# Kinematics

Kinematics is the study of motion without regard to its causes – it is the motion of a rigid body within a defined reference system – can be translation which occurs when all particles within the rigid body undergo an equal linear displacement with a specified time period or it can be rotation which occurs when all particles within the rigid body move in a circular path about the same centre. Motion can take three forms – translation, rotation or a combination of the translation and rotation.

Osteokinematics – describes motion of bone relative to the three body planes.
Arthrokinematics – describes the motion that occurs between the articular surfaces of joints

Kinematic Data

Biomechanics labs use …

Reference systems:
Need a spatial reference system – most laboratories use a Cartesian co-ordinate system – can be 2 or 3 dimensional.

Linear kinematics

Distance:
• in mechanics, ‘distance’ is the length of a path travelled by a body
• scalar quantity

Displacement:
• the straight line that connects a point’s position from one instant in time to another
• how far a body has moved from a starting point
• vector quantities (has magnitude and direction)

Speed:
• the rate of change of distance over time
• scalar quantity (no direction)
• average speed = distance/time

Velocity:
• the rate of change of displacement
• vector quantity (as displacement is a vector)
• average velocity = change in displacement/duration of time

Acceleration:
• rate of change of velocity
• also, rate of change of rate of change of displacement
• vector quantity
• often difficult to grasp concept, but fundamental to understanding kinematics

Angular/rotational kinematics

Uses same concepts as linear kinematics, but applies them to a body rotating about an axis.

Angular Displacement:
• defined as the changes in angular position
• angular quantities measured in radians (rad) – a rad is the angle formed by a arc length on a circle equal to the radius of the circle

Angular velocity:
• the rate of change if angular displacement (radians per second)
• can be calculated as the average angular velocity or the instantaneous angular velocity

Angular acceleration:
• the rate of change of angular velocity (radians/second2)
• can be calculated average angular acceleration or instantaneous angular acceleration

Instant centre of rotation (ICR):

Joint axes of rotation:
Bones rotate about a joint in a plane that is perpendicular to an axis of rotation.

Degrees of freedom:
The number of independent movements allowed at a joint – a joint can have up to 3 degrees of freedom.

Screw/Helical axis:
In 3 dimensional analysis instead of using an ICR, the concept of a screw or helical axis is used – that is that any rigid body’s motion may be described as a combination of rotation about an axis and translation along that same axis.

Projectile Motion:
The motion of a body under the influence of gravity only

Measurement of Gait Kinematics:

Electiogoniometers:

Video:

2D Kinematics:
Limitations of 2D:
• Parallax error
• Perspective error

3D Gait Analysis:

Models:
The modified Helen Hayes model