1. Advanced Biomechanics of Physical Activity (KIN 831)
Lecture 2
Biomechanics of Tendons and Ligaments
* Material included in this presentation is derived primarily from two sources:
Enoka, R. M. (1994). Neuromechanical basis of kinesiology. (2nd ed.). Champaign, Il: Human Kinetics.
Nordin, M. & Frankel, V. H. (2001). Basic Biomechanics of the Musculoskeletal System. (3rd ed.). Philadelphia:
Lippincott Williams & Wilkins.

2. What do you know about the macroscopic structure and function of tendons and ligaments?

3. What do you know about the microscopic structure and function of tendons and ligaments?

4. Functions of Ligaments and Joint Capsules
connect bone to bone
act as static restraint to:
help with joint stability
guide joint motion
prevent excessive motion

5. Functions of Tendons
connect muscle to bone
transmit tensile loads from muscle to bone to:
produce joint torque
stabilize joint during isometric contractions and in opposition to other torques
cause joint motion during isotonic contractions
act as a dynamic joint restraint
interact with ligaments and joint capsule to mitigate loads that they receive
Interesting points:
tendon extends the reach of muscle
tendon may conserve muscle tissue mass (i.e., muscle tissue not required to extend from origin to insertion)

6. Tendons and Ligaments
Dense connective tissues (parallel-fibered collagenous tissues)
Sparsely vascularized
Composed primarily of collagen (fibrous protein which gives tendons and ligaments strength and flexibility)
Consist of relatively few cells or fibroblasts (≈ 20% of total tissue volume)
Contain abundant extracellular matrix
≈80% of total tissue volume
≈70% of extracellular matrix is water and ≈30% solids (collagen (≈75% of extracellular matrix), ground substance, and small amount of elastin)
Structure and chemical composition identical to other animal species (extrapolate behavior from animals)

7. Tendons and Ligaments Tendons Ligaments Join muscle to bone
Organization of collagen fibers to accommodate specialized function
Fibers longitudinal and parallel
Transmit tensile muscle forces
Ligaments
Join bone to bone
Organization of collagen fibers to accommodate specialized function
Fibers generally longitudinal and parallel, some oblique and spiral
Primarily transmit forces in functional direction, but also multidirectional

8. How can you make string able to support a large load?

9. How do manufacturers of string make it able to support a large load?

10. Collagen Molecule
Synthesized by within fibroblast as procollagen (precursor to collagen)
Consists of 3 polypeptide chains ( chains) each coiled in left hand helix
3  chains combined in a right handed triple helix
Bonding (cross-linking) between  chains enhances strength of collagen molecules
Develops extracellularly into collagen molecules

11. Collagen Groups of 5 collagen molecules form microfibrils
Cross links formed between collagen molecules that aggregate at the fibril level
Cross links between collagen molecules give strength to tissues (e.g., tendons and ligaments) they compose
Fibrils aggregate further to form collagen fibers
Fibers aggregate to form bundles

12. Collagen Fiber Arrangement in Tendons and Ligaments

13. Macroscopic and Microscopic Structure of Tendon and Ligaments

14. Macroscopic and Microscopic Structure of Tendon and Ligaments

15. Macroscopic and Microscopic Structure of Tendon and Ligaments
Epitendidium -outer covering
Fascicle - bundle of fibrils
Fibril - basic load bearing unit of tendon and ligaments
Microfibril - 5 rows of triple helixes in parallel (see figure)

16. Schematic illustration depicting the hierarchical structure of collagen in ligament midsubstance

18. Macroscopic and Microscopic Structure of Tendon

19. Schematic representation of the microarchitecture of a tendon

20. Structural hierarchy of a tendon
Structural hierarchy of a tendon. Connective tissue layers or sheaths envelop the collagen fascicles (endotenon), bundles of fascicles (epitenon), and the entire tendon (paratenon)

21. Macroscopic and Microscopic Structure of Tendon and Ligaments
Collagen molecule - triple helix in series; 5 rows stacked side-by side (parallel)
Triple helix - cross links occur both between and within rows of triple helixes  strength (# and state of cross links influence strength)  determined by age, gender, and activity level

