Angular Kinetics Review

• Readings: – Hamill Ch 11 esp pp 382-410– Kreighbaum pp 318-324, 326-331– Adrian 33-40 (COM calculations)

• Homework problem on calculating MOI of lower extremity will be distributed in class

Angular Kinetics Outline• Torque and motion relationships Musculoskeletal

machines – Mechanical advantage– Length-tension relationship

• Center of Mass – segmental method• Angular analogue of Newton’s third law• Angular impulse and momentum• Conservation of angular momentum• Calculating moment of inertia of body segments using

cadaver data• Homework problem – calculating MOI of lower extremity

Torque and Motion Relationships• Relationship between linear and angular motion

– displacement, velocity, and acceleration

• Angular analogue of Newton’s third law (F=ma), the instantaneous effect of a force or torque– Torque = moment of inertia (I) X angular acc (

What is torque? • What is moment of inertia ? • What is radius of gyration • Changing moment of inertia and radius of gyration in the body

Calculations using a 3-segment system• Homework problem

What is torque, or a moment of force?

Torque is the turning effect of a force and is the product of force magnitude and moment arm, or perpendiculardistance from the force’s line of action to the axis of rotation:

Angle of Pull of Muscle & degree of force application

Turning component equalsForce times sin θ

Mechanical Advantage of Elbow Flexors

Length of Elbow Flexors as Joint Angle Changes

Length-tension, angle of pull combined

Sine of

Sample Problem #2, p 433

Example of total body

torques

Torque and impulseabout the center ofmass

What is the COM and why is it important?

• What is COM (or COG) and why is it important?– It simplifies mechanical analysis of a complicated system– It is the point at which all of the mass of the system may be

considered to be located– It is the only point that represents movement of the total system

The acceleration of the COM is proportional to the net force and inversely proportional to the mass.

– It is the only point that follows a parabolic flight pattern when free of contact with earth

– External forces through the COM cause produce only linear motion– External forces not through the COM (eccentric forces) create a

torque, or moment, and produce both linear and rotary motion

COM/COG Concept and Calculation Method (Adrian pp 33-41)

• Concept of balancing segmental torques

• Segmental Calculation of COM – General calculation method– Information needed

• Proportionate mass of each segment

• location of COM of each segment

Segmental concept of center of mass

Hanavan Model used for Segmental Calculation of COM and MOI

Segmental concept of center of mass

Information needed: 1. Segmental COM location 2. Segmental proportionate mass

Instantaneous effect of net torque: Moment of Inertia (MOI) Constant

What are torqueand MOI?

T = I

Instantaneous effect of net torque: Torque is constant

Instantaneous effect of net torque: Ang acc constant

What is Moment of Inertia?

Here, r (the radius of rotation) is equal to k (the radius of gyration), but that is not the case with extended bodies

It is the resistance of a system to rotational acceleration, and is calculated at follows:

What is radius of gyration (k)?

An indicator of distribution of massabout the axis. It is the distance fromthe axis to a point at which all themass of a system of equal masswould be concentrated to have the MOI equal the original system. Itis, then, the average weighted distance of the mass of a systemto the axis.

Equivalent systems

k 35

k 35

Determining MOI & K • Simple 3-segment system:

– I = mi di2 = m1 d1

2 + m2 d22+

m3 d32 + . . . . . . .+ mi di

2

– I = mk2 ; k = (I/m).5

• Irregularly shaped bodies

But we can’t measure all of these small masses!

Physical pendulum method of determining MOI and K

• Suspend object at axis• Measure mass (m), and distance from axis to COM, r• Measure period of oscillation (T)

– Moment of inertia (I) = T2 mr * .248387 m/sec

– Radius of gyration (K) = ( I/m).5

MOI & K – Geometric Objects

Changing I and k in the human

body

Changing I and k in the human body

MOI around principal axes of human body in different positions

Angular Impulse and Momentum

• Impulse-momentum relationship - effect of force or torque applied over time– Linear: Ft = mv Rotational: Tt = I

• What is angular impulse? • Torque X time• What is angular momentum?

• Amount of angular movement: I • Conservation of angular momentum • Angular momentum is constant if net impulse is zero

Total body torque and angular impulse: Mediolateral axis

Angular Impulse around vertical axis

What is angular momentum (L)?

Example of angular momentum

Conservation of Momentum

Conservation of Momentum

Addendum to angular kinetics: estimates of body segment parameters

• The calculation of the linear and rotational inertial properties (mass, moment of inertia) of the human body requires estimates of body segment parameters

• Chapter 3 of Roberson provides an excellent summary of these estimation techniques

• Each of you will be assigned selected portions of this chapter to summarize for the class on February 27

Next topic: Biomechanics of Skeletal Muscle and Electroymography

• Biomechanics of skeletal muscle– Readings: Hamill pp 76-81, 103-109

• Electromyography– Readings: Hamill pp 81-85; Cram pp 32-37, Ch 3;

DeLuca website tutorial (http://www.delsys.com ),