Kinetics Flashcards
Ground reaction forces during gait
- changes magnitude
- GRF moves horizontally as stance progresses into propulsion
Horizontal: How fast
Vertical: lifting foot up and down
Mediolateral: rotation side to side
coP
Location of average GRF vector showing displacement
Measured by force plate
VERTICAL force component of stance phase
- Vertical propulsion of the Com
- Fp + or F-mg
shape of curve:
- confidence to load stance foot
- Height change of centre of mass
- indicates speed of movement
*when mass os lower - force is greater than actual mass
Anterior (Propulsion) - posteriror (breaking) force
- Heel strike to posterior peak: -ve, 0.2/20% body weight
- Posterior peak to cross over: moving over stance limb, reducing horizontal force @55%
- cross over to anterior peak: +ve force to propel forwards, 0.2/20% of body weight
- anterior peak to toe off:
What is claw back?
Inital contact with ground - representing positive force
- hamstring used
e.g. more on ice than carpet
compensates or adjusts after an initial impact or force application. For example, after the heel strike, there is a phase where the forces and movements adjust to stabilize and continue the forward motion
What does the posterior peak signify?
Maximum GFR in posterior direction relative to bodys motion
(breaking force, heel strike)
Negative force represents posterior (it is coming back at us and working against forward movement)
0.2 times / 20% body weight
When does cross over occour durnig gait?
55% of stance phase
Horizontal force = 0
only vertical ground reaction force
What does anterior peak signify?
Midstance to toe off
propulsion force (+ve)
positive force to propel us forwards
Force partly comes from momentum, part from muscle force = propulsion force
Anterior peak to toe off
-Force is being transfered to front foot and anterior force is reducing
-The length of time the force takes to reduce and offload - effects foot loading of next foot contact
Medioloteral forces during gait
Early stance: Lateral GRF
Early single support: Pronation (inwards)
Late stance: Medial GRF
Mediolateral forces help to understand control of muscles (hip abductors, hip adductors )
What % of mediolateral force is from muscles (control of gait)
92%
(hip abductors, hip adductors )
Overpronation
IS A MYTH
foot does not move, it is the shape of the shoe rotating, effecting the persons foot
orthotics: alter the trajectory of the food during the gait cycle which effects the hip joint muscle responses (NOT FOOT) -> causing hip and knee issues
What is the relationship between force and velocity?
Increasing force can increase velocity in biomechanics
Impulse - momentum relationship: Greater force applied over time = greater acceleration -> increased velocity.
esp in performance contexts like running or jumping
What is the nature of a force-time curve in terms of performance outcome, change of direction
Inertia
The natural tendency of an object in motion to keep moving in the same direction and speed
- Or if at rest, to stay at rest
- Resistance to change in motion state
- Resistance is depended on an objects mass
- Bigger = harder to move
Momentum (L)
Momentum = mass x velocity
L =mv
100kg x 8m/s = 800kg m/s (L)
-Describes the quantity of motion a mass has based on its velocity
Once overcome inertia of mass and we are moving/motion
Heavy = more momentum than light object (traveling at same speed)
What is the centre of mass?
when force is applied - no rotation
What is the gentre of gravity
resultant torques are zero
no rotation
only translation
equal and opposite forces
locating the centre of mass
Mass x moment arm (sum of all forces and moment arms) / total mass of object = position of com
*have to make assumptions
How is torque influenced?
- The magnitiude of the force
- The direction of the force (line of action)
- Point of application of the force
T = Fr
*distance (r) is measured perpendicular from i.e. right angle or 90deg to the line of force/action
Free axis of rotation
Centre of Mass
Fixed axis of rotation
- Door hinges
- forearm is fixed to elbow axis
If there is no rotation occurring, what is the sum of the torques?
