Torques and Moments of Force

Question 1

What are Torques?

Answer 1

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

Question 2

Torque

Answer 2

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

Question 3

Torque Force

Answer 3

Magnitude
Point of application
Direction
Line of action

Question 4

Torque Moment Arm

Answer 4

Perpendicular distance between line of action of force and axis of rotation

Question 5

Calculating Torque

Answer 5

T=Force x Moment Arm

Rotary vs non-rotary

Switch Coordinates

Break Resultant Force into component parts

• horizontal
• vertical

Question 6

Moment Arm

Answer 6

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

Question 7

Lever Arm

Answer 7

Distance between the point of force application (perpendicular component) and the axis of rotation

T = F⊥∙d
d = lever arm

Question 8

Torque (In Humans)

Answer 8

Muscles attach at some distance away from joint center of rotation

Therefore, all muscles produce torque about the joints they cross

Question 9

Joint Moment

Answer 9

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

Question 10

Muscle Force Component -Rotary

Answer 10

Rotary Component
⊥ to bone segment
Creates internal moment
Causes motion

Question 11

Muscle Force Component - Non Rotary

Answer 11

⊥ to rotary component, // to bone

Does not contribute to internal moment

Causes joint compression or distraction
Stabilization or Dislocation

Question 12

Muscle Force Components

Answer 12

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

Question 13

Internal Movement created by a muscle is dependent upon:

Answer 13

Muscle Force (F)
Lever Arm (d)
Angle of pull (θ)

Question 14

Joint Torque - Muscle Contributions (Net Internal Moment)

Answer 14

Muscle Contributions (net internal moment)

Muscles may produce co-contraction
-Creates opposing joint torques (opposite direction)

Motion occurs in direction of net internal moment

Question 15

What is the purpose of antagonist muscle co-contraction?

Answer 15

Control velocity of joint motion

Increase stability at joint

Question 16

What type of contraction are the muscles producing?

Answer 16

Quadriceps = concentric contraction
Hamstrings = eccentric contraction

Question 17

Muscle Co-Contraction- Good or Bad?

Answer 17

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

Question 18

Joint Torque Non-Muscle Contributions (External Moment)

Answer 18

Segment mass will cause a joint moment
Any external object attached or held by segment will produce a joint moment
Ground reaction forces

Question 19

Knee Adduction Moment in OA Patients

Answer 19

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

Question 20

Equilibrium and Stability

Answer 20

Equilibrium and stability are not the same thing

Question 21

Clinical Stability

Answer 21

Response of a joint to an injurious perturbation

Ex – Valgus stress to a knee; Rolling an ankle etc.

Question 22

Biomechanical Stability

Answer 22

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

Question 23

Equilibrium

Answer 23

State characterized by balanced forces and torques
-No net forces and torques

Static Equilibrium
-When a body is completely motionless

Question 24

3 Conditions of Static Equilibrium

Answer 24

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

Question 25

Static Equilibrium

Answer 25

Also applies to angular analogs of Newton’s Laws Examples Isometric contraction Parking brake

Question 26

Levers

Answer 26

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

Question 27

3 Component of Lever

Answer 27

``` -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

Question 28

Classes of Levers

Answer 28

Classified according to the relative positions of the axis, motive force and resistive force Acronym: A-R-M

Question 29

1st Class Lever

Answer 29

Axis between the motive force and resistive force (MAR) e.g. = see-saw Examples in body Agonist/Antagonist action Elbow extension Plantar flexion*

Question 30

2nd Class Lever

Answer 30

``` 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? ```

Question 31

3rd Class Lever

Answer 31

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

Question 32

Mechanical Advantage/Disadvantage

Answer 32

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

Question 33

Mechanical Disadvantage

Answer 33

``` MA < 1 Mechanical disadvantage -Motive force = Muscle force Muscle force greater than resistive force All 3rd class levers and some 1st class ``` Fmuscle >> 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

Question 34

Mechanical Advantage

Answer 34

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