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Biomechanics

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... study of human motion (kinematics) and the forces or moments causing the motion (kinetics. ... The study of physical measurements of the human body to ... – PowerPoint PPT presentation

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Title: Biomechanics


1
Biomechanics
  • MECH 393

2
Biomechanics
  • The study of human motion (kinematics) and the
    forces or moments causing the motion (kinetics.)
  • Applications
  • Assessment disease, rehabilitation, athletic
    performance, workplace assessments,
  • Design implants, mobility/lifting/workplace
    aids, sports training aids, fracture fixators,
    artificial tendons, assessment tools (i.e. x-ray
    frames), human interfaces, athletic equipment and
    training aids,

3
What do we measure/calculate?
  • Anatomical landmarks
  • Position (displacement), velocity, acceleration,
    orientation (angles).
  • Segment parameters
  • Size (length, diameter, etc.), mass, centre of
    mass, moment of inertia, bone geometry (for
    muscle moment arms)
  • Joint kinetics
  • Joint reaction forces, net joint moments, joint
    contact forces
  • Muscle activity
  • Material properties
  • Bone, cartilage, muscle, tendon, ligament, fat,
    skin

4
Analysis
  • Simplify anatomy
  • Break the body into segments. Each segment will
    have its own coordinate system. Segments form
    the link-segment model that is then used in the
    inverse dynamics approach.
  • Motion analysis
  • Position and orientation of segments
  • Calculate displacement, velocity, acceleration
  • Force measurement
  • External forces applied to the body
  • Sum the forces and moments on each segment
  • Muscle Activity
  • Measured using electromyography (EMG)
  • Sum the forces and moments on the joint to
    calculate joint reaction forces.

5
Anatomy Simplification (Link-segment model)
  • How do we decide what segments to include in the
    link-segment model?
  • Need proximal and distal joints
  • What do we want to know?
  • Is there a significant amount of movement at the
    joint?
  • Get parameters for each segment
  • Measure dimensions
  • Mass, centre of mass, moment of inertia estimated
    from regression equations based on dimensions,
    total body mass, etc.

6
Anthropometrics
  • The study of physical measurements of the human
    body to determine differences in individuals and
    groups (Winter, 1990).
  • Applications
  • Human interfaces cockpits, body armour, desks,
  • Kinetic measures mass, moment of inertia,
    centre of mass locations, origin and insertion of
    muscles,

7
Anthropometric relationships are often based on a
group of fit, young males
8
Winter, 1990
9
Motion analysis - Electrogoniometers
  • Measures angle between two segments based on
    potentiometer output
  • Adv Simple concept, easy to use/collect data
  • Disadv No absolute reference, skin motion

10
Motion Analysis - Video
  • Adv No wires, easy data collection
  • Disadv Passive markers, time-consuming data
    analysis, 2D, skin motion

11
Motion Analysis - Electromagnetic Systems
  • Adv
  • Disadv

12
Motion Analysis Optoelectric systems (1895)
13
Motion Analysis Optoelectic systems
  • Passive (VICON)
  • Active (OPTOTRAK)
  • Adv
  • Disadv
  • Adv
  • Disadv

14
Choosing a motion analysis system
  • Things to consider
  • Will you be able to see the markers?
  • Do you need more than one camera?
  • What is your measurement volume?
  • How will the system interfere with the subject?
  • What activities are being analyzed? Do you need
    3D information?
  • Environment, cost

15
Skin motion
  • Soft tissue motion skin, muscles, fat and
    tendons/ligaments move relative to the underlying
    bone
  • Overcoming skin motion
  • Bone pins, fluoroscopy, RSA

Baker, 2007
Benoit et al., 2006
Source Upstate Medical University
16
Data analysis
  • The output of the motion analysis system is the
    marker motion
  • Calculate position and orientation of segments
  • Requires a coordinate system (position of the
    origin and orientation of the axes) for each
    segment
  • Filter!
  • Differentiate to get velocity and acceleration

17
Filtering
Differentiating unfiltered data
Differentiating filtered data
18
External forces
  • Gravity
  • Lifting loads
  • Force plates (force platforms)
  • Often used to measure ground reaction force under
    the foot
  • Six channels three force channels and three
    moment channels. Output is in volts.
  • Using a sensitivity matrix, transform the voltage
    readings into three forces, one moment and the
    centre of pressure on the force plate

19
Force plates
  • Centre of pressure gives the position on the
    force plate (x,0,z) where the ground reaction
    force is applied to the foot.

