery stiff material can tolerate high loads (stress) with
only small deformations (strain)
• Ahigher value for Eis indicative of astiffer material
• Compliance is sometimes used instead of stiffness and is
simply the inverse of stiffness i.e. the ratio of strainchange
to stresschange 52

13. Hysteresis and Stress -Strain
Theamount of
energy stored may
not all be givenback
subsequent to
unloading – this can
be illustrated viathe
stress– strain curve
and canbe asa
result of damping
35

14. Creep
• With prolonged
loading amaterial
may exhibit creep
•Here strain
increasesunder
constantprolonged
loading
58

15. StressRelaxation
• When amaterial experiences aconstant strain the
stress will tend to decrease with time
20

16. Material Fatigue
• Amaterial canwithstand
afinite numberof
stresses above agiven
level after which failure
or rupture is likely e.g.
stressfracture of bone
• Below endurancelimit
the materialcan
withstand aninfinite
number of stresses
1

17. TendonInjury
43
Researchperformed using isolated tendons:
Damagemay occur at:
• 15-30%strain (Haut & Pawlinson,1990;
Stäubli et al.,1999)

18. Factors Affecting Mechanical
Properties of Tendon
3
• Age
• Gender
• Stretching
• Training
• Fatigue
• Chronic disease
• Time of day

19. Influence of TendonMechanical
Properties on Muscle Function
37

20. Why study tendonmechanical
properties?
25
Function of tendons:
• Tensileforce transmission
• Storage and release of energy duringlocomotion
(Maganaris and Paul, 2002)
The mechanical properties of tendon significantly affect
muscle output andfunction

21. Tendon properties can influence the
force-velocity relationship of muscle
Tendonacts asaseries viscoelastic component in the muscle tendon
complex
Tendonstiffness (K)can effect the relationship betweenforce
and velocity in muscle
0.10
0.05
0.00
0.15
0.20
0.25
0.30
0 0.5 1 1.5 2 2.5 3
V e lo city
Force
If atendon is relatively compliantit
canresult in areduced ability to
generate force
51

22. Tendon properties can influence the
length-tension relationship of muscle
Theamount of muscle filament overlap canalso be changedwith
changes in tendon stiffness
Muscle length tension
relationship
All things being equal a
more compliant tendon
will require agreater
amount of filament
sliding before external
force isgenerated
33

23. Tendon properties caninfluence
changes in pennationangle
Here if we consider apennate muscle in series with atendon under
isometric loading:
Asforce is developed and the tendon stretches the muscle fibrecan
change its angle of Pennation
Rest
59
Contracted

24. Tendon properties can influencechanges
in pennation angle – thus resultantforce
Thischange in angle effects the effective force seenexternalto
the muscle – tendon complex
θ
18
mf
Ultrasound image of
muscle fibres
showing pennation
angle

25. Tendon properties can influencerate
of forcedevelopment
In some instances it is required to generate forces rapidly e.g. to
correct a trip or in many sporting situations especially where an
explosive effort isrequired
Low K
HighK
31

26. EMD
Thisalso hasan effect on electro mechanical delay (the time lag
between muscle activation and muscle forceproduction)
Thiscould effect the ability to carry out anumber of motor tasks
due tothe delay between muscle activation and external movement
Compliant tendons would delay action of muscle spindles (stretch
reflex)
26

27. Energy storage and release -SSC
FromKawakami
et al. J.Physiol.
(2002)
40
and released during
concentric contraction –
up to ~93%of energy is
returned (Alexander,2000)
Movement economy canalso be modulated asenergy is capable
of being stored and released from thetendon
Stretch-shorten-cycle (SSC)
Activation of muscleduring
lengthening of muscle –
increased lengthening of
tendon
Energystored in tendon

28. EnduranceTraining
46
• No effect of endurance training on mechanical
properties (i.e. K/Young’sModulus) of the PTorA
T
(Rosageret al., 2002; Hansenet al., 2003;
Karamanidis andArampatzis, 2006;Arampatzis et
al., 2007)

29. Measurement of TendonMechanical
Properties (in vivo) during Dynamic
Movements
12

30. Measurement of TendonProperties
36
In order to estimate tendon mechanical properties
(stiffness) both elongation and force in the tendon
haveto bedetermined
In order to measure the mechanical properties of
tendon in vivo we useacombinationof:
• Motion Analysis
• Ultrasonography
• Electromyography
• Dynamometry
• Force

