Final Flashcards
Kinematics
- the study of motion without regard for the forces that cause the motion
- motions include positions , velocities, acceleration, angles, angular velocity and angular acceleration
kinetics
study of motion and forces that caused it
motion capture
- measurement of kinematics
Numerical diffrentiation
- Sample a function at small time steps and calculate the slope between adjacent points. The resulting time series of slopes is the time derivative of the function
Numerical integration
- Divide the area under the function into small rectangles. The integral of the function is a time series that is the cumulative sum of the areas up to each time point
how does an accelerometers measure acceleration
- They measure the acceleration of the sensor itself. When attached to an object (ex. the wrist) there is no allowed motion between the object and sensor
- While there are very many, their principle operation is the same: there is a mass inside that when accelerated stretches a “spring”. The deflection of that spring can be measured and since we know the stiffness and mass we can determine acceleration
how are accelerometers used in biomechanics
- can be attached to different parts of the body to analyze gate patterns
activity classification
- to use the signals from the accelerometers that are worn somewhere on the body to not only count steps, but to figure out what activity you are engaged in as well as how intensely you are performing this activity
What is movement artifact
- motion capture is limited by movement artifact
- this means the movement of your marker on the skin is influenced by the movement of your segment, and the bone moves with respect to other tissues, so it is sometimes not very accurate
Why do we need integration constants
an integration constant is needed to get the starting point of a function being integrated
What are the methods used in activity classification
- cluster analyses are used to learn the relationship between the acceleration signal and the activity
- then once you know how things cluster we can use decision trees to assign activities
Give 3 reasons why we can’t just integrate accelerometer signals to determine speed and position.
- measured accelerations have contributions from not only motion of the limb but also gravity
- accelerometers “drift” can quickly cause large inaccuracies in the estimated displacement because its contribution grows proportional to the square of time
- the initial condition for each integration must be known
Give an example of how improper joint angles can lead to injury, and explain the cause of the injury.
- certain wrist joint angles can result in compression of the medial nerve
- chronic compression reduces blood supply, and thus starves the nerve of oxygen, this results in pain, numbness, loss of function and even cell death
- the cause of the injury can be ulnar deviation and wrist extension and can lead to carpal tunnel syndrome
the importance role of falls in injury and death of the elderly
- over 30% of ages 65+ living independently fall at least once a year and 10-15% falls result in injury
- falls are the leading cause of unintentional injuries among older adults in Canada, including 90% hip fractures and wrist fractures and over 60% of traumatic brain injuries
- falls are also the number 1cause of injury-related death among older adults
- falls and injuries they cause cost the canadian government over $2 billion annually
- 20% of older adults hospitalized fora hip fracture die within the year and 50% do not return to their pre-fracture mobility
What causes falls in elderly
incorrect weight shifting followed by loss of support and collapse
Why can’t you stand with your heels against the wall, and then bend over at the hips to pick something up off the ground without falling over?
The movement would shift your centre of pressure outside of your base of support, causing you to fall over.
Explain what a stable and unstable system is. Give an example of each.
- a stable system is one that rejects the perturbation because gravity provides a restoring force back towards equilibrium position
Ex. Normal pendulum - an unstable system is one that doesn’t reject the perturbation because gravity acts to amplify the original perturbation
Ex. Inverted Pendulum
Does a normal pendulum without damping go unstable
no it will oscillate but never go unstable
What is the base of support
the area within an outline of all ground contact points
how do you use base of support and COM height to determine system static stability
- as long as the height COM sits above the base of support our system is statically stable because gravity will act to reject perturbations, but if COM passes outside of the base of support the system becomes unstable because gravity acts to cause the system to fall over
How to make yourself more stable
- lower your COM and widen your base of support
Explain the biomechanics of the ankle strategy used to stabilize standing
in order to maintain our balance after slightly being perturbed, we are required to shift the ground reaction force to generate a moment that corrects the gravitational moment (i.e shift the centre of pressure)
What is the zero moment point?
The Zero-Moment Point (ZMP) is the point on the ground where the line of action of the gravitational force acting on the COM intersects the ground. It is called this because if the GRF acts at the ZMP, no ground reaction moment is required for stability.
Explain the biomechanics of the hip strategy used to stabilize standing.
