Advanced Biomechanics of Physical Activity (KIN 831)

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

5. 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) Functions of Tendons

6. 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) Tendons and Ligaments

7. Tendons 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 Tendons and Ligaments

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. 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 • 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 • Ligaments • Vascularity • Originates from ligament insertion sites • Small size and limited blood flow • 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 ---------------------------------------------------------------- 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 • Ligaments surrounded by very loosely structured connective tissue (not named) • Vascularity • Originates from ligament insertion sites • Small size and limited blood flow • 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

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 = 224.8 pounds Note that text gives value of failure of ACL between 76.4 and 87.67 lbs (340-390 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  2 - 5 % 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