Winter, 1990
20
3D inverse dynamics
  • Work your way up from the external force (usually
    a ground reaction force on the foot) to the joint
    of interest
  • Start with the foot to analyze the ankle, add the
    shank to analyze the knee and so on.

Winter, 1990
21
2D Example Swing leg
  • Determine the joint reaction forces and the net
    joint moments at the ankle and the knee.

22
2D Example
L6
L2
L5
L1
L4
L3
23
Given
  • Body mass, M 80kg

24
(No Transcript)
25
Results
26
Limitations of the Link-Segment Model
  • Many inputs to the model are approximate or
    estimated (e.g. segment mass, palpation of
    landmarks)
  • Calculated moments and forces are the net effect
    of the action of many muscles and
    support/resistance of many soft-tissue
    structures.
  • Insufficient for calculating individual muscle
    forces or joint contact forces.
  • Muscle forces can be estimated using
    electromyography.

27
Electromyography
  • Muscle contraction is triggered by a motor unit
    action potential (an electrochemical signal)
  • Measure the signal conducted through the muscle
    using surface or internal transducers
    (electrodes)

As the output muscle force increases, the
amplitude of the EMG signal increases. More
muscle fibers are activated and the firing rate
of the fibers increases.
DeLuca, 2006
28
Processing the EMG signal
  • Raw signal
  • Full-wave rectify
  • Absolute values
  • Linear envelope
  • Low-pass filter

29
Relating EMG to Muscle Force
  • Maximum Voluntary Contraction (MVC)
  • Encourage the subject to contract a particular
    muscle using his/her maximum effort.
  • Measure the force exerted at maximum effort.
  • Represent all EMG readings as MVC. Assume a
    linear relationship between muscle force and EMG
    magnitude
  • Can be used to estimate muscle forces (then joint
    contact forces)

Nagura et al., 2006
30
Why are joint contact forces important?
  • Joint pain, osteoarthritis
  • Can we decrease the joint contact force using
    training or intervention?
  • These are the forces that a joint implant will be
    subjected to during locomotion
  • Needed for implant development finite element
    modeling, simulator testing
  • NOT the same as a joint REACTION force!

31
Joint contact forces
  • Larger than reaction (net) force due to
    co-contraction of muscles
  • Agonist and antagonist muscles act simultaneously
    for stability.
  • Results in an indeterminate problem there are
    too many muscle forces (unknowns) for the number
    of available equations.

32
Options for dealing with indeterminacy
  • Ignore co-contraction.
  • When EMG shows very little antagonistic muscle
    activity
  • Group muscles so that you have less unknowns
  • Use optimization
  • minimize some value such as muscle stress based
    on the muscle cross-sectional area.
  • Use EMG
  • Relate EMG signals to muscle force
  • Make muscle forces zero when the muscle is
    essentially off.
  • Use muscle ratios

33
Challenges with EMG
  • Cross-talk when using surface electrodes
  • Invasive nature of indwelling electrodes
  • Noise biological or man-made
  • The magnitude of an EMG signal can be very small
    (µV)
  • Calibration

34
Biomechanics Summary
  • Why study biomechanics?
  • Disease, joint implants (design and testing),
    design of human interfaces, determining factors
    in optimal performance (sports, lifting),...
  • Kinematics requires a motion analysis approach
    appropriate to the activity and research question
  • Kinetics requires anthropometric data, external
    force data
  • Link-segment model, kinematic data,
    anthropometrics and external force measurements
    are used in the inverse dynamics approach
  • Results in net joint reaction forces and net
    joint moments
  • Calculation of net joint contact forces requires
    more information
  • One way of obtaining this information is to
    collect EMG data on the muscles surrounding the
    joint.
  • Be aware of the limitations of various data
    collection techniques and analysis approaches.
    Often the choice of technique is a trade-off.
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