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Advanced Biomechanics of Physical Activity (KIN 831)

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Title: Advanced Biomechanics of Physical Activity (KIN 831)


1
Advanced Biomechanics of Physical Activity (KIN
831)
  • Lecture 8
  • Biomechanical Models and Modeling
  • Some of the material included in this
    presentation is derived from
  • Nigg, B. M. (1994). In B. M. Nigg W. Herzog
    (Eds.), Biomechanics of the Musculo-Skeletal
    System (pp.365-379).
  • West Sussex, UKJohn Wiley Sons.

2
What is a model?
3
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4
Definitions
  • Model an object, plan, or theory that
    represents or imitates many of the features of
    something else (an attempt to represent
    reality)
  • Deduction logical reasoning from a known to the
    unknown, from the general to the specific
  • Induction logical reasoning from particular
    facts or individual cases to a general conclusion
  • Validation (of a model) providing evidence that
    a model is strong and powerful

5
Definitions (continued)
  • Biomechanical model a representation
    (microscopic or macroscopic) of a biological
    system
  • Free body diagram simplified drawing of a
    mechanical system, isolated from its
    surroundings, showing all force vectors and
    torques
  • Generalizable the ability to make broader
    application of a process or results

6
Working in groups of two,carefully develop a
model (drawing) of a inanimate (not a biological
structure) mechanical structure which you can
show to the class and define its
function.These models should be relatively
simple mechanical systems.
7
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8
Example
9
2. Using the example provided, what do you know
about the model? List several assumptions that
have been made about the model and indicate why
these assumptions may have been made.
10
Model
  • Known Facts
  • Assumptions

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12
KNOWN FACTS (?) F1s1 F2s2 or F1
F2(s2/s1) s1 ½(s2) ASSUMPTIONS 1. 2. 3.
13
Refine the model (drawing) in order to remove
some of the assumptions about the model. You
will be asked to show your revised model and its
assumptions.
14
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15
4. Attempt to simplify the model and redraw it.
Be prepared to show and explain your simplified
model to the class.Hints - possibly
representing parts of the structures as a point
masses and/or lines- possibly representing the
parts of the structure by its behavior according
to the laws of physics
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17
Why are biomechanical models used?
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19
Why are biomechanical models used?
  1. to simplify the understanding of the structure
    and function of a biological system
  2. to simply the kinematics and/or kinetics analysis
    of the biological system
  3. to remove the biological system from exposure to
    potential adverse effects by exposing a
    representative model and observing its behavior

20
Purposes of models and modeling
  • to increase knowledge and insight about reality
  • to estimate or predict variables of interest
  • The fact that insight and knowledge are
    prerequisites for the development of a
  • model, but are also the purpose of using a model,
    seems contradictory.

21
Information used to construct a model
  • Knowledge of the system being modeled
  • Using knowledge of the system being modeled, to
    move from general principles to specifics, is a
    deductive process.
  • Experimental data that constitute system inputs
    and/or outputs
  • Using experimental data, in an attempt to arrive
    at a general conclusion that explains the data,
    is an inductive process.

22
Deduction versus Induction
knowledge
experimental data
Information used
deduction
induction
Method used
unique solution
no unique solution
Expected results
 Constructing a model information, method, and
results   1. Knowledge may be preliminary
assumptions. 2. In deductive model there may be
many assumptions. 3. In inductive model there
may be many possible answers.
23
Simplification
  1. In general, simple is better.
  2. Simple may not agree with reality.
  3. The key to a model (modeling) is to know what to
    include and what to eliminate there is a science
    and art to creating a model.

24
Validation of a model
  1. Validation of a model means that evidence is
    provided that the model is strong and powerful
    for the task for which it has been designed
    (i.e., provision of cases for which the results
    of the model corresponds to reality).
  2. Validation may lead to increased confidence in a
    model, but it never confirms that the model
    corresponds to reality.

25
Three ways to validate a model
  1. direct measurement comparison of estimated
    results from a model with actually measured
    results (e.g., predicting projectile distances of
    javelin from information about angle of
    projection, height of release, and velocity of
    release and comparing it to actually measured
    projectile distances)

26
Three ways to validate a model(continued)
  • 2. indirect measurements measurements of
    another variable may be made and compared with
    the value predicted for this variable from the
    model (e.g., use of IEMG of the hamstring muscles
    compared with the models predicted value of knee
    flexion force)

27
Three ways to validate a model(continued)
  • 3. trend measurements the quality of a model
    depends on how well the trends predicted agree
    with the trends measured (e.g., if the model
    predicted that measured girths of the forearm
    were linearly related to grip strength,
    validation would require several input variables
    and subsequent out put values)

28
Types of models
  1. Analytical - deductive
  2. Semi-analytical many assumptions are used
    because there are more unknowns than equations to
    solve for the unknowns
  3. Black box regression models functions used to
    determine relationships between input and output
  4. Conceptual used in hypothesis testing

