Biomechanics Exam 2 Flashcards
What do force plates do?
Measure three orthogonal reaction forces, Fx, Fy, Fz and three moments about these orthogonal axes, Mx, My, and Mz.
How do force plates measure force?
Through transducers sandwiched between rigid plates that convert force into voltage.
Ground Force Reaction
The reaction force provided by the support surface on which the movement is performed.
Calculate the resultant of the 3 reaction force vectors
Square root of the sum of the component forces squared
GRF Angles
Theta yz = inversetan(Fy/Fz)
Theta xz = inversetan(Fx/Fz)
Theta xy = inversetan(Fx/Fy)
Center of Pressure
Point on the force plate that the resultant force originates
COP(x)
My/Fz
COP(y)
Mx/Fz
Newton’s 1st Law
A body continues in its state of rest or uniform motion in a straight line unless compelled to change the state by external forces exerted upon it. Due to mass or inertia.
Inertia
property of a body to resist a change in its state of motion.
Moment of Inertia
Sum of m*r^2
Angular Momentum
Moment of inertia * Angular velocity
Newton’s 2nd Law
An unbalanced force applied to a body causes acceleration of that body (sum of the forces equals mass times acceleration)
Newton’s 2nd Law - Angular Acceleration
Sum of the torque equals Inertia time angular acceleration
Torque
Radius cross the force
Newton’s 3rd Law
For every action there is an equal and opposite reaction (torque and/or force)
Forces always come in pairs
Momentum
mass times velocity
Elastic collisions
objects rebound
Inelastic collisions
object stick together and usually become distorted and generate heat.
Coefficient of Restitution
describes the relative elasticity of an impact
perfectly elastic e = 1
inelastic e = 0
e = velocity after / velocity before
Kinetic Energy
(1/2) m*v^2
Power
Force * velocity
1 hp
550 ft lbs / s = 746 W
Potential Energy
mgh
Motor Units
motor neuron and muscle fibers it innervates
Fine control
small motor units contain as few as 20 muscle fibers per nerve fiber
Strength control
a large number of muscle fibers per nerve fiber
Erector Spiane Muscle
extends down the back, responsible for maintaining back posture in any “sitting” position.
Basic strategies to prevent overexertion injury?
Design, select, train
Recommended Weight Limit
(RWL) Equation
LCHMVMDMAMFMCM
Lift Index (LI)
Task Load / RWL
What does Lift Index (LI) mean?
LI < 1 - lifting task is safe for most workers
LI > 3 - many workers are at elevated risk
Load Constant (LC)
max recommended weight for lifting at standard 23 kg or 51 lbs
Horizontal Multiplier (HM)
HM (metric) = 25/H
HM (english) = 10/H
H = horizontal distance from hands to midpoint
Velocity Multiplier (VM)
VM = (1-0.003 (V-75)) (cm)
VM = (1-0.0075 (V-30)) (in)
V is vertical distance of hands from floor
Distance Multiplier (DM)
D is total vertical distance moved between origin and destination
Asymmetric Multiplier (AM)
A is angle of symmetry (angular displacement in sagittal plane)
Frequency Multiplier (FM)
based on work duration and V (vertical distance from floor)
Coupling Multiplier (CM)
based on V (vertical distance of hands from floor in cm) and quality of coupling
Area under a force distance curve is ____ .
Work
Units associated with impulse are ____ .
Newtons * sec
Torque is angular acceleration * ____ .
Inertia
Angular velocity * Inertia
Angular Momentum
Which multiplier in the NIOSH lifting equation has the most influence on determining the safety of a given task?
HM (Horizontal Multiplier)
