Everything

what is the structure of tendons?

tendon is a dense fibrous tissue, connects muscle to bone (force transmission), relatively stiff and strong under tension, common for tendon to slide under extensor retiniculum to help guide movement and increase efficiency

How is the PCSA oriented in relation to the muscle?

Perpendicular to muscle fibres

how does tendon relate to poissons ratio?

experience compressive strain in transverse direction due to poissons ratio but doesnt experience compressive stress

what is the extracellular matrix of tendon made of?

80%, water (55-70%), solids (collagen 30-45%, type I collagen 97-98%, type III collagen 3%, elastin 2%, proteoglycans and structural glycoproteins 1%), cellular material (tenocytes 20%)

what is the cellular material of tendon made of?

20%, tenocytes

what are the characteristics of water in tendon?

viscous behaviour, lubrication/fascicular sliding, nutrient transportation

what are the characteristics of collagen in tendon?

responsible for tensile strength, force transduction (type I, more tough), high tensile resistance (type III)

what are the characteristics of elastin in tendon?

provide elasticity

what are the characteristics of proteoglycans and structural glycoproteins in tendon?

stabilize collagenous system

what is the function of tenocytes in tendon?

mechanoreceptors to mechanical loading

what does type II collagen relate to?

more about smooth gliding surfaces

what is key to note about the structure of tendons?

hierarchical tissue starting with amino acids that form collagen then microfibrils (aka tropocollagen with helix pattern)

why is there a toe region in the stress strain curve of tendons?

partly because of unwinding of triple helix pattern

how do microfibrils bind in tendon?

series or parallel (forming fibril)

what are the regions of tendon?

external/free tendon (middle of tendon), aponeurosis/internal tendon (attachment area for muscle fibers, myotendinous junction), bone tendon junction (osteotendinous junction), muscles have tendons on both sides

what is another name for osteotendinous junctions?

enthesis

what attaches to each end of the free tendon?

osteotendinous junction

what. isa myotendinous junction?

muscle joins with tendon

why do myotendinous junctions have a wavy shape and what does it represent?

due to increased surface area which decreases stress, see spike when we pull apart with shear (pull apart strength), see longer regions due to tensile forces

what are the characteristics of myotendinous junctions?

increased surface area/contact area (decreased stress, changes form of stress from tensile to shear), golgi tendon organs (specialized nerve endings, mechanoreceptors for muscle tension

what is an osteotendinous junction?

tendon joins with bone, see a lot of woven bone turnover

explain the transition of osteotendinous junctions

happens over microns, from tendon to fibrocartilage (mostly type I cartilage so a lot of strength), fibrocartilage to mineralized fibrocartilage to bone

where are ruptures common in tendons?

in free tendon, sometimes in MTJ happening over course of microns

what can material testing measure?

force, displacement, CSA

what can material testing help us compute?

stress, strain

what does material testing help us determine?

stiffness, elastic modulus, hysteresis, fatigue life

explain the general behaviour of tendon failure

toe region (microfibrils unravel), linear (not completely), partial failure (issues with viscoelasticity), total failure

what are key points about the mechanical properties of tendons?

its really tough, stretches more than bone, modulus less than bone, kind of like mineralized tissue

what are the general values for the mechanical properties of tendon?

ultimate tensile stress (80-120MPa), ultimate tensile strain (12-15%), youngs modulus (0.8-2GPa), toe region (3%)

what are the characteristics of the achilles tendon structure and function?

two strong muscles, single tendon insertion, applies large forces to bone for efficient movement 

what are the characteristics of the tibialis posterior tendon structure and function?

one weak muscle, 4-5 tendon insertions, stabilizing effect, locking of mid foot

how does increasing the cross sectional area of a tendon impact its stiffness?

increases stiffness

how does increasing the length of a tendon impact its stiffness?

decreases stiffness

what does the slope of a force-change in length diagram show you in tendons?

stiffness of tendon

how does the thickness of a tendon impact its stiffness?

thicker tendon is stiffer and needs more force to be stretched

how will the stiffness of a tendon be impacted when increasing the CSA by a factor of 2?

stiffness increases by a factor of 2

how will the stiffness of a tendon be impacted when increasing the length by a factor of 2?

decreases the stiffness by a factor of 2

if the modulus of elasticity of a tendon is unchanged how will stress and strain be impacted?

unchanged

how does the length of a tendon impact the properties of force and stiffness?

longer tendons is less stiff and needs less force to be stretched

stiffness is related to strength and CSA, what about stress and strain?

unrelated

what are the viscous properties of tendons?

