Biomechanics 2 Flashcards
Kinematics definition?
describing motion without regard to the force producing the motion
What is rotation about an axis?
Angular motion to produce linear motion
When rotating around a bar what do different body positions do?
Influence resistance to rotation
What’s generalised motion?
Most sporting movements
Inlvolves both translation and rotation
How is curvilinear translation achieved?
via angular motion about joint centres
Whats a minute?
1/60 of a degree
Standard unit for angular velocity?
Radians
Its the ratio of the length of a circular arc (s) to the radius of the arc(r)
θ = 𝒓/s
How to convert degrees into radians?
divide by 57.3
What’s angular distance?
length of angular path
What’s angular displacement?
Difference between initial and final positions
What’s angular speed?
Angular distance / time
What’s angular velocity?
w = (change in angular position) / (change in time)
Units are radians/s
What’s angular acceleration?
Change in angular velocity / change in time
Units are radians/s^2
What’s a relative angle?
Between 2 different segments
Knee, ankle
What’s an absolute angle?
Angle defined relative to a line in space
Describes orientation of segment in space
Thigh, foot
When given 3 points with coordinates how to calculate the angle behind the knee?
Cosine rule: a^2 = (b^2 +c^2) - (2bc cos angle)
But first have to determine the 3 lengths via Pythagoras theorem
Rearrange to find the angle
= cos angle = (b^2 + c^2 - a^2) / (2bc)
Or you can use right angled triangles find the angles next to the one you want then do 180 - the 2 angles you have found using SOH CAH TOA
Is the first central difference method different using angles?
No
The ones around the subject subtract the divide by time, same for velocity and acceleration
Why does heel lift reduce potential for achilles tendon strain?
There is reduced ankle angle and no change in knee angle
Results in reduced ankle dorsi flexion so less likely to strain
Assumptions you make when finding angles on lower part of body?
Skin markers represent joint movement on the: Hip Knee Ankle MTP Heel
Rigid body segments
two-dimensional rotations
What are the phases of ground contact in running?
Footstrike - heel just landed
Initial support - foot flat
Mid stance - heel begins to lift off
Toe - off
Dixon and Kerwin (1999) found that heel lift devices reduced ankle dorsi-flexion while having no significant influence on knee joint motion. What did they conclude regarding Achilles tendon strain?
That Achilles tendon strain was reduced when using heel lift devices
What’s a good method of breaking down a performance?
Identifying of kinematic variables at key points, for example in a golfers swing just look at the backswing
What’s the kinetic chain?
For example when looking at a tennis serve and the maximum velocity of the tennis ball its been found that to achieve this the timing of peak angular velocities of the contributing body segments occur in an order from proximal to distal = the kinetic chain
Each segment produces a force and also acts as a stabilising structure for the next segment
So each segment has a stabilising phase followed by an acceleration phase
if segment fails other segments are compensated reduce in performance and increased chance of injury
Relationship between linear and angular motion?
Required to optimise performance in sports involving rotation
VT = rw
VT = tangenital velocity
r = radius of rotation
w = angular velocity
hence the relationship can be increased via increasing the radius of rotation or angular velocity
2 types of athletic injury?
Acute - are associated with a traumatic event such as those from a hard tackle or from falling
Chronic (overuse) - from result from repetitive action such as long distance running or repaired throwing
What are intrinsic factors (within the body)?
Age Sex Previous injury Aerobic fitness Muscle strength Reaction time Anatomical alignment Postural stability
What are extrinsic factors (outside the body)?
Footwear
Surface
Competition level
What’s dorsi flexion of the ankle?
Put your toe up
What’s Plantar flexion?
Putting toe down
What’s abduction of the foot?
Toe moving away from body
What’s adduction of the foot?
Toe moving towards the body
What’s rear foot inversion?
Sole of foot points inwards and upwards
What’s rear foot eversion?
Sole of foot points outwards and upwards
Parts of the foot and ankle complex?
Talocrural (Ankle) Joint (joins distal ends of the Tibia and Fibula with the proximal talus
Allows sagittal plane motion (plantar/dorsiflexion)
(should be drawn in the exam)
Subtalar joint:
Allows frontal and transverse motion
Possible advantages of allowing Pronation (eversion)?
