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
(No Transcript)
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
(No Transcript)
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

11
(No Transcript)
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
(No Transcript)
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
16
(No Transcript)
17
Why are biomechanical models used?
18
(No Transcript)
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
(No Transcript)
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

?p
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