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Stress

Stress. “The external forces that act on the body are resisted by internal forces and cause deformation of the body. The amount of deformation produced is related to the stress caused by the forces and the material that is loaded.”. Mechanical Stress.

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Stress

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  1. Stress • “The external forces that act on the body are resisted by internal forces and cause deformation of the body. The amount of deformation produced is related to the stress caused by the forces and the material that is loaded.”

  2. Mechanical Stress • Internal force divided by the cross-sectional area of the surface on which the internal force acts. σ = F/A • 3 principal forms of stress • Tension or tensile force (axial) • Compression or compressive stress (axial) • Shear (transverse)

  3. The Effects of Loading • Two potential effects through force application • Acceleration • Deformation (change in shape) • Diving board example • Board accelerates & changes shape • Change the axis of the board • External force applied to the human body • Magnitude and direction of the force • Amount of stress • Material properties of the loaded body tissues

  4. Load-Deformation Curve

  5. Demonstrations • Elastic region • Return to normal shape • Yield point • Permanent change but not complete violation of structural integrity • Ultimate failure point • It’s History!! • Examples • Pencil • Hanger

  6. Repetitive vs Acute Loading • Acute loading • Single force application that causes macrotrauma • Repetitive loading • Submaximal chronic force application that causes microtrauma

  7. Measuring Kinetic Quantities • Electromyography • Electrical activity produced by muscle • Dynamography • Force platforms • Examples • Rocky IV • Punching bag

  8. Torsion & Combined Loads • Torsion • A force with opposing rotational forces (torque) along the long axis of the object. • Combined load • Muscles, tendons and ligament deal with uniaxial tension • Bones and cartilage experience combined loads • Figure 9.13

  9. Strain • The quantification of the deformation of a material. • Linear strain is the difference between the undeformed and the deformed lengths divided by the original undeformed length. Є = l – lo / lo

  10. Shear Strain • Figure 9.14 • Shear strain is represented by a sliding force between adjacent molecules • Represented by lambda (λ) • Poisson’s Ratio • Range is from .25 to .35 for most materials • Specific ratio of strain in the axial direction to strain in the transverse direction • Example • Intervertebral discs

  11. Mechanical Properties of the Musculoskeletal System • Connective tissue • Bone, cartilage, ligament and tendon • Passive elements • Muscle tissue • Active elements • Key terms • Isotropic (Synthetic materials) • Materials have the same mechanical properties in every direction • Anisotropic (Biological materials) • Materials have dynamic mechanical properties depending on the direction of the load

  12. Mechanical Properties • Bone • Cortical or compact bone • Dense and found in outer layers of long bones • Cancellous or trabecular bone • Less dense and porous in appearance found at the ends of long bones • Figures 9.23 & 9.24 • Cartilage • Hyaline or articular cartilage • Fibrocartilage • Elastic cartilage • Figures 9.25, 9.26, & 9.27

  13. Mechanical Properties • Tendons & Ligaments • 70% water, 25% collagen, 5% ground substances • Ligaments contain more elastin • Figures 9.27 & 9.28 • Ligaments are weaker than tendons • Ligaments can withstand nonaxial loads better • Muscles • Contractile elements • Connective tissue sheaths surrounding the muscle • Figure 9.29

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