0
What is the net effect of the torques equal to?
the sum of the torques acting on the axis
how do we know if we have located com?
sum of torques = 0
Mass moment of inertia
Mass moment of inertia = resistance to rotate
InertiaCom = Mass x axis through CoM (squared)
explains how difficult it may be to generate rotation about a given axis
How do we explain the fact that velocity about the axis can change with re-distribution of mass about the axis (i.e. decreasing, increasing)
Angular momentum (H)
H = mr^2 x ω
H = inertia x angular velocity (ω)
(inertia = mass x moment arm^2)
Parallell axis theorem
Multi-segmented bodies have both remote (rotation of gyration (k) and local terms
total inertia (I) = local (m1r1^2 + m2r2^2) + remote (mk^2)
Angular momentum (H) total = Icmω + mk^2 ω
Angular momentum (H)
Quantity of angular motion possessed by a body; measured as the product of movement of inertia (I) and angular velocity (ω)
H =mk^2ω or H = Iω
units: kg m^2 rads/s or kg m^2/s
Factors affecting the magnitude of a bodys angular momentum (H)
- Mass (m)
- distribution of the mass in respect to the axis of rotation (k or r)
- angular velocity (ω) of the body
How does the angular velocity change in the air? e.g. diving
changing the moment of inertia
H = inertia x velocity
Decrease inertia, automatically increase velocity (trade-off = conservation of momentum)
Inertia and velocity changes - total stays the same
Why?
Due to gravity acting through the CoM (which only causes linear motion - NOT rotation
Inertia
Newton’s First Law of Motion, which states that an object will remain at rest or in uniform motion unless acted upon by an external force.
Inertia itself is the resistance of an object to a change in its state of motion, and it’s directly related to the object’s mass.
Explain the impulse momentum relationship
The impulse is the product of force and time. ‘ the area under the curve’
which is equal to the change in momentum (L) which is derived from mass x velcoityf - inital.
The force applied changes the momentum of the object, affecting both magnitude and direction of its motion.
Describe how acceleration is related to changes in velocity?
acceleration is the rate of change in velocity over time.
Acceleration occours when a net force is applied to an object, causing it to speed up or slow down.
F =ma
Discuss the significance of the force-time curve in performance outcomes.
Impulse (f x t ) = change in momentum (L)
Increasing force (GRF) enhances performance outcomes
Increasing time of application force, reduce injury risk.
e.g.
The impulse-momentum theorem states that the change in momentum is equal to the impulse applied. By applying a strong force over a short time, Alex increases his momentum, allowing for rapid acceleration in the new direction.
A rapid application of force over a short duration can lead to high peak forces, increasing the risk of injury. If the time taken to apply or absorb these forces is minimized, it may not allow adequate adaptation by the tissues involved.
How can understanding biomechanics improve athletic performance?
- Maximizing ground reaction force (GRF) for better velocity
- Analyzing force-time curves to improve movement efficiency
- Balancing force application to reduce injury risk
- Applying impulse-momentum principles to optimize techniques like long jumping
Using the impulse-momentum theorem, determine the net force applied by the player during this maneuver.
Net force = change in L (Lf - Li) / time
L initial = 75kg x 8m/s = 600kg m/s. L final = 75kg x -5m/s = -375kg m/s. -375kg m/s - 600kg m/s = -975kg m/s. -975kg m/s / 2s = -487.5 N
What principle explains the relationship between applied force and change in momentum?
The Impulse Momentum Theorem explains that the change in momentum of an object is equal to the impulse applied to it. This means that the force applied over a period of time alters the object’s momentum, crucial for maneuvers like changing direction in soccer.
How can the long jumper optimize their performance during take-off?
Maximizing ground reaction force (GRF) during take-off allows the long jumper to convert horizontal momentum into vertical lift effectively. This enhances jump distance while maintaining speed, crucial for optimal performance.
How can Sarah maximize her performance in the long jump by applying the principles of the impulse-momentum relationship?
increase initial horizontal velocity and then Increasing ground reaction force, change horizontal velocity into vertical velocity for maximum distance. increase impulse by applying more force within a shorter amount of time.
What is the most important aspect for long jump performance?
Speed of run up (initial momentum)
The best jumpers are able to redirect horizontal velocity into vertical velcoty with minimal loss of forrward momentum
what does τ = Iα mean?
Relationship between torque and angular acceeration (α).
where I is the moment of inertia.
This shows how the torque applied to an object results in its angular acceleration, analogous to F = ma in linear motion.
What is the angular analogue of mass in biomechanics?
Moment of inertia
it quantifies the object resistance to rotate (angular acceleration)
depending on the distribution of mass around its axis of rotiaion (com) and radius of gyration (remote axis) and the mass
What does the moment of inertia depend on?
