Torques and Moments of Force Flashcards
What are Torques?
The turning effect produced by a force
Also called a moment
Think of it as an angular or rotary force
Directly proportional to the magnitude of force as well as the distance between the line of action of the force and the axis of rotation
Torque
Motion of a restrained system -Has an axis of rotation -One side is fixed in space Force is applied away from the axis Line of action is not through the axis
Torque Force
Magnitude
Point of application
Direction
Line of action
Torque Moment Arm
Perpendicular distance between line of action of force and axis of rotation
Calculating Torque
T=Force x Moment Arm
Rotary vs non-rotary
Switch Coordinates
Break Resultant Force into component parts
- horizontal
- vertical
Moment Arm
Shortest distance between the axis of rotation & line of action of the force
Perpendicular to force’s line of action & axis of rotation
T = F∙d⊥ d⊥ = moment arm
Lever Arm
Distance between the point of force application (perpendicular component) and the axis of rotation
T = F⊥∙d d = lever arm
Torque (In Humans)
Muscles attach at some distance away from joint center of rotation
Therefore, all muscles produce torque about the joints they cross
Joint Moment
Muscle Contributions (internal moment) Muscle force (tension) Muscle lever arm
Concentric Muscle Action:
Muscle internal moment & motion same direction
Eccentric Muscle Action:
Muscle internal moment & motion opposite direction
Muscle Force Component -Rotary
Rotary Component
⊥ to bone segment
Creates internal moment
Causes motion
Muscle Force Component - Non Rotary
⊥ to rotary component, // to bone
Does not contribute to internal moment
Causes joint compression or distraction
Stabilization or Dislocation
Muscle Force Components
Joint position influences the magnitude & direction of muscle force components
Angle formed by line of action and bony segment influences magnitude of rotary and non-rotary components
Internal Movement created by a muscle is dependent upon:
Muscle Force (F) Lever Arm (d) Angle of pull (θ)
Joint Torque - Muscle Contributions (Net Internal Moment)
Muscle Contributions (net internal moment)
Muscles may produce co-contraction
-Creates opposing joint torques (opposite direction)
Motion occurs in direction of net internal moment
What is the purpose of antagonist muscle co-contraction?
Control velocity of joint motion
Increase stability at joint
What type of contraction are the muscles producing?
Quadriceps = concentric contraction Hamstrings = eccentric contraction
Muscle Co-Contraction- Good or Bad?
Adds stability and control
Increases compressive force
-Greater Co-contraction in patients with OA, ACLR/ACLD, Obesity
Contributes to stiff gait and reduced knee flexion Disease progression
Joint Torque Non-Muscle Contributions (External Moment)
Segment mass will cause a joint moment
Any external object attached or held by segment will produce a joint moment
Ground reaction forces
Knee Adduction Moment in OA Patients
GRF vector is medial to the knee
Places an adduction moment on the knee and increases medial joint compression
- Increases disease progression and severity
- Worse in obese individuals and those with ACLR
Equilibrium and Stability
Equilibrium and stability are not the same thing
Clinical Stability
Response of a joint to an injurious perturbation
Ex – Valgus stress to a knee; Rolling an ankle etc.
Biomechanical Stability
Ability of a loaded structure to maintain static equilibrium
Statically maintained as long as the vertical projection of the COG remains within the base of support
Equilibrium
State characterized by balanced forces and torques
-No net forces and torques
Static Equilibrium
-When a body is completely motionless
3 Conditions of Static Equilibrium
Sum of all horizontal forces (or force components) acting on body must be 0
Sum of all vertical forces (or force components) acting on body must be 0
Sum of all torques must be 0
∑Fx = 0 ∑Fy = 0 ∑T = 0
Static Equilibrium
Also applies to angular analogs of Newton’s Laws
Examples
Isometric contraction
Parking brake
Levers
Simple machine consisting of a relatively rigid bar that may rotate about a fulcrum
- Bone = rigid bar
- Joint = fulcrum
Force applied to lever moves a resistance
-Muscle = force
3 Component of Lever
-Axis of rotation (fulcrum) Lever rotates around this axis -Motive forces (from our muscles) Cause rotations -Resistive forces (from weight of limbs or objects) Resists rotation
Three arrangements of force, resistance, and axis of rotation for a lever
Classes of Levers
Classified according to the relative positions of the axis, motive force and resistive force
Acronym: A-R-M
1st Class Lever
Axis between the motive force and resistive force (MAR)
e.g. = see-saw
Examples in body
Agonist/Antagonist action
Elbow extension
Plantar flexion*
2nd Class Lever
Resistance in middle (ARM) Torque advantage usually exists for motive force Not as versatile as 1st class lever e.g - Push-up What is the A, R, and M?
3rd Class Lever
Motive force in middle (AMR)
Most joint complexes act as 3rd class levers
Muscle is motive force
Advantage in ROM and speed of movement but disadvantage in force
Most joint complexes are of this type
Mechanical Advantage/Disadvantage
Ratio of the lever arm of the motive force to the lever arm of the resistive force for a given lever
MA = lever arm motive/ lever arm resistive
Mechanical Disadvantage
MA < 1 Mechanical disadvantage -Motive force = Muscle force Muscle force greater than resistive force All 3rd class levers and some 1st class
Fmuscle»_space; Fresistance
To hold a 44.5 N (10 lb) barbell you would need a muscle force of ?
1272 N
Muscles at a mechanical disadvantage
Muscle’s line of action very close to joint axis
Small lever arm
Mechanical Advantage
MA > 1
Mechanical advantage
Muscle force less than resistive force
All 2nd class levers and some 1st class
A motive force can balance a larger resistance when the motive force lever arm is longer than the resistance force lever arm
Muscles do however have an advantage in movement covered
-Point further from axis must move through a greater range of motion
A motive force can move a resistance through a larger range of motion when the motive force lever arm is shorter than the resistance force lever arm