29
Cyclical interaction between facts and theory in
scientific activities
theories
deduction
prediction
induction
Science must start with facts and end with
facts, no matter what theoretical structures it
builds in between.
description
evaluation
observation
facts
facts
1. Start with observation to build upon what is
known. 2. Describe what is known 3. Use the
known facts to come to general conclusions
induction) 4. Develop and test the predictions of
theories (models) deduction
  • 5. Compare results with actual facts
  • 6. Evaluate the process
  • 7. Seek additional facts
  • Refine theories (models) and possibly repeat the
    process

30
Why are biomechanical models used?
  • 1. to simplify the understanding of the structure
    and function of a biological system
  • 2. to simply the kinematics and/or kinetics
    analysis of the biological system
  • 3. to remove the biological system from exposure
    to potential adverse effects by exposing a
    representative model and observing its behavior

31
Why are biomechanical models used? (continued)
  1. to obtain information on the structure and
    function of the biological system
  2. to simplify the presentation of a complex
    biological system

32
General steps in developing a biomechanical
modelPrior to developing a biomechanical model
the scientist musta) have a thorough
understanding of existing facts,b) make
observations of the phenomena to be studied, and
c) develop an understanding of the integration
of facts and observation.
  1. Define question to be answered
  2. Define the system of interest
  3. Review existing knowledge (literature review)
  4. Select procedure (model) to be applied to solve
    research question research methods
  5. Make simplifications and assumptions decide what
    to include and what not to include based on
    defendable reasons

33
General steps in developing a biomechanical
modelPrior to developing a biomechanical model
the scientist musta) have a thorough
understanding of existing facts,b) make
observations of the phenomena to be studied, and
c) develop an understanding of the integration
of facts and observation.
  • 6. Formulate mathematical approach (e.g.,
    statistical methods) to be applied to data
  • 7. Develop mathematical solution (results)
  • 8. Evaluate the model
  • 9. Discuss, interpret, and apply the results
  • 10. Draw conclusions

34
What are categories of biomechanical models?
  • Static versus dynamic
  • static implies constant linear and/or angular
    velocity (linear and/or angular acceleration 0)
  • dynamic implies changing linear and/or angular
    velocity (linear and/or angular acceleration ? 0)
  • Object Dimension
  • a. point-mass (0 dimension)
  • b. line (1 dimension)
  • c. plane (2 dimensions)
  • d. solid (3 dimensions)
  • Space Dimension
  • a. uni-axial
  • b. bi-axial
  • c. tri-axial (three dimensional Cartesian
    coordinate system)

35
What are categories of biomechanical models?
(continued)
  • 4. Kinematic versus kinetic
  • a. kinematic implies a description of position
    without regard for forces and torques
  • b. kinetic implies the forces and torques that
    cause linear and/or angular accelerations
  • 5. Uni-segment versus multi-segment
  • a. uni-segment implies internal and external
    forces and torques
  • b. multi-segment implies reactive forces (joint
    reaction force) and torques (mutual net muscle
    moments across joints) between segments

36
Input parameters for biomechanical models
  • 1. Direct measurement actual measurements of
    parameters used in the model (e.g., height,
    weight)
  • 2. Indirect measurement measures predicted from
    other sources of information (e.g., location of
    the center of mass of a segment of the body,
    segments proportion of total body height,
    estimate of density of a body segment
  • 3. Inverse dynamics the use of linear and
    angular acceleration parameters and information
    about segmental mass and moment of inertia to
    determine forces and torques
  • a. mass X acceleration force (equation of
    linear motion)
  • b. moment of inertia X angular acceleration
    torque or moment (equation of angular motion)

37
What is a free body diagram?
38
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39
Free body diagram simplified drawing of a
mechanical system, isolated from itssurroundings,
showing all forces vectors and torques
  • Particle (or point-mass) free body diagram
    assumed that kinematics of object can be
    represented by its center of mass and its linear
    movement (e.g., parabolic path of the center of
    mass of a long jumper in flight) Note that there
    is no angular motion associated with a point
    mass.

40
Free body diagram (continued)simplified drawing
of a mechanical system, isolated from its
surroundings, showing all forces vectors and
torques
  • 2. Segmental - can represent the human body
    mechanically as a linked system of rigid segments
    moving about an axes of rotation through joints

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41
Assumptions
  • Rules for making assumptions
  • a. assumptions should not adversely or
    substantially affect the results of the model
    (e.g., knee joint is pinned)
  • b. assumptions should generally simplify the
    model (e.g.,
  • c. assumptions should be able to be justified
    (e.g., linear and angular accelerations are small
    and therefore can be neglected)
  • d. assumptions should usually be made for
    unknowns (e.g., knee joint is frictionless)
  • Reasons for making assumptions
  • a. remove complexity from model
  • b. simplify computation and/or understanding
  • c. lack of knowledge
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