View Full Version : Re: about definition of moment

02-27-2007, 12:55 AM
G'day everyone from Vientiane, People's Democratic Republic of Laos (he
says, just to get Laos into BIOMCH-L for the first time...).

I would like to take the opportunity to ask a question one of my students
once raised. When we calcuate/estimate joint moment, we model the
muscle-tendon-joint system as a "torque motor", implying a free-spinning
spindle driven by a rotation torque. I wonder what the assumptions aee
behind this model - e.g. does it necessarily assume that the tendon moment
arm is small?

I ask because this student showed me a very complex analysis (beyond my ken)
showing that the torque-motor assumption broke down when the tendon moment
arm >> 0.


On 2/24/07, David Smith wrote:
> Dear Sae Yong Lee
> I can see why you can get confused with all the terminology and many
> papers
> don't state whether they are presenting external or internal moments and
> in
> what terms.
> I will assume you are working in a 2D frame.
> This is my explanation:
> 1) Moments are the result of a minimum of two opposing offset forces that
> have lever arm about a point of interest acting on a body, known as
> a force
> couple, and cause a tendency for that body to rotate. (This would be a lot
> easier if we could post diagrams) So usually in the human body the applied
> forces cause rotation at the joints. This as a result of the force couple
> between Joint force (bone and soft tissue) and internal or external forces
> E.G. muscle or GRF.
> So a moment is (Force * lever arm) but only if there is an equal and
> opposite (force * lever arm) (lever arm also known as moment arm)
> It seems simple but is very useful to remember that all forces are equal
> and
> opposite and the sum of all forces is zero and this is is true for
> moments.
> F-m*a=0 applied forces or moments - resisting forces or moments = 0 The
> forces and moments in any mechanical system, dynamic or static, are always
> in eqilibrium.
> Therefore when speaking of the magnitude of forces and moments it is
> necessary to define when we mean applied or resisting since the sum of the
> two is always zero and the product of each are always equal..
> Terminology:
> Applied force: That force acting on the body of interest
> Resisting force: That force resisting the applied force
> External force: In terms of biomechanics is usually the force applied by
> or some force extrinsic to or outside the body EG the weight of a kettle
> held in a hand or the force of a boxing glove applied against a boxers
> head.
> This can also be thought of as an external moment.
> Internal force: Is usually the force that resists the external force and
> is
> exerted by forces intrinsic or inside the body EG muscles, inertia,
> ligaments, bone.
> This could also be thought of as an internal moment.
> Clockwise moment: Is a way of defining the rotation or tendency to rotate
> of the moment (force * lever arm) in a 2D plane.
> Anticlockwise moment: (Anticlockwise and clockwise moments can also be
> thought of as positive and negative moments as can the forces but stick
> with
> the former for now)
> You may be able to see from the above descriptions that a moment in a
> given
> direction (say anticlockwise) about a joint of interest may be the sum of
> external and internal forces.This may be where you get confused since
> External forces * lever arm + Internal forces * lever arm = internal and
> external moments acting in the same direction, when intuitively you might
> imagine they would be in opposite direction.
> The easiest way to imagine and summate the moments is to find all the
> forces
> * lever arms causing an anticlockwise rotation minus the forces * lever
> arms causing a clockwise rotation about the joint of interest. This
> negates
> (cancells the importance of) the fact of whether they are internal or
> external. Can you see that Internal and external are not absolute terms
> but
> more a comparative term. So therefore in terms of the body as a whole a
> given force or moment of interest may be internal or intrinsic but in
> terms
> of a given joint of interest the same force or moment may be external or
> extrinsic to that joint.
> Nett: That sum remaining after all deductions from the gross or grand
> total.
> Gross sum: = total sum of everything
> Nett sum: = the gross sum less a certain individual sum of interest.
> Therefore it could be said that the individual sum is the nett sum. EG
> Gross
> of all stock value of a shop = 117.50 - Gross sum net of Value Added Tax
