Week 6 Flashcards
Base of Support (BoS)
Area defined by straight lines connecting the peripheral
most points in contact with the surface
Centre of Mass (COM)
Point around which the mass and weight of a body are
balanced
Centre of gravity (COG) or Line of Gravity (LOG)
Line of action of the force of gravity
Always acts at the COM and always downwards
Who has a higher centre of mass, males or females and why?
Males have a higher centre of mass generally as they have more muscle mass in the upper body
How can the COM be manipulated in humans?
Changing the position of our torso over our legs or moving our arms away or toward the BoS.
What is a concentric force?
A force whose line of action passes through the COM of the body on which it acts
What does a concentric force do?
Causes a change in linear motion (translation) but no changes in rotational motion
What is an eccentric force?
A force whose line of action does not pass through the COM of the body on which it acts
What does an eccentric force do?
causes a change in rotational motion as well as a change in linear motion
What is a force couple?
Equal, opposite directed and parallel forces
Define Torque
Tendency of a force to cause rotation
How do you calculate torque?
T = F*MA (Nm)
F = applied force (N) MA = moment arm (m)
What is a moment arm?
Perpendicular distance from the force to the axis of rotation
Draw a line from the force line of action to the axis of rotation. This will give you the moment arm.
Equilibrium
ΣF & ΣT = 0
Σ = sum of
Describes the state of a system that is not changing its speed or direction
How do you calculate the COM of the forearm?
Work out the length of the forearm and multiply it by the proximal or distal length % from Winter’s table.
How do you calculate the mass and weight of a forearm?
Determine body weight of person and multiply it by the forearm represented number in Winter’s Table. = 0.016
How would we calculate the COM of a forearm if the length was 35cm?
0.35 x 0.43 (winter’s table)
= 0.15 m from elbow as we used the proximal length
How would we calculate the mass and weight of the forearm if someone weighed 58kg?
58 x 0.016 (winter’s table)
=0.928kg for mass and weight:
0.928 x 9.81 = 9.1N
How to calculate the centre of mass with a XY graph?
Calculate the coordinate for each segment (body part) and times each by their % BW from Winter’s table. Once all segments or body parts are calculated, we some all of the XY x % BW’s together to come up with COM X and Y coordinate to plot on the graph/picture
Define balance
A body’s ability to control or maintain its state of equilibrium
Define Stability
The resistance of a body to change its state of equilibrium
What are the factors that affect stability or balance
- Line of Gravity (LOG) within BOS
- Size of Base of Support (BOS)
- Height of Centre of gravity (COG) / Centre of Mass (COM)
- Principles of moments/torque
Types of stability
Linear stability and Angular stability
Define linear stability and what factors play a role
Resistance to being moves in a straight line
Inertia (mass), friction
Define angular stability and the factors involved
Resistance to being ‘tipped’ over
COM, BOS, LOG
How do you increase linear stability
Greater inertia/mass or increased friction
What is The Principle of Moments
When a body is balanced, the total clockwise moment about a point equals the total anticlockwise moment about the same point
ΣT = 0
Why is the person on the right more stable?
COM is lower, BOS is bigger in an Anterior-Posterior direction, LOG in the middle of BOS
Explain how the application of stability can be applied to these three real-world examples:
Teaching balance activities in gymnastics
Assisted chairs
Crutches
Teaching balance activities in gymnastics - Floor balances to beam balances, start with lower COM and larger BOS and slowly increase COM and decrease BOS.
Assisted chairs - mechanically move COM over BOS
Crutches - Increase BOS
