Tissue Mechanics- Tendon Biomechanics

Function of Tendon

1. transmit tensile load from muscle which produce motion

2. provides leverage for muscle without requiring excessive muscle tissue (store energy for explosiveness)

3. Help modulate speed of movement

   • closer the attachment of tendon to the joint axis, faster the movement

The farther the attachment of a tendon is from the joint axis, the faster the movement.

true

Describe class 3 levers

the force applied is between the fulcrum and the load

What are the three main sections of a tendon

Bone-tendon junction

Muscle-tendon junction

Mid-substance tendon

Bone tendon junction

4 Zones

• end of the tendon

• collagen fibers intermesh with fibrocartilage

• fibrocartilage gradually becomes mineralized fibrocartilage

• merges into cortical bone

  Sharpey’s fibers located at junction- important for bone healing

Myotendinous Junction

There are interdigitations between muscle cells and tendon tissues, these occur in two ways

• transmembrane proteins

• other protein-cell membrane-ECM interactions

It will increase surface area (reduce stress concentration)

Ensures that junctions are loaded in shear rather than tension (provides friction)

tenoblasts

located between the collagen fibers along the long axis of the tendon

specialized fibrocytes

more blood and plasma promote healing and realignment

What has to happen before collagen can bear a load

collagen must be organized and cross-linked before it can function as a load-bearing unit

what is the smallest tendon unit capable of resisting tensile load

tropocollagen

What is the role of epitenon in tendon, and which types of tendon is in present in

outer-most layer that surrounds the entire tendon in some cases, will enclose the nerve, lymphatics, and blood vessels supplying the tendon

only present in tendons with very high friction loads e.x. finger flexor tendons

what makes up a fascicle

nerve, lymphatics, blood vessel and tendon

associated with specific muscle fibers

what does endotenon become as it attaches to bone and why is it important

continues into the perimysium of the muscle and as it attaches to bone it becomes Sharpey’s fibers

improves continuity and decreases stress concentration

what is paratenon and how does it help tendons function

areolar tissue that covers all tendons

allows gliding of tendon

how do tendons receive blood flow, and what role do tendon sheaths play in this process

receive blood from both ends

diffusion through the tendon sheaths

tendon sheaths do not have innervation

false, they are innervated in order to feel pain

Toe region

1.5% elongation

increase length is from the uncrimping of fibers

ADLs tendon will function at 1-3% strain

elastic region

4% of tendon length

defined by young’s modulus and is linearly proportional

Plastic region

the stress distribution is not uniform

starts at one end and builds up progressively as more and more shearing stress is exerted by the interfibrillar matrix

strength is directly proportional with the diameter of the collagen fiber

beyond yield point tears begin and will continue until the fracture point

When does a tendon typically fail

8%, this is because collagen fails because of the adjacent tropocollagen molecules pull apart rather then the molecules themselves rupturing

Mechanical properties that will differ in healthy and unhealthy tendon

collagen fibril organization

differences in cross-linking

varying ground substance concentration

different collagen types

Why do tendons of different muscles such as hamstrings and vasti-muscles, differ in structure and function?

tendons of the hamstrings are longer and have smaller cross-sectional area compated to the vasti-muscles

fusiform v. pennate muscle

speed v. force development

(hamstrings can only handle 60% of what the vasti can)

Describe how tensile strength in tendons compares to the force produced by muscle

4x greater than force produced by muscle

Tendons adaptation to strength training

increase in cross-links

increase in cross-sectional area

increased collagen content

stronger tendon material

tendon adaptations to endurance training

same cross sectional area

may decrease cross links

sane or decreased collagen content

same or weaker tendon material

failure of tendon in children

avulsion injury

failure of tendon in adults

mid-length region

intrafibrillar slippage

occurs between molecules

cross-linking is responsible

interfibrillar slippage

occurs between fibers

ground substance and gross disruption by these factors plus fibril size