22. Elastin tendons and ligaments contain protein elastin
influences elastic properties of tendons and ligaments (↑ elastin  ↑ elasticity)
proportion varies by function
little in tendons and extremity ligaments
much present in ligamentum flavum between laminae of vertabrae
protect spinal nerve roots
pre-stress the motion segment
provide intrinsic stability to spine

23. Ground Substance in Tendons and Ligaments
amorphous material in which structural elements occur
in connective tissues, composed of proteoglycans, plasma constituents, metabolites, water, and ions between cells and fibers
Ground Substance in Tendons and Ligaments
Proteoglycans act as cement-like substance between collagen microfibrils contributing to overall strength of tendons and ligaments

24. Water and Proteoglycans
Forms a gel
Viscosity decreases with activity
Thixotrophy (property seen in catsup)
Increased ability to accommodate higher velocity stretches
Advantage of a warm-up

25. Vascularization of Tendons and Ligaments
Dual Pathway for Tendons
Vascular (tendon surrounded by paratenon)
receives blood supply from vessels in perimysium, periosteal insertion, and surrounding tissues
Avascular (tendon surrounded by tendon sheath)
Synovial diffusion
Healing and repair in the absence of blood supply
Ligaments
Vascularity
Originates from ligament insertion sites
Small size and limited blood flow
Take home message:
Amount of tissue vascularization is directly related to rate of tissue metabolism and healing
Tendons and ligaments have limited vascularization

26. Macroscopic and Microscopic Structure of Tendon and Ligaments
Tendons surrounded by loose connective tissue (paratenon)
Paratenon forms sheath
Protects tendon
Enhances gliding
Epitenon
Synovial-like membrane beneath paratenon in locations of high friction
Absent in low friction locations
Surrounds several fiber bundles
Endotendon
Surrounds each fiber bundle
Joins musculotendinous junction into perimysium
Ligaments surrounded by very loosely structured connective tissue (not named)
Vascularity
Originates from ligament insertion sites
Small size and limited blood flow

27. Tendon Insertion in Bone

29. What comes to mind when you hear the word “toe”?

30. Load Deformation Relationships in Collagenous Tissues
Toe - collagen fibrils stretched to line up, from zigzag to straighten
linear region - elastic capability of tissue; elastic modulus
failure region - fibers disrupted
Hysteresis – failure to return to resting length

31. Stress-Strain Relationship in Collagenous Tissues

32. Collagen Fibers – Unloaded (Toe) and Loaded (Elastic Region)

34. Typical Load-Elongation Curve

35. Load-Elongation Curve of Ligaments with High Levels of Elastin
Elastin (protein) scarcely present in tendons and extremity ligaments
Ligamentum flavum:
Substantial proportion of elastin
Connect laminae of adjacent vertebrae
Function to protect spinal nerve roots
Provide intrinsic stability to spine

36. Load-Deformation Relationships for Connective Tissues
* 1kN = pounds
Note that text gives value of failure of ACL between 76.4 and lbs ( N)

38. Is there any movement in isometric contractions?

39. Physiological Loading of Tendons and Ligaments
P (max) of ligaments and tendons not achieved during normal activities
normally 30% of P (max) achieved
upper limit during running and jumping  % P (max)

40. Ligament and Tendon Injury Mechanisms
Injury mechanisms similar in tendons and ligaments
Microfailures take place before yield point
After yield point, gross failure results and joint begins to displace abnormally
Joint displacement can also damage surrounding structures (e.g., joint capsule, other ligaments, blood vessels)

41. Anterior Drawer Loading the ACL to Failure

42. Anterior Drawer Loading the ACL to Failure
Microfailure begins before physiological loading range is exceded

43. What is the numerical categorization system used by athletic trainers to differentiate between levels of ligamentous injury?