31. Motion Analysis – SagittalPlane
44
• Can use 2D or 3D motion capture depending on
information required for research
• Markers placed on lateral aspects of ankle, knee
and hip joints (marker on tendon insertion may
also be necessary)
• Sagittal motion of the above joints required to
calculate instantaneous MTU length and tendon
moment arms using regression equations
obtained from cadaver studies (Hawkins and Hull,
1990; Visseret al.,1990)

32. Example of 3D MotionAnalysis
15

33. Ultrasound – Mode ofOperation
2
• B-mode ultrasound is a useful tool for the
imaging of softtissue.
• Its mode of operation is via the transmission and
reception of soundwaves.
• Ultrasound waves are produced by oscillating
crystals at a frequency that is inaudible to the
human ear.
• Transducers located in the probe produce sound
(for example) at 7.5mhz which is then pulsed at
intervals which occur every 20micro-seconds.

34. Ultrasound – Mode ofOperation
23
• These sound waves penetrate and encounter the
different tissue interfaces as they travels through
the body.
• When sound encounters tissues or tissue planes,
part of the wave is reflected back to receivers in
this sameprobe.
• The transducer must be in contact with the
medium scanned, in this case skin, so a
"transmission jelly" is used to insure a complete
"union". The ultrasound produced can not travel
through the air and then into the body.

35. Ultrasound – Mode ofOperation
53
• This mode analyses the intensity of the returning
ultrasound signal as well as the direction and depth
from, which it wasreflected
• A two-dimensional grey-scale image is constructed
with different intensities from the returning signals
being assigneddifferent levels ofbrightness
• Generally, a high-density structure such as
tendon/bone will reflect ahigh-intensity signal back to
the probe and be displayed aswhite on thescreen
• Weuseultrasonography to measure tendonelongation

36. TendonElongation – Method 1
Usedmainly during
isometric assessmentof
tendon stiffness, but can
also be used to measure
tendon
60

37. Instantaneous MTU length is determined from
sagittal joint angle data (Hawkins and Hull,
1990)
TendonElongation – Method 2
θ
4
Instantaneous
muscle length is
determined by
multiplying
muscle fascicle
length

38. Electromyography(EMG)
The resultant signal from many action potentials is
what we measure with the surface EMG(sEMG)signal
34

39. EMG
• sEMG allows the determination of when a muscle
is switched on or off
• The root mean square (RMS) value of a sEMG
signal has been suggested to be a measure of the
strength of muscleactivity
• For some muscles it has been shown that there is
essentially a linear relationship between sEMG
RMSand force output (Lippold,1952)
41

40. EMG
• Relationship between
RMSsEMGand force
output ofmuscles
9

41. EMG
How do we measure this electricalactivity?
• For simple single differential measurement (to
reduce noise) 2 electrodes are placed over the
muscle belly of interest
• Thesignal is then
amplified and
filtered before
being sampledby
acomputer to be
saved
17

42. EMG’suse in
determining tendon properties
• To determine levels of co-contraction and
hence co-contraction force.
• During agonist muscle contraction antagonists
are also active and producing force.
• The
force
agonists must overcome this
before external torque is
‘hidden’
recorded
through forcereadings.
44

43. Dynamometry
• Used to determine EMG activity of agonist and antagonist
muscles during MVC– used to calculate antagonistforce
Co-contraction effort (CT)definedas:
(EMGduring extension / Max flexor EMG)*Max flexor torque
Total extensor torque =CT+Extensor torque
• Allows EMG activity attained during dynamic movement to
be normalised to EMG activity attained during MVC (when
comparing groups)
• Canbe assessedover arange of joint angles specific to the
range demonstrated during the dynamictask 38

44. Ground Reaction ForceData
Required to calculate tendonforces
Tendonforce is derived
by multiplying
instantaneous joint
moment (asdetermined
using inverse dynamics)
by instantaneous
tendon moment arm
(Visser et al.,1990)
50

45. Calculating TendonStiffness
Tendon stiffness (N·mm-1) is
then determined from the
slope of the elongation –
force relationship
Remember westated
stiffness was
∆force/∆length?
27

46. Results
Determination of tendon stiffness and separation of
muscle and tendon components from the wholeMTU
Solid line =MTUlength, dotted line =tendon
length, broken line =muscle length 57

47. Normalizing Values
• Differences in tendon length and/or cross
sectional area canaffect the stiffnessvalues
• It is therefore important to normalise the
stiffness to account for these dimensional
factors when comparing different groups
Young'smodulus is suchavalue:
K*(L/CSA)or stress/strain
29