The mass x radius ^2 (the distribution of mass around its axis of rotation)
greater mass = greater resisitance to changes in angular velocity
greater distance = greater resistiance to changes in angular velocity
Describe the significance of locating the center of mass in relation to torque.
Locating the center of mass is crucial for torque because:
- A force through the center causes only linear motion
- The center of mass is where the sum of torques equals zero
- It helps understand resistance to rotation and angular acceleration, impacting performance in sports like gymnastics.
Discuss the importance of angular kinetics in optimizing human performance in sports.
ability to increase angular velocity at joints by reducing the radius to the axis of rotation. Enhancing control and movement efficiency, resulting in faster movements in running and increase step frequency without increasing energy expenditure. also for tumbling sports for greater control of limbs while airbone and ability to manipulate mass around axis of rotation i.e. the centre of mass
Why is the location of the COM important in tumbling?
The centre of mass affects the distribution of forces acting on her body
- if the force is directed through her com she will have liner rotation and without rotation - increasing her stability and control
if Sarah applies a force at a distance from her center of mass while rotating, what effect will this have according to Newton’s laws of motion?
it creates torque, leading to angular acceleration
What is the significance of the mass moment of inertia?
determines how easily angular velocity can be changed
What factor primarily influences the resistance to rotation?
Moment of inertia
depends on the mass of the object and how that mass is distributed relative to the axis of rotation, affecting angular acceleration and performance in sports like gymnastics.
How do we explain the fact that the velocity about the axis can change with re-distribution of mass about the axis? i.e. increase/decrease?
Angular Momentum
Linear momentum (L) = Mass x velocity
Angular Momentum (H) = mass x radius^2 x angular velocity
(inertia x angular velocity)
What is velocity? - displacement divided by time
What does angular momentum mean for athletic performance?
J curve approach in high jump to try and generate angular momentum
Change direction of linear momentum from forward to upward (linerar) ALSO need to add a rotational force around long axis to back to towards the bar (after left the ground for optimal energy transfer) torque of internal to create force around long axis is done by taking J approach
HJ: Need both upward and rotation around long axis
If don’t get enough rotation around long axis, shoulders/hips need to be parallel with bar.
How is GRF increased within a shorter amount of time?
increasing explosive strength and reducing contact time
What factors influence the mass moment of inertia and angular momentum in sprinting?
length of limbs
body composition (fast twitch muscle)
mass distribution
center of mass position
What role does the center of mass (COM) play in the transition from acceleration to maximal velocity for Athlete A?
The position of the COM influences balance and propulsion; maintaining an optimal COM allows Athlete A to maximize force application against the ground, facilitating a smooth transition to maximal velocity.
Why is the acceleration phase crucial for sprinters?
establishes the foundation for reaching maximum speed. A strong start and effective propulsion during this phase enable a smoother transition into the maximal velocity phase, enhancing overall performance.
What distinguishes the maximal velocity phase from the acceleration phase?
The maximal velocity phase is characterized by achieving higher speeds with less focus on propulsion, as the athlete has already accelerated. In this phase, maintaining speed becomes more important than generating additional force.
transitions from the acceleration phase to the maximal velocity phase, what key change occurs in his running mechanics?
hifting focus from propulsion in the acceleration phase to optimizing stride length and frequency for maximum speed.
What is essential for cycling the limb quickly through the recovery phase?
Powerful muscle contractions provide the necessary force for quick limb cycling, while flexibility allows for a greater range of motion, both essential for optimizing the recovery phase and enhancing sprinting performance.
i.e. fast twitch muscle fibers
How might improving the efficiency of the limb recovery phase impact the sprinter’s overall performance?
Improving the efficiency of the limb recovery phase can increase step frequency and movement economy.
Quicker limb cycling during recovery leads to improved speed in subsequent strides, enhancing overall performance in sprinting.
What are the two types of acceleration profiles mentioned for track and field athletes?
QAHP (Quick Explosive Acceleration with High Peak Force) and QAMP (Quick Acceleration with Moderate Peak Force). These profiles help match athletes to appropriate events based on their acceleration characteristics.
How do variations in contact time influence the vertical ground reaction forces observed during the sprinting phases?
Longer contact times generally lead to higher vertical ground reaction forces, which can enhance acceleration but may negatively impact overall speed if prolonged.
