Lab Test Flashcards
What are the three approaches to the analysis of human movement?
“subjective”, “objective”, and “predictive”
Describe each approach: subjective, objective, and predictive
predictive approach = “What if …” , computer simulations and forward dynamics
objective approach = measurement of kinematics and inverse dynamics to calculate forces and moments that cause movement
subjective analysis of human movement in which subjective estimates are made and biomechanical principles are applied without exact measurements and equipment.
List the 10 Biomechanics principles
1) Summation of joint torques
2) Continuity of joint torques
3) Impulse - Momentum
4) Action - Reaction
5) Equilibrium and Balance
6) Summation of body segment velocities
7) Production of Rotational Momentum
8) Conservation of Momentum and Energy Transfer
9) Manipulation of Rotational Inertia
10) Manipulation of Segmental Rotational Momentum
What is included in the Pre-Observation phase?
Determine Performance Goal
Determine Mechanical Variables
Identify and Select critical variables and determine acceptable range for these variables
Then Develop Observation plan
What makes an object more stable? And does it positively correlate with humans?
Higher base of support and lower centre of mass.
No in Humans this is not always the case
What is the Summation of Joint Torques Principle?
When you think about how our muscles generate moments of force/torques about joint centres to apply force to smt.
So for any task you ask if you’re using all available muscle groups to maximize the summation of the joint torques
What is the Continuity of Joint Torques Principle?
Thinking about the timing or sequence of the joint torques and if they are optimal. Example: throwing a ball (proximal to distal sequence) has a different sequence than vertical jumping (simultaneous)
What is the Impulse - Momentum Principle?
Momentum is mass times velocity and impulse is the change in momentum.
In order to maximize a change in momentum you need to generate a high impulse. Impulse is also force times time so: are the forces being applied for as long as possible?
You can apply force for longer period of time by maximizing range of motion.
Explain why the definition of Stability not Sufficient for humans?
Tightrope artists raise their arms when walking on a tight rope. This raises Centre of Gravity but they are more stable
This occurs because you increase moment of Inertia which decreases tendency to rotate, allowing for more time to balance
What is the Action-Reaction Principle?
Newton’s 3rd law: for each action there is an equal, opposite reaction
think abt throwing a ball in the air, if the pitcher throws the ball in air the force on the ball would not be as large. contribution from the legs is gone b/c contact with the ground is lost.
Also think of friction.
Summation of Body Segments (Velocities):
Velocities of body segments can be added. If desired movement is not achieved, then can see which body segment didnt have the expected magnitude and velocity to perform the action
What is the Equilibrium, Balance, and Stability Principle?
Stability of object looked at by ‘righting moment’.
The larger the base of support / lower the center of mass, the more stable the object is
when tilting the objects, the least stable one requires an additional moment to keep from falling while gravity provides an adequate righting moment for the others
but for dynamic systems this isn’t sufficient.
Tightrope balancing –> raise arms –> base of support and CoM don’t change but you’re more stable.
This is because you have increased your moment of inertia which decreases tendency to rotate and gives you more time to make corrections/ ‘righting moments’ before losing your balance. This is why moving bicycle more stable than still.
What is Angular Impulse? And how does it relate to rotational momentum?
Angular impulse is: applying a large moment of force over a period of time
To have rotational momentum, angular impulse has to occur
Would applying a force over a prolonged period of time increase or decrease impulse? Would this force cause any rotational force?
It would increase Impulse .. and a change in momentum ..?
If this force was directed at centre of mass then it wont create any rotational force
When two objects collide what happens to Momentum?
Momentum is conserved in collisions or impacts
What happens to energy as you walk?
Energy can be transferred between limb segments
When you walk, leg goes into full extension and slows down, pulling the thigh
When is momentum conserved?
Impact, Collision, Airborne phase in running long jump
What happens if you lose mass during a running long jump (during airborne phase)
Velocity would increase. This is because during the airborne phase the momentum stays the same
P (momentum) = M (mass) x (Velocity)
Thus, if mass decreases, velocity has to increase to keep the same momentum
What is rotational inertia dependent on?
