Title: Biomechanics
1Biomechanics
2Biomechanics
- 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,
3What 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
4Analysis
- 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.
5Anatomy 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.
6Anthropometrics
- 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,
7Anthropometric relationships are often based on a
group of fit, young males
8Winter, 1990
9Motion 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
10Motion Analysis - Video
- Adv No wires, easy data collection
- Disadv Passive markers, time-consuming data
analysis, 2D, skin motion
11Motion Analysis - Electromagnetic Systems
12Motion Analysis Optoelectric systems (1895)
13Motion Analysis Optoelectic systems
14Choosing 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
15Skin 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
16Data 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
17Filtering
Differentiating unfiltered data
Differentiating filtered data
18External 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
19Force 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
203D 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
212D Example Swing leg
- Determine the joint reaction forces and the net
joint moments at the ankle and the knee.
222D Example
L6
L2
L5
L1
L4
L3
23Given
24(No Transcript)
25Results
26Limitations 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.
27Electromyography
- 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
28Processing the EMG signal
- Raw signal
- Full-wave rectify
- Absolute values
- Linear envelope
- Low-pass filter
29Relating 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
30Why 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!
31Joint 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.
32Options 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
33Challenges 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
34Biomechanics 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.