- To maintain balance after slightly being perturbed we can try to create a ground reaction force that pushes back on the body. This means we must generate a forward pushing force on the ground in order for it to push back on us
- we can use our muscles to flex our hip and try to drive our feet forward, if we don’t slip, the ground will push back rejecting the forward perturbation this is called the hip strategy
Explain the biomechanics of the stepping strategy used to stabilize standing.
To maintain balance after being perturbed we can take a step in the direction of the perturbation. We can use this step to:
1. increase our base of support in the direction of the perturbation
2. dissipate some of the energy added from the perturbation
3. can capture some of the motion to come to a new balanced position
Explain the role of a Biomechanical model in the scientific method
-
How are reflexes and preflexes different? Why are reflexes important for stability?
- a REFLEX is a stereotyped response to a stimulus (muscle stretch) that involves neural sensing, conduction, and computation before a response can be generated. They are the fastest mechanism for active control, but still suffer from delay
- a PREFLEX is the response of a biological tissue from a stimulus that arises from tissue biomechanics that help stabilize motion. They give an appropriate response to a stimulus with essentially zero delay
Energy is
is a property of all physical things
What is the principle of Conservation of energy?
energy can be transferred from one form to another, but it can’t be created nor destroyed
Positive mechanical work
- when a force performs positive mechanical work on system it increases its mechanical energy
- the force component that acts along the displacement points in the same direction as the displacement
Negative mechanical work
- when a force performs negative work on its system, it decreases its mechanical energy
- the force component that acts along the displacement points in the opposite direction of the displacement
positive/negative power
- Positive power when the point at which the force is being applied is moving and if it is is moving the velocity component has to be moving in same direction as force
- negative power when velocity component is moving in opposite direction as force
What type of stabilization is the ankle and hip strategy used for
static stabilization
What kind of stabilization is the stepping strategy used for
dynamic stabilization
energy dissipation
- the total energy in an isolated system is conserved, but when converting energy from one form to another, energy is also constantly being converted to thermal energy (heat) due to factors like friction
- systems can’t convert all this thermal energy back into useful forms, thus all systems must continue to take in energy to survive
Motors
perform positive mechanical work
breaks
preform negative mechanical work
struts
generate force, but remain isometric performing no mechanical work
What is the work loop technique
- evaluates the mechanical work and power output of muscle contractions via muscle testing
Why is efficiency always less than 100%
because some of the input energy ends up as heat rather than mechanical work
efficiency
a dimensionless number that describes how much work you get out of a system for the energy you put in
explain when muscles can behave like motors, brakes and struts
- muscles act as motors when shortening (positive mechanical work)
- muscles act as breaks when lengthening (negative mechanical work)
- muscles act as struts during isometric contraction (generate force, but perform no mechanical work)
tendons act as _______
springs
What is the muscle tendon unit (MTU)
- muscles acting in series with tendon
How does the muscle tendon unit change length
- the muscle fibres can lengthen or shorten while tendon stays the same length
- muscle fibres are controlled to be isometric and tendon lengthen or shortens
Why are these behaviours important for understanding movement
because it creates an advantage to reduce muscle mechanical work and energetic cost when we use the tendon to store and return elastic energy
The maximum mechanical power output of a muscles is
is maximized when the muscle shortens at about 1/3 of its maximum velocity
What determines the maximum power output a person can sustain?
The duration of the activity. Max power starts out very high and quickly drops and plateau’s much lower than maximum.
How is ATP used to power cellular processes
- mitochondria converts energy stored in food to energy stored in ATP with a maximum efficiency of 50%
How do we convert ATP into positive mechanical work
- cross bridges convert the energy released from ATP into positive mechanical work with a maximum efficiency of 50%
what is the efficiency of the entire process including cellular respiration and cross bridge cycling
25%
max efficiency of performing positive work
25%: 4J of food yields 1J of positive work
max efficiency of negative work
- 1J of negative work will require a minimum of 0.83J of metabolic energy (min efficiency=120%)
statically stable
maintaining one position where there is no movement of individual
dynamically stable
using dynamics as a mean to achieve stability: movement occurs
passive dynamic stability
perturbation that produces a mechanical response that automatically rejects perturbation
example: taking an extra step that dissapates the energy added from perturbation
active control
- opposite of passive stability
- involves sensing of perturbation, calculation and implementation of the response by actuators (muscles)
What are the ideal characteristics of a method for measuring movement?
Motion capture
Provides: high quality resolution, high spatial resolution and quick analysis
