Linear Kinematics

Question 1

What is video analysis?

Answer 1

Video analysis is our route to describing and explaining human movement.

Qualitative- involves ‘systematic’ observation of that relies on relevant knowledge to analyse human movement (describing the action of a joint).

Quantitative- involves the measurement of biomechanical variables to analyse human movement (measuring the ROM at a joint); requires biomechanics knowledge

Video- a 2D representation of a 3D scene; a sequence of images at constant time intervals

Question 2

How can movement be analysed?

Answer 2

Movement can be analysed:

By assessing the anatomical contribution to the movement (functional anatomy)

By describing the motion characteristics (kinematics)

By determining the cause of motion (kinetics)

Kinematics and Kinetics make up Biomechanics.

Question 3

What needs to be done when setting up the camera?

Answer 3

Make sure the camera has a fixed position (no extra movement), needs to be perfectly perpendicular to the sagittal plane

Standard video in the UK works with the standard video format, PAL (Phase Alternating Line)

Standard frame rate= 25 frames per second (25Hz), need more frames per second for quick, sporting actions- some movements may be missed otherwise.

Shutter speed indicates how long the sensor is ‘open’ and exposed to light- how long it takes to collect a still image. The higher the shutter speed, the faster it will open and close to collect the still image. The shutter speed doesn’t change the sampling rate, just how quickly each sample is taken.

A low (slow) shutter speed increases risk of blurred image if the movement is fast. A high (fast) shutter speed can lead to dark images.

Aperture influences amount of light and depth of field- affects brightness of image. Large aperture, much light (smaller F-stop number). Small aperture, less light (larger F-stop number) better depth of field.

Digital gain can be used to increase image brightness, but this is artificial and can make the image grainy (adding noise to the image).

If you want to quantify movement beyond describing what you see, requiring quantitative analysis.

Question 4

What is camera orientation and calibration?

Answer 4

Camera positioning/orientation:
Perspective error- the error caused in the position of an object when the viewing angle is not perpendicular to the object.

Consideration 1- camera-object distance ‘as far as possible’
Consideration 2- camera is perpendicular to the plane of motion

Calibration:
Need to record a ‘known distance’ so you can scale the video (pixels) to real world distances, to obtain 2D (or 3D) coordinates. It helps identify the distance of each pixel.

Question 5

What is biomechanics?

Answer 5

Biomechanics- the study of the structure and function of biological systems by means of the methods of mechanics.
We look at the dynamics section of mechanics, which splits further into kinematics and kinetics (linear and angular).

Question 6

What are forms of motion?

Answer 6

Translation (linear)- all parts of the body move through the same distance in the same time.

Rectilinear- translation occurs along a straight line

Curvilinear- translation occurs along a curved line

Rotation- all parts of the body move through the same angle in the same time. General motion is a combination of rotation and translation.

Question 7

What is trajectory, position, distance and displacement?

Answer 7

Trajectory- the path followed by the object in motion

Position- location in space relative to a reference; described by a vector, measured in (m), reference is always to the right-horizontal.

Distance- length of a trajectory path (scalar quantity, only magnitude)

Displacement- change in position between two points in time; described by a vector (magnitude, direction, sense), measure in (m)- the length of the line

Vector displacement: magnitude- length of vector, direction- 90 degrees
Horizontal displacement: magnitude- length of vector, direction- 0 degrees

Question 8

How is trigonometry used when working with vectors?

Answer 8

Sin of the angle is the opposite (vertical) over hypotenuse (resultant)
Cos of the angle is the adjacent (horizontal) over the hypotenuse (resultant)
Tan of the angle is the opposite (vertical) over the adjacent (horizontal)

Pythagoras: horizontal squared + vertical squared = resultant squared

Question 9

What is displacement, velocity and acceleration?

Answer 9

Displacement- change in position (m)

Velocity- rate of change in position between 2 points in time (rate of displacement). Described by a vector, measured in m/s. v = s / change in t. The gradient of a line on the displacement-time graph is the average velocity. Displacement (m) / change in time (s)

Acceleration- change in velocity (m/s) / change in time (s), rate of change of velocity, described by a vector, measured m/s/s. The gradient of the velocity-time graph is the average acceleration.

Question 10

How do displacement, velocity and acceleration work together?

Answer 10

Local peaks in displacement = changes in sense of velocity (crosses zero)

Local peaks in velocity = changes in sense of acceleration (crosses zero)

Steepness dictates magnitude.

An object has a negative velocity (moving down) but positive acceleration- deceleration takes place (acceleration sense opposite to velocity sense). Deceleration will mean velocity will be moving towards zero (coming to a stop).

If displacement-time trace is positive/negative slope, velocity will be positive/negative.

If velocity-time trace is positive/negative slope, acceleration will be positive/negative.

Question 11

How can the kinematics of running be described and how can running velocity be increased?

Answer 11

Running (or other forms of gait) is a ‘cyclical’ form of human locomotion.

The kinematics of running can be described by other parameters:
Stride/step length
Stride/step frequency
Stride/step velocity

Running velocity can be increased by:
Increased length
Increase frequency
Or increasing both

When approaching maximum velocity, a trade-off between frequency and length occurs, they either prefer one or the other.

Running velocity = Step/stride length x Step/stride frequency

Question 12

What is uniformly accelerated motion and SUVAT equations?

Answer 12

Uniformly accelerated motion- when a body experiences the same acceleration, the acceleration of that body is constant. Most commonly, the acceleration due to gravity (projectile motion).

Equations when constant acceleration:
Vf = Vi + at
S = Vit + (at^2)/2
Vf^2 = Vi^2 + 2as

S- displacement
Vi- initial velocity
Vf- final velocity
A- acceleration
T- time