Course topics Muscle biomechanics Tendon biomechanics Bone biomechanics

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Transcript of Course topics Muscle biomechanics Tendon biomechanics Bone biomechanics

  • Slide 1
  • Course topics Muscle biomechanics Tendon biomechanics Bone biomechanics
  • Slide 2
  • Bone Provide mechanical support for each body segment Act as a lever system to transfer muscle forces Must be stiff yet flexible strong yet light
  • Slide 3
  • N&F, Fig 1-2
  • Slide 4
  • Compact bone (40X)Cancellous bone (30X) N&F, Fig 1-3 trabeculae Haversian canal
  • Slide 5
  • Classifications Biomechanical properties similar, difference is in density (porosity) Cancellous is less dense (weaker) Made of trabeculae oriented in direction of forces commonly experienced Irregular lamellae layers of mineralized matrix Cortical Cylindrical lamellae Functional unit is the osteon
  • Slide 6
  • Bone Synonyms Compact = cortical Cancellous = trabecular
  • Slide 7
  • Definitions Load (N) Deformation (mm) Stress (N/m 2 ; Pa) Strain (mm/mm; mm/mm*100%) Stiffness (N/m) Elastic Modulus (Pa)
  • Slide 8
  • Tissue Mechanics: Equations and Values Force = F = k L Stress = F / A Strain = L / L Elastic modulus = E =Stress/Strain Stiffness = k = EA / L Elastic energy = 0.5k( L) 2 Elastic energy = 0.5 F L 10,000 cm 2 = 1 m 2 Tendon: E (tendon or ligament) = 1.5 10 9 Pa Tendon safe limits: Stress (Ultimate strength) = 100 MPa Strain = 8% strain Bone: E (bone) = 17 x 10 9 Pa Bone safe limits: Tension = 150 MPa stress, 0.7% strain Compression = 190 Mpa stress, 1% strain
  • Slide 9
  • B,B,B* C,C,C* D,D Energy needed to yield? Energy needed to fracture?
  • Slide 10
  • Bone is a Composite Material One phase: mineral (strong and brittle) Other phase: collagen (weak and ductile) Strong vs Weak: Ultimate Stress Ductile vs Brittle: Deformation before Failure
  • Slide 11
  • Slide 12
  • Bone is a Composite Material Chicken wing bones: some baked in oven, denatured protein, only mineral left brittle some soaked in vinegar, removed mineral, leaving only collagen ductile (rubbery)
  • Slide 13
  • Bone mechanics Depend on Type of loading Compression, tension, & shear Duration, frequency, number of repetitions Bone density Compact vs. Cancellous bone Age/gender, use/disuse
  • Slide 14
  • Tension (longer and thinner) Compression (shorter and fatter) Bending (tension & compression) Shear (parallel load) Unloaded N&F Fig 1-10 Torsion (primarily shear)
  • Slide 15
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  • Bending: Tension + Compression Compression Tension
  • Slide 17
  • Mechanical properties of bone: Stress-strain relationship Stress = F / A Strain = L / L L L F
  • Slide 18
  • Stress-strain for compact bone loaded in tension 3 0.7 Strain (%) 150 Stress (MPa) Yield point Elastic Plastic Ultimate strain Elastic: no permanent deformation Plastic: permanent deformation Yield point: strain where plastic range begins Ultimate strain/stress: fracture occurs
  • Slide 19
  • Compact bone vs. tendon/ligament in tension 3 Bone E = 17 GPa Ult. stress = 150 MPa 0.7 Strain (%) 150 Stress (MPa) 100 0 0 6 9 Tendon/ligament E = 1.5 GPa Ult. stress = 100 MPa yield
  • Slide 20
  • Tendon vs. bone strain in running Achilles tendon strain ~ 6% (vs. 8%) Tibia Strain ~ 0.07% (vs. 0.7%)
  • Slide 21
  • Compact bone in compression & tension same modulus, but different yield points Stress (MPa) Strain (%) Compression 190 1 2.6 3 Tension 0.7 Strain (%) 150 Stress (MPa) yield ult. strain
  • Slide 22
  • Compression: ~190 MPa Tension: ~150 MPa Shear: ~ 65 MPa Ultimate stress of compact bone
  • Slide 23
  • Bone mechanics Depend on Type of loading Compression, tension, & shear Duration, frequency, number of repetitions Bone density Compact vs. Cancellous bone Age/gender, use/disuse
  • Slide 24