Natural cushioning
Can adapt to surfaces
Allows tibial rotation
Rearfoot movement typical values?
4 degrees = 3.3 m.s^-1 (Willems et al 2005)
11 degrees = 3.7 m.s^-1 (Pohl et al 2009)
Limitations of injury research?
Many are cross sectional and retrospective
Large sample sizes are required
Injuries grouped together as overuse or running related
characteristics highlighted in shoe advertising?
Shock absorption / cushioning
Motion control / stability
Traction
Weight / energy
Comfort / fit
In rear foot motion the steps of supination and pronation?
Supination:
Inversion
Adduction
Plater-Flexion
Pronation:
Eversion
Abduction
Doris-Flexion
What is shoe stability?
The ability of the shoe to resist excessive or unwanted motion of the ankle
Quantified by rear foot motion
Shoe design features?
Shoe shape - heel flare
Shoe supportive features-
Medial posting - built up bit of the shoe on the medial side (stops you rolling inwards) - decreases rear foot motion
Dual density - doesn’t deform on a certain area
Orthotic devices - inserts in the shoe - reduce rear foot motion
What does increased medial heel flare do?
Reduced rear foot movement
What does lateral heel flare do?
= reduced leverage
= reduced moment causing rotation
= reduced initial pronation in first 10% of ground contact
=Lower initial pronation velocity
On a shoe what does medial post and high density cause?
Reduced rear foot motion
Problems with prescribed orthotic devices?
They measure feet them in a static state
What does cushioning do?
Reduce peak impact forces
Provide Cushing to the athlete during ground contact
Loading rates shown to reduce with increased cushioning
But cushioning is a trade off between stability
How does footstrike, initial support, mid stance, toe off relate to ground reaction force?
N on y, Time (ms) on x
Peak impact force at foot strike - initial hump
Initial support - down then up
mid stance - large hump
toe off - slopes down
What changes when you run barefoot?
More likely to see a mid or forefoot strike
Foot presented flatter to the ground to compensate for higher impact at heel
Reduced rear foot motion
No cushioning
Increased calf activation
What does shoe surface influence?
Coefficent of friction
The surface Is more influential than the shoe
Arguments for barefoot running?
- “For most of human evolutionary history, runners were either barefoot or wore minimal footwear…”
- “Habitually barefoot endurance runners often land on the fore-foot before bringing down the heel…”
- “Habitually shod runners mostly rear-foot strike”
Why are their differences on studies on the 2 studies on barefoot running?
In US force data and indoor track was used
In Kenya video data and hard dirt outside track used, they also went at self selected speed
4 types of runner?
Heel strike
Midfoot strike
Forefoot strike
Toe runner
What does De Wit et al. (2000) say about impact peaks/loading rates in shoes?
- Greater impact loading rate in barefoot v shod
- Barefoot landed with a flatter foot
- Runners ran both shod and barefoot – with a rearfoot strike
What did Hamill et al (2011) say about impact peaks in shoes?
Reduced impact loading rate in barefoot v shod
• Barefoot landed with a plantar flexed ankle
• Shod landed with a dorsiflexed ankle
• Midfoot strike adopted when barefoot
Difference between prospective and retrospective?
Prospective studies usually have fewer potential sources of bias and confounding than retrospective studies. Retrospective. A retrospective study looks backwards and examines exposures to suspected risk or protection factors in relation to an outcome that is established at the start of the study.
What does a higher cadence do (running in shorter steps)? Heiderscheit et al 2011, Luke et al 2016?
Reduces knee loading
Reduces risk of shin injury
In conclusion does barefoot running cause a forefoot strike?
Not necessarily. Conflicting evidence
• What is the influence of surface on impact peak?
No difference in impact peak across surfaces
What is the influence of surface on foot strike?
Fewer rearfoot strikes on hard surface compared with soft
Do higher vertical loading rates increase risk of injury?
Some retrospective studies indicate an association. One prospective study supports this to an extent.
Does barefoot running reduce injury risk?