fluids resistance to fluid flow, 55-70% of a tendon is water, internal frictional forces between adjacent fibers/fibrils/fascicles etc.

what are the elastic properties of tendons?

ability to return to original shape once unloaded, collagen triple helix structure, crimp of fibers, elastin

what do the viscoelastic properties of tendon result in?

time/history dependent relationship, mechanical behaviour of tendon is dependent on loading

how does loading change the strength of tendon?

increased loading causes increased tensile strength and strain, can withstand more stretch and stiffness

what are the rate dependent properties of tendon?

high strain rate (high ultimate tensile stress and strain-can stretch further, higher young’s modulus-stifffer)

what is meant by hysteresis in tendon?

loading and unloading profile is not identical (different going up vs down), area between the lines, difference in curves represents energy loss (heat due to friction, disruption of crosslinks, 6-11%))

what is a stress relaxation test in tendons?

apply a constant deformation (strain), stress (force) decreases with time

what is a creep test in tendons?

apply constant force, deformation creeps with time

can we observe creep outside of testing in tendons?

yes when loading (ex: cyclical loading shows constant state of stress, recoverable)

what are the impacts of repeated loading in creep tests for tendons?

deformation increases each cycle, stiffness decreases each cycle, steady state after 10-20 cycles 

can we see stiffness degradation creep in vivo?

yes

what are the other main forms of creep tests?

repetitive loading, stiffness degradation, residual strain

what is meant by residual strain in a tendon creep test?

plastic creep, when tendon goes down on loading cycle theres residual strain so you’re damaging material, recoverable in lab and in vivo

what does evidence suggest regarding material damage in tendons?

damaging of material serves as stimulus for adaptation

what is meant by plastic deformation in creep?

see mechanical fatigue (accumulation of microdamage in response to repetitive loading, accumulative exercise), characterized by stress life plot

if we decrease loading by 10% how will life cycles change and what does this imply?

increase cycles to failure by 100%, small changes in loading magnitude produce large changes in cycles to failure

what is meant by fatigue life in tendons?

number of cycles of submaximal stress/strain a material can withstand before failure

how can length relate to fatigue life and cycles to failure?

increase in cyclic tasks result in shorter tendon survival before it fails, magnitude of loading is key in number of cycles it can experience before failure 

what is meant by fatigue behaviour in linear elastic regimes?

due to hierarchical nature of biological material, strain amplified at lower scales, elastic behaviour at structural level doesn’t mean you won’t see plastic at micro-structural level

how does fatigue life relate to ultimate stress?

failure below materials ultimate stress, see decrease in stiffness and increase in residual strain

what are the three regimes (in order) of a fatigue life plot?

non-linear increase in strain forming damage, damage is growing, failure

what are the four phases in fatigue life cycles?

unloaded crimped fibers, load - stretching of local tendon fibers into plastic region, plastic deformation seen as kinked fibers (indicates low damage), leads to re-allocation of load to remaining fibers (load borne primarily by other fibers still intact)

what is fatigue life sensitive to?

stress magnitude, small increase in stress = non-linear (large) increase in loading cycles to failure, implications for overuse injuries

what are the limitations to mechanical testing in tendons?

strain not necessarily uniform (stress/strain concentrations present within tendon structure, underestimation of youngs modulus with crosshead displacement), results influenced by clamping method, limited to ex-vivo specimens

explain the following limitations of material testing in tendons: limited to ex-vivo specimens

research based on specimens from animal models (subtle differences in structures and molecular content to humans), cadaveric specimen (older donors), specimen can dry out (become brittle), freeze/thaw of ex-vivo specimen (altered mechanical properties maybe), requires a-priori estimate of loads exerpeinced during activity, do not capture biological response to load (adaptation)

how do we material test in vivo for tendons?

direct measurement (ultrasound imaging-find stiffness and modulus, shear wave tensiometery-approximate force of tendon by seeing where tapping signals is picked up), indirect measurement (MRI, motion capture, dynamometry/US/EMG, musculoskeletal model)

what do MRIs show you in material testing of tendons? What does it look at?

patellar tendon moment arm and length

what do Motion captures show you in material testing of tendons? What does it look at?

ankle and knee moments (3 shoes x 3 flooring types)

what do dynamometry/US/EMG show you in material testing of tendons? What does it look at?

achilles/patellar tendon stiffness, achilles tendon moment arm and length, measure stiffness and calculate force

what do musculoskeletal muscles show you in material testing of tendons? What does it look at?

achilles/patellar tendon force 

what is acute tendon failure and what is a result of?