  • Compact vs. cancellous bone in compression (effects of density) Stress (MPa) Strain (%) 200 510 Compact ( = 1.8 gm/cm 3 ) 100 Cancellous ( = 0.9 gm/cm 3 ) Cancellous ( = 0.3 gm/cm 3 ) 0 15 20 0
  • Slide 25
  • Bone density effects on ultimate strength Density (g / cm 3 ) 0.10.20.5 12 1 10 100 Ultimate compressive stress (MPa) Strength 2 Cancellous Compact
  • Slide 26
  • Broken Back? A smokejumper (mass = 70 kg) hits the ground with 25x body weight. If the load is concentrated on the facet joints, which have an area of 1 cm 2, will they break? (F = mass x g; g = 9.81 m/s 2 ) A)Yes B)No C)It depends
  • Slide 27
  • Bone mechanics Depend on Type of loading Compression, tension, & shear Duration, frequency, number of repetitions Bone density Compact vs. Cancellous bone Age/gender, use/disuse
  • Slide 28
  • Failure Modes Single load/high stress Tensile fractures usually induced by rigorous muscle contractions Compression fractures induced by impacts Most fractures involve bending, torsional, or combined loads Multiple loads (repetitive)/low stress
  • Slide 29
  • Repetitive loading: Tension Fracture stress (MPa) 150 60 100 1,000 10,000 Repetitions # of repetitions important Running: SF = 1.3 strides/s ~ 2 hours of running 10,000 strides But bone repairs during recovery
  • Slide 30
  • Bone remodelling Bone remodelling is dependent upon mechanical loading Wolffes Law (1892) Bone laid down where needed Resorbed where not needed bone response is site specific, not general bone responds to high loads and impact loading trabecular bone lost most rapidly during unloading (bed rest, spaceflight etc.)
  • Slide 31
  • Repetitive Loads -> Fatigue Number of repetitions important Time between repetitions is important Muscle fatigue increases stress on bones Bone cannot repair rapidly enough
  • Slide 32
  • Peak bone stress on anteromedial surface of tibia Walk (1.4 m/s): Peak values compression: 2 MPa tension: 3 - 4 MPa Run (2.2 m/s): Peak values compression: 3 MPa tension: 11-12 MPa See N&F, Fig. 1-30 Ultimate stresses C: 190 MPa T: 150 MPa
  • Slide 33
  • Lifting a box Calculate how much force in the back extensor muscles is needed when lifting a 1 kg box with the arms outstretched (r = 30 cm), compared to when the arms are beside the body (r = 5 cm). The muscles effort arm: (r effort = 5cm).
  • Slide 34
  • Lifting a box Calculate how much force in the back extensor muscles is needed when lifting a 1 kg box with the arms outstretched (r = 30 cm), compared to when the arms are beside the body (r = 5 cm). A)6 times less force A)6 times more force A)the same force A)150 times more A)I dont understand
  • Slide 35
  • Vertebra Surface Area Vertebral bodies are the primary weight-bearing components of the spine Progressive increase in vertebral size (area) from cervical region to the lumbar region Variation serves a functional purpose: Stress-reduction
  • Slide 36
  • Bone mechanics Depend on Type of loading Compression, tension, & shear Duration, frequency, number of repetitions Bone density Compact vs. Cancellous bone Age/gender, use/disuse
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  • Aging: reduced bone density/quality Greater porosity in compact & cancellous bone Compact bone tensile strength Age 20: 140 MPa Age 80: 120 Mpa So most of the problem is with density in cancellous bone (less dense, not poor quality) Geometry changes as well Data from Burstein et al.
  • Slide 38
  • Can Exercise Help? cross sectional studies indicate + highest BMD in weight lifters BMD proportional to body weight Higher tibia BMD and CSA in runners prospective training studies, modest +
  • Slide 39
  • Bone mechanics Depend on Type of loading Compression, tension, & shear Duration, frequency, number of repetitions Bone density Compact vs. Cancellous bone Age/gender, use/disuse