Not strong enough evidence to suggest this as a general rule Some evidence of increased risk of MSF in minimal shoes
Does foot strike influence injury risk?
Growing evidence to suggest transition to a forefoot strike can reduce symptoms of existing conditions
Is there a performance benefit to barefoot running?
Improved running economy, seemingly through reduced shoe mass.
What is an eccentric force?
Force whose line of action does not pass through the centre of gravity
Always causes a rotational movement and therefore a moment
Can also cause translation
What is a couple?
2 equal and opposite parallel forces
No translation
But rotation occurs about the centre of gravity
What is the moment of a couple?
turning effect that a couple produces
amount of rotation depends on:
Magnitude of one of the forces (the forces are equal)
perpendicular distance between lines of action
Moment (M) = F1 . x (perpendicular distance)
What is a moment arm?
The perpendicular distance between the lines of action of the forces of a couple
What is a moment influenced by?
Length of the moment arm
Magnitude of the force
Direction of the force
What is the resultant moment?
Sum of all moments about a defined point
Have to take into account negative direction
Will let you know the direction of rotation
What is equilibrium?
When sum of all forces is 0
So if told this can find out unknowns in a moments question
What is the centre of gravity?
Point about which mass is equally distributed - an imaginary theoretical point
3 methods for the calculation of total body centre of gravity?
Suspension:
A cardboard cut-out of the body shape is suspended from a point. In equilibrium, a vertical line is drawn on the cardboard, through the suspension point. This line passes through the centre of gravity. The procedure is repeated using a second suspension point. The point where the two lines intersect is the centre of gravity of the body.
Reaction Board:
A reaction board is supported by a block of wood at one end and a set of scales at the other. A subject lies on the reaction board. Since the board and subject are in equilibrium, the sum of the moments is zero. Knowledge of the reading of the scales and the length of the reaction board, allows calculation of the location of the centre of gravity. (calculations in lab book)
Segmentation Method:
This method uses the relationship: the sum of the moments about a point is equal to the moment of the resultant force. Mass and centre of gravity locations for each body segment are obtained from the research literature and used to calculate total body centre of gravity location. (calculations in lab book)
Moment of resultant about OY =? (the same for OX)
Sum of the moments about OY
= Sum of (mass of segment x y coordiante of segment of gravity)
The same is for x
3 experimental methods for the determination of body segment mass and centre of mass?
Cadaver Studies (Ratio and Regression) Several researchers have used cadaver segments to obtain segmental data.
Dempster (1955) used eight frozen cadavers. The limbs were cut through appropriate joint centres and the trunk was divided into three segments.
Clauser et al. (1969) used 13 cadavers. The body was divided as by Dempster, with the trunk as one unit. The bodies were not frozen.
Mathematical Modelling Represent body segments as geometric shapes • Truncated cones (arm, foot etc.) • Cylinders (trunk) • Elliptical spheres (head, hand etc.) (Hanavan, 1964) (Yeadon, 1990)
Scanning
Scanning techniques, such as gamma scanning and CT scanning, have been used to determine the mass per unit surface area for body segments. This information has been used to generate regression equations for the calculation of segmental (Hinrichs, 1975) mass and centre of gravity (Zatsiorsky and Seluyanov, 1983).
With reference to the work of researchers (e.g. Dempster, 1955; Hanavan, 1964; Hinrichs, 1975; Jensen, 1978; Yeadon, 1990; Zatsiorsky, 1990
Difference between kinematics and kinetics?
Kinematics = no force
Angles, distances, velocities, etc.
Kinetics = forces
Moments, inertia, pressure
What is a moment (or torque)?
The tendency of a force to cause rotation about a specific axis
It isn’t a force, it is the effect of a force in causing a rotation
Calculated by
• The product of the magnitude of a force and the perpendicular distance from the line of action of the force to the axis of rotation
so M (N.m) = force (N) x distance (m)
distance is referred to as the moment arm
Which of Newtons laws apply to angular motion?