44. Categorization of Ligamentous Injury
Negligible clinical symptoms, some pain, microfailure of some collagen fibers
Severe pain, clinical detection of some joint instability, progressive collagen fiber failure resulting in partial ligament rupture, strength and stiffness may decrease 50% or more, muscle guarding, perform clinical testing under anesthesia

45. Categorization of Ligamentous Injury
3. Severe pain, joint completely unstable, most collagen fibers ruptured, loading joint produces abnormally high stress on the articular cartilage  correlated with osteoarthritis

46. Additional Factors in Injuries to Tendons
Amount of force of contraction produced by muscle attached to tendon
Tensile stress on tendon directly related to force of muscle contraction
High levels of tensile stress can be produced by eccentric contraction, possibly reaching failure

47. Additional Factors in Injuries to Tendons
Cross sectional area of tendon in relation to cross sectional area of its muscle
Cross sectional area of muscle directly related to force of contraction
Cross sectional area of tendon directly related to tensile strength
Tensile strength of healthy tendon may be more than twice that of force of muscle contraction (clinically, muscle ruptures more common than tendon ruptures)
Large muscles usually have large tendons

48. Viscoelastic Behavior (Rate Dependency) in Tendons and Ligaments
Increased strain  increased slope of stress-strain curve (i.e., greater stiffness at higher strain)
Higher strain rate  more energy stored, require more force to rupture, undergo greater elongation

49. Typical loading (top and unloading curves (bottom) from tensile testing of knee ligaments. The two nonlinear curves, called the area of historesis, represents the energy losses within the tissue.

50. Two Standard Tests of Viscoelastic Behavior*
Stress-relaxation test
Loading halted in safe region of stress-strain curve
Strain kept constant over extended period of time
Stress decreases rapidly at first, then gradually
Decrease in stress less pronounced with repeat tests
*Viscoelastic – variation in mechanical properties of tissue with different rates of loading

52. If you were asked to develop a creep test, what would you use to make measurements?

53. Two Standard Tests of Viscoelastic Behavior
2. Creep test
Loading halted in safe region of stress-strain curve
Stress kept constant over extended period of time
Strain increases rapidly at first, then gradually
Clinically used in casting club foot and bracing in scoliosis

54. Schematic creep curve for ligament

56. Influence of Loading Rates on Bone-Ligament-Bone Complex
At slow loading rates (60 sec.; much slower than in vivo injury mechanism), avulsion produced
At fast loading rates (0.6 sec.; simulates in vivo injury mechanism), ligamentous injury typical

57. Factors Affecting Biomechanical Properties of Tendons and Ligaments
Maturation and aging
Up to 20 years of age,
number and quality of cross-links in collagen molecules increases  increased tensile strength
Collagen fibril diameter increased  increased tensile strength
After maturation,
Collagen content of tendon and ligaments decreases  decreased tensile strength

58. Factors Affecting Biomechanical Properties of Tendons and Ligaments
Pregnancy and postpartum period
Clinical observation – increased laxity of tendons and ligaments in pubic area during latter stages of pregnancy and during early postpartum period  hormonal influence
Research studies of rats– increased laxity of tendons and pubic symphasis during latter stages of pregnancy and during postpartum period; stiffness of these structures later returned

59. Factors Affecting Biomechanical Properties of Tendons and Ligaments
Pregnancy and postpartum period (continued)
Hormones may have influence on ligament laxity in women at various stages of menstrual cycle  influence ligamentous injury rates in females (e.g., higher incidence of injury in women in basketball and soccer in comparison to men)

60. Factors Affecting Biomechanical Properties of Tendons and Ligaments
Mobilization and immobilization
Tendons and ligaments remodel in response to mechanical demands
Become stronger and stiffer when subjected to increased stress
Become weaker and less stiff when stress removed
Physical training found to increase tensile strength of tendons and ligament-bone interface

61. Factors Affecting Biomechanical Properties of Tendons and Ligaments
Mobilization and immobilization
Immobilization found to decrease tensile strength of ligaments
Immobilization decreased mechanical properties of bone-ligament-bone complex in knee of primates (8 weeks of casting)
Considerable reconditioning required in primate knees to regain former complex strength (approx. 12 months) (see figure)

62. Influence of Immobilization on Primate ACL Ligament

63. Influence of Immobilization on Primate ACL Ligament

64. Factors Affecting Biomechanical Properties of Tendons and Ligaments
Nonsteroidal Anti-Inflammatory Drugs (NSAID) (e.g., aspirin, acetaminophen, indomethacin)
In animal studies, short term administration of NSAIDs (indomethacin) found to increase the rate of biomechanical restoration of tissues (tendons)