> at
> 17.5% = 100 - Nett sum of VAT = 17.50
> Joint moment: is the sum of moments acting about the joint of interest,
> which is ZERO. The NET sum of joint anticlockwise moments = X and the NET
> sum of joint clockwise moments = Y (X - Y = 0)
> 2) So you may now see that although all moments within the mechanical
> system must be in equilibrium we cannot easily directly measure the
> internal
> moments and forces of the muscles and joints and bones etc. We can however
> measure the external forces (external and internal in terms of the whole
> body) of GRF for instance.
> Then by using the laws of Newton to caculate the external moments about a
> joint of interest we can, by knowing that moments are equal and opposite,
> calculate the forces required from the internal structures for equlibrium
> of
> moments to be established. This is known as inverse dynamics using linked
> segment models. It is inverse because the action is reversed whereas in
> real
> life (some would say) muscles are the initiating or driving force and not
> GRF for instance. (For me the two are inseperable as the muscles could not
> move the body without the opposite action of external forces).
> 3) There are two basic systems for studying Human movement, a) Kinetics
> =forces and b) Kinematics = motion, Vicon or Motion monitor systems
> characterise the kinematics (motion) of the joint and limb. Therefore they
> do not measure related forces, which are Kinetics, which can be measured
> using a force plate for instance, moments can then be calculated. It is
> possible to calculate the forces and moments from the kinematics by
> finding
> the product of the accelerations of a limb and its body parameters. This
> is
> known as the Forward solution or Forward Dynamics.
> This brings the Anthropometry into the picture and Anthropometry is the
> knowledge of the body parameters such as Dimensions, Mass, CoM and Radius
> of
> Gyration of a segment and the location of its joint centres. These are
> necessary for both forward and inverse dynamic calculations.
> A good reference for this is Biomechanics and Motor Control of Human
> Movement, David Winter, Wiley interscience NY.isbn 0-471-50908-6
> Does this help? all the best Dave Smith.
> ----- Original Message -----
> From: "Sae Yong Lee"
> To:
> Sent: Friday, February 23, 2007 4:07 PM
> Subject: [BIOMCH-L] about definition of moment
> > Dear Biomch-L subscriber
> >
> > My name is Sae Yong Lee. I am doctorate student studying at University
> of
> > Virginia in Kinesiology program.
> > I have question related to moment. Are there anyone help me to clarify
> the
> > definition of moment related termiology?
> > 1. Difference between joint moment, net joint moment, external moment,
> > muscle moment, and internal moment. I'll write down my understanding of
> > those terms.
> > Joint moment: The moment acting at the joint to overcome external
> moment.
> > net joint moment: Sum of all moment acting at the joint including
> internal
> > (active (muscle) + passive (ligament, joint capsule...), external
> moment.
> > muscle moment: Moment produced by the muscle (different to quantify
> using
> > motion analysis system).
> > Internal moment: Moment produced by active (muscle) + passive (ligament,
> > joint capsule...). part of joint moment.
> > external moment: Moment produced by external forces such as ground
> > reaction force.
> > Please give me some comments whether or not my understanding is right.
> >
> > 2. What is general Newtonian inverse dynamics calculating? I have been
> > believed that it is internal moment which is moment produced by passive
> > and active structures crossing the joint. But I got confused these days
> > while I was reading articles. It seems like quite a lot of articles
> > alternatively use all those terms I mentioned. Even though the several
> > articles use same system to calculate Moment, one said it is external
> > moment and the other said internal moment. Is it joint moment, net joint
> > moment, external moment, muscle moment, or internal moment.
> >
> > 3. If anyone use Vicon system or Motion monitor system, what kind of
> > moment does this system calculating? Are there any reference article
> that
> > mention about the calculation of moment of these systems?
> >
> > Thanks
> >
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Dr. Chris Kirtley MB ChB, PhD

608 Dockside
44 Ferry St.
Kangaroo Point
Queensland 4169
Tel. (07) 3891 6644 x 1608
Fax 3891 6900

Clinical Gait Analysis: http://www.univie.ac.at/cga