rupture, result of excessive stress/strain, frequently occurring during sport esp with rapid eccentric activity due to high loads (running, sprinting, jumping, agility activities), load approaches but is often below ultimate strength (modified tissue)

how are tissue properties compromised in acute tendon failure?

functional weakness (torn collagen fibrils, damaged crosslinks-decrease number of fibers and those intact experience more strain, decreased CSA-increase stress), material weakness (altered composition, inflammation, neo-plastic or scar tissue)

what are the risk factors for acute tendon failur?

combo of load and material, age, gender, weight, sport, sociological background, lifestyle, personality

how does age correlate with acute tendon ruptures?

see spike around 30-40 (achilles, because Achilles experiences some of the highest forces in body ) and 60 years old (other) then decline (less active, negative adaptions like decreased CSA, collagen fiber breakdown, decreased modulus and stiffness, loss of collagen fibers, more cross link gin so more brittle)

what is tendinopathy?

clinical syndrome characterized by pain swelling, and functional impairments

what are the characteristics of tendinopathies?

gradual onset of pain which may increase with continued activity, decrease in performance, premature cessation of activity, morning stiffness + creptus in acute cases

how do tendons respond to exercise in tendinopathies?

tenocytes respond to exercise induced strain (2-3 fold increase in collagen synthesis peaks, 24hr post exercise), matrix of metalloproteinase elevated in response to exercise (degrade collagen, response large in magnitude but shorter in duration that tencocyte mediated collage synthesis), result (negative net balance in collagen synthesis up to 36 hrs post exercise, net synthess >36hrs post)

what is the process of tendon repair in tendinopathies?

inflammation and infiltration (immediate, RBC, leukocytes, platelets with growth factors, tenocytes), proliferation of new ground substance + collagen (24-48 hrs, synthetic onstruction of type III collagen + ground substance, mediated by tenocytes), remodeling of new tissue (1-2 months, collagen type I synthesis, alignment of ECM)

what are some key factors about the pathophysiology of tendinopathies?

damage formation (highly dependent on loading magnitude), tissue (improved material and structural properties), D>Dc (critical damage threshold), injury (detrimental to material/structure)

what is the molecular pathophysiology of tendinopathise?

decreased levels of type I collagen, increased type III collagen proteins (scar tissue), increased matrix metalloproteinase - degrade connective tissue in matrix

what is the histological pathophysiology of tendinopathise?

disorganization of collagen matrix, rounding of collagen fibrils, increase in proteoglycans, glycosaminoglycans and water

what are some other pathophysiologies in tendinopathies outside of molecular and histological?

hyper-vascularization, ingrowth of sensory nerve fibers (nociceptors), delayed healing response

how does disuse impact tendons?

decreased stiffness and modulus, mitigated by exercise

how does aging impact tendons?

decrease in stiffness with age

how does exercise impact tendons?

tendon can increase stiffness and decrease hysteresis with resistance exercise even in older individuals, strain on tendon, tenocytes respond to strain via collagen synthesis, appropriately balanced with rest adaptation occurs

how does tendon adapt to training?

adapt to mechanical loading (increased stiffness, modulus, and small increase in CSA), resistance training leads to greatest changes, inducing large strains leads to greatest changes

what are advanced forms of imaging for in-vivo testing?

biplanar videoradiography 

what are the three types of muscles?

smooth, cardiac, and skeletal

what are the characteristics of smooth muscle?

typically found surrounding the lumen of tubes within the body such as blood vessels, control of smooth muscle is largely involuntary 

what are the characteristics of skeletal muscle?

attached to bones via tendon, responsible for locomotion and body motion, voluntary, crosses joints

what is the role of muscles?

force generating structure, active, 40% of body mass

what are our passive structures?

tendon, ligament, bone, cartilage 

what do muscles produce force for?

creating movement, provide joint stability, act as passive force transmitter

what is the amount of skeletal muscle in the body?

434 muscles, 40-45% of total body weight, 75 muscle pairs responsible for bodily movements and posture

how do moment arms differ for different muscles?

extrinsic muscle have larger moment arms, intrinsic have smaller moment arms and can change motion quickly 

whats the anatomy of muscles?

hierarchical structure (makes it easier to repair and create), muscle bundle (fascicle): several muscle fibres, muscle fiber (single muscle cell made of myofibrils):responds to nerve impulses

what is the function of the sarcolemma?

transmit electrical impulses (action potentials)

what do myofibrils create in the muscle? What key structure?

repeating units of sarcomeres (fundamental contractile unit of muscle) 

what does skeletal muscle consist of?

blood vessels, connective tissues, and muscle cells also known as muscle fibers

how does the amount of muscle fibers impact other aspects of the muscle?

more sarcomeres, more force production capacity