The amount of mass and how that mass is distributed about the axis of rotation
What is the second stage of the flow chart?
Make the observation and compare it to the desired movement
What is the Analysis phase of the flow chart?
1.Identifying and ranking errors
2.Using various principles to dictate why these errors occur
Ranking the errors can help one focus on which error to focus on first
What is the Remediation phase of the flow chart?
- communication of the error to the suggested strategy that may lead to improved performance
- Following intervention, the action is observed again and then you go through flow chart again (continuous process)
What are the 3 orthogonal components that the ground reaction force resolves into
Vertical, anterior-posterior, medial-lateral
What is a force platform/plate?
It’s an instrument used to measure the ground reaction forces generated by a body standing /moving across the surface
placed on the floor often
What is a force plate used for?
Used in many biomechanics situations such as gait analysis, sport and impact situations and testing/ medical diagnoses.
When combined with video, it can be used to obtain joint kinetics
How does a force plate work ?
transducers are placed in the 4 corners that measure strain due to loads placed on the surface
What are the 3 components measured in the force platform?
Fz is the total vertical force
Fx and Fy are 2 horizontal forces due to friction
If a strain was applied in the center of the platform what would the transducers measure?
vertical force applied to center of plate would produce an equal amount of strain on each corner
if not the transducers can measure the tendency to twist the plate abt the x and y axes
What happens during a typical Gait analysis?
Typically use 2 force plates because double support phase has both feet on the ground at the same time
What information does the force platform produce?
3 orthogonal forces (Fx, Fy, Fz) and 3 orthogonal moments (Mx, My, Mz)
But the plate resolves the contact into a single reaction force with a point of application which is the center of pressure
Whats the difference between a Force plate and a Pressure Sensor?
Force Plate has a single resultant force vector applied to the plate
Sometimes we want to see the distribution of forces in the area. In this case, We use pressure sensors
Pressure sensors are similar to tiny force plates that measure vertical forces based on pressure applied to small area
No horizontal forces measured, low accuracy (compared to force plate), but has diagnostic info for podiatrist to make orthotics
What are the equations for center of pressure?
CPy = Mx/Fz
CPx = My/Fz
How does the force plate measure the center of pressure ?
It is calculated by dividing the moment by the vertical force
for CPy it is Mx/Fz and for CPx it is My/Fz
Why is the center of pressure important?
In a static position, the center of gravity is directly above the CP
in dynamic movement, it is rarely above CP, and it is continually moving
What is ISO?
It is another way to alter exposure. Does this by increasing sensitivity of CCD to light
What is a typical ISO value?
Typical Value: 100
But if biomechanist needs shutter speed of 1/500 and largest aperature of lens is f/4.0 then ISO Increased to 400. This allows for more light available making image brighter
..(Idk if this is important)
What happens when you increase ISO too much?
Can make the image “noisy” or “grainy”
Auto Focus, Good or Bad?
Biomechanist usually dealing filming with same plane of motion, so subject is always same distance from camera and focus does not need to change.
Auto focus should be turned off so camera doesnt get confused with any movement in background or foreground
Focal Length (Zoom)
Measured in mm and Human Eye has focal length of about 80mm
Wide angle (<50mm) lens increases distortion (Barrel effect).
Longer lens (>200mm) has less distortion but harder to keep it steady, requires high shutter speed to prevent motion blur from unsteady hand
Biomechanists dont have this limitation since they use tripod
Usually larger lenses cant handle aperatures larger than f/4.0. This means more light (or higher ISO) is needed
What happens when you have more light?
With more light: greater freedom to choose aperture, exposure time, ISO, depth of field, focal length etc.
In lab usually extra light from spot lights but some lab settings do not have this and have to be prepared
What is the most common method of measuring human movement
use film or video recordings with a method called motion capture
What variables can be measured by a video or film analysis?
Position and time
But if we are careful we can derive other things like joint angles, velocities, accelerations, centers of mass, etc
What are the 4 requirements for motion capture?
- camera placement
- body segment landmarks
- scaling the image
- frame rate
Describe camera placement
the camera must be placed perpendicular to the plane of motion.
