6.8 Specific Examples Of Deficiencies

  • It is taken on a subject population, such as 4000 Male United States Air Force personnel (ANSUR)
    • It is a small sample size relative to the entirety of the population
    • Military personnel are mostly young, healthy, adults
    • It may not include people who are very small or very large
    • That’s it may not be generalizable to the population at Large
  • Gender bias
    • Much of the existing data is from the military
    • Under represents women (due to their historical under-representation in the military)
  • Racial bias
    • Differences in Height Around the World
    • As designed previously there are differences among the races in their anthropometric data
    • If you are designing a products for a specific market, then you may consider using only using data from that region
    • However, there are many things to consider as you do that.

6.8.1 Mathematical models for mass properties

  • If we assume body segments can be approximated by common shapes, we can empirically define mass properties
    • Ellipsoidal cones (truncated)
    • Ellipsoids

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6.8.2 Muscle and ligament forces

  • A critical aspect of modeling muscles and ligaments is to determine appropriate lines of action
  • Cadaveric measurements are available
  • In-vivo reconstructions are possible with current medical imaging, however, appropriate ethical practices are a must

6.8.3 Lines of action

Public Domain work of the US federal government

  • Simple method: muscles can be modeled as strings connecting two points by a straight line
  • More complex: muscle and tendon wrap around bones (ie patella)
  • In some cases, parts of muscles can be independently activated

6.8.4 Muscles cross joints

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  • Muscles cross joints, thus, we must account for the joint motion on the line of action
    • Example: cam-like action of knee

6.8.5 Efficient use of muscles

  • Muscle redundancy suggests that more than one muscle can perform the same task
  • It is theorized that efficiency might drive “muscle selection”
    • Size of muscle (force limit) and moment arm both effect which muscle is recruited for a task
  • Thus, an accurate estimate of muscle generating area is required
  • Length of muscle changes during contraction, however, volume is nearly conserved
  • Thus, we can calculate an effective area (and force) on the basis of measured length and volume