Newtons 1st (linear): An object remains at rest or constant velocity unless acted upon by an external force
Newtons 1st (angular) - a rotating object will continue to do so at constant angular momentum unless acted on by an external force
Newtons 2nd (linear): A force applied to an object will cause the object to accelerate in proportion to that force (F = ma)
Newtons 2nd (angular):
An external moment produces an angular acceleration that is:
Proportional to the moment
In the direction of the moment
Newtons 3rd (linear): Every action has an equal and opposite reaction
Newtons 3rd (angular): For every applied moment , there is an equal and opposite reaction moment
What is inertia?
An objects tendency to resist change in motion
If we apply a sufficient to a body it will accelerate and can move in which ways?
Translation (If force is applied through the centre of gravity)
Translation and rotation
Definition of moment of inertia?
the resistance of a body to changes in angular motion = sum of all mr^2
m = mass r = distance from point of rotation to mass units = kgm^2
We are measuring the distribution of mass about an axis
It’s the rotational equivalent of mass, so greater mass means greater inertia, and more difficult to change linear motion
Also a greater inertia if mass concentrated further down the axis
Experimental methods for the determination of segmental moment of inertia?
Cadavers have been used to determine segmental moments of inertia experimentally using the compound pendulum method. The object is suspended and oscillated about the point for which moment of inertia is required. The time period of one complete oscillation is recorded for use in the calculation of moment of inertia (I) using the formula:
Inertia = (WhT^2) / 4pi^2
W = weight h = distance from suspension point to CG T = time period of one complete cycle
How can the moment of inertia be calculated by mathematical modelling?
By modelling the human body as geometric shapes
Overall what is inertia determined by?
Mass of body
Distribution of mass about the centre of gravity
Why is it more difficult to rotate in a straight position than when tucked?
Because more mass is distributed further away from the axis of rotation so larger moment of inertia
Does the moment of inertia vary for which axis you measure it about?
Yes
In flight what does the body rotate about?
It’s centre of gravity
Segments rotate about their own joint centre
Equation for angular momentum?
Angular momentum (kg.m^2.s^-1 = moment of inertia (kg.m^2) x angular velocity (radians.s^-1)
What is the transfer of momentum (see Hay, 1993 for examples)?
Since angular momentum is conserved during flight, if the angular momentum of one body part is reduced, there is an increase in the angular momentum of other body parts
Relationships of angular acceleration?
Directly proportional to the net moment
Inversely proportional to the moment of inertia of the object
In the air is total angular momentum constant?
Yes
If the angular momentum of one part of the body is decreased another part must increase to conserve total angular momentum
Total length jumped = ?
Take off distance + flight distance + landing distance
Determined by take off velocity, take off angle, and relative height difference between take off and landing
During running the relative ankle angle at ground strike is typically?
90 degrees
What is angular acceleration?
Change of angular velocity with respect to time
How do you work out average angular velocity?
The amount of degrees it’s displaced / by time
Dorge et al 2002 showed?
Foot velocity was influenced by knee linear velocity and shank angular velocity
Supination is characterised by?
Inversion, plantar flexion and adduction
Intrinsic factors associated with Ankle eversion injuries include?
Propprioception and muscle reaction time
when filming in the sagittal plane for someone running what markers do you place for digitising?
Hip Knee Ankle Heel Ball of foot
What are absolute angles?
Defined by the line of a segment with relation to an imagined line in vertical or horizontal space
What’s a relative angle joint?
The angle between 2 segments
Do heel inserts reduce the chance achilles tendon injury?
Yes as puts less strain on tendon
Equation for range of motion?
Peak inversion - touchdown angle
Equation for Average rate of inversion (deg/s)?
(Peak inversion - touchdown angle) / (Time of peak - time at touchdown)
Placing markers for rear foot movement during treadmill running?
Need to identify posterior shank and calcaneus vectors that make up the rear foot angle
Marker 0 - bottom of calf muscle belly
Marker 1 - lower achilles tendon
Marker 2 - superior calcaneus
Marker 3 - inferior calcaneus
Equation for average loading rate (N.s^-1)?
Peak impact force (N) / time of occurrence (s)
As surface hardness increases peak impact and average loading rate will?
Increase
What indicates the start of ground contact?
Vertical force greater than 10N