The camera should be stationary so that any motion from one frame to the next would be due to the movement of the subject
Since the camera cannot zoom in or out during filming, it must be paced far enough away so as to “see” the required frame in its entirety
There are methods of using a moving camera but they require a calibrated background so that the position and orientation of the camera can be calculated for each frame
Describe body segment landmarks
they are placed on the subject in a biomechanical analysis so that joint centers can be digitized more easily
Often, the markers are reflective tape so when a light is shined on the subject, the video shows bright lights at each marker
Describe how the image is scaled
The image is scaled using an object of known size in the plane of motion
Often, a meter stick or calibrated object of a known size is placed in the field of view and in the plane of motion
Explain why we need a frame rate
Exact timing information of frame rate is needed to establish velocity and acceleration
A falling object in the background was used in the old days but this was replaced by light emitting diodes within the cameras and the quartz technology used in modern watches ensures exact precise frame rate in today’s video cameras
What is an aperture ?
Aperture refers to the size of opening that the lens uses to allow light expose film or reach the CCD of a digital camera
How is aperture measured
It is measured in f-stops with the larger numbers (settings) being used when a lot of light is present and lower numbers are used in low light situations.
For example F8 is often used outdoors in sunlight while F2.8 or lower is often used indoors with room lighting
Besides aperture, what is another factor that affects the amount of light that hits the film or CCD?
Exposure time which is regulated by shutter speed
Describe exposure time
The shutter opens to the set aperture for a period of time and then closes.
In biomechanics, this exposure time is measured in a fraction of a second
choose a time that is sufficiently short to prevent motion blur yet sufficiently long to get a proper exposure. If the time is too short, the image will be under exposed and be too dark
Explain exposure time in still photography
Still photography often uses a flash to increase the amount of light such that a short exposure time can be used and in video, extra lights such as flood lights or TV lights are used
What shutter speed is good for slower vs faster motion?
A shutter speed of 1/100th of a second is often sufficient for walking and slower movements while 1/500th or higher is often used for more dynamic movements
What is the depth of field?
It refers to the range of distances from the camera that will be in focus
Objects that are closer or further away from this range will not be in focus
Explain how brightness of light affects aperture and thus depth of field?
if light was low and the photographer needed to open the aperture to f/2, the depth of field is smaller – only a narrow range of objects will be in focus
If more light is available, and an aperture of f/16 can be used and the depth of field is larger
Why is depth of field important for biomechanists?
biomechanist must be careful to ensure that all objects that need to be digitized are in focus
It is possible for the right arm to be in focus but the left arm is not in focus of a sagittal plane
What is the area under an acceleration graph
velocity
Where is max take off velocity on the force time graph?
Max take off velocity is when force crosses bodyweight line
What is the area under a force-time trace
The area under a force-time trace is a measure of the impulse or change in momentum of an object
What are factors that increase impulse?
- Increase average force over time of force application (Increase Strength). Ex Weight training
- Increase time which force is applied (increase ROM)
- Increase Impulse with decrease in time (Utilize stored elastic energy)
- Increase impulse with pattern of force application
WHat does it mean if the force is greater or less than bodyweight?
If the force is greater than his body weight, he has an upward acceleration and creates positive impulse which in turn indicates a positive change in momentum and a positive change in velocity.
Conversely, if the force is less than his body weight, he has a downward acceleration, creates negative impulse, has a negative change in momentum and a negative change in velocity
When does the jumper have maximum upward velocity/ downward velocity and the lowest point in their squat?
Max downward = where slope is steepest (downward) in first bump (but acceleration is positive)
Max upward = where the slope is steepest in the first bump (but acceleration is negative)
lowest point = the first tiny bump on the big one (muscles eccentrically contract to stop you from going into ground)
What is formula for impulse
I = F* t or m*delta v (which is change in momentum)
Why is measured and calculated time different and which one is usually larger?
Measured is usually larger b/c there are a lot of uncertainties in the calculated time (e.g. number of boxes, dimensions of boxes, difference in position of CoM when taking off and landing)
