Chapter 14: Injury Mechanics

  • Differentiate between force and pressure or stress
  • Explain how the stress continuum relates to tissue adaptation and injury
  • Describe the concept of the stress threshold
  • Differentiate between intrinsic and extrinsic factors related to injury development
  • Identify intrinsic factors that predispose an individual to injury
  • Identify extrinsic factors associated iwth high stress during skill performance
  • Explain the concepts of "cross-training" and "within-activity cross-training"
  • Suggest interventions to decrease the risk of injury when performing a task

Mechanical stress and injury
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Pressure = F/A

Wolff's law: A tissue adapts to the level of stress imposed on it; the level of adaptation in a tissue reflects the level of typical loading

Where does Wolff's law fail?

Can this curve be altered? How?

Vibram Shoe and Bone Injury
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What type of stress causes injury?

Yield strength stress: 153 MPa

Yield strength strain: 0.009 or 0.9%

Young's Modulus: 17 GPa


Ultimate strength stress: 165 MPa

Ultimate strength strain: 0.032 or 3.2%


  • About 30% of bone is collagen (fibrous protein)
  • About 70% hydroxyapatite (inorganic mineral)
Tendon and ligament
  • 70% water
  • 25% collagen
  • 5% elastin


Tension Compression Shear
Compact Bone Stress (MPa) 90-170 100-280 50-100
Strain (%) 0.7-5 1-2.4
Ligament Stress (MPa) 1-2
Strain (%) 30-125
Tendon Stress (MPa) 15-100
Strain (%) 10-17

Intrinsic and extrinsic factors related to injury development

Model for overuse injuries

Within-activity cross-training

  • Alter-g treadmill
  • Different surfaces (grass, turf, ashphalt)
  • Contact versus non contact days
  • Different strokes in swimming

Thought questions:
  1. How does the magnitude of imposed stress relate to the size of the applied force and the area over which the force is distributed? Give examples of interventions to alter stress by changing the size of the imposed force and examples of interventions to alter stress by changing the area over which the force is distributed.
  2. Differentiate between tensile, compressive, and shear stress. Give an example of each type of stress imposed on the human body, identifying the source of the force and the tissue loaded.
  3. What types of injuries do you expect to see in these sports:
    • Swimming
    • Football
    • Basketball
    • Cross-country
      • Are these injuries caused by intrinsic or extrinsic factors