Paul Guy

04-26-1996, 07:36 AM

Hi Kieran,

In your letter (some parts deleted):

>=20

> I have what may be a very simple question, but the answer to which is

> very important to my studies. My PhD studies partly involve the

> evaluation of Bi-lateral symmetry of the lower extremities during

> vertical jumps. The results are being evaluated at the joint moment

> level and, therefore, I require the determination of the point of

> application of force.

>=20

> I have access to two platforms which are located parallel to each

> other ( and seperated by 5cm in the Z direction). With my subjects

> facing in a positive X direction the point of application of force in

> the X direction for each plate is equal to the moment of force about

> the Z axis (Mz) divided by the total vertical force (Fy):

>=20

> Ax =3D Mz / Fy (where moments are taken about the plate surface).

>=20

...Make sure also to remove any components in that moment that could =

be caused by shear force and the effective depth of the gauges. Not all =

force plates have gauges at zero height. Torgue about the vertical axis =

can cause a similiar problem if the mechanical and 'electrical' centres =

aren't coincident. Tables of correction values should be supplied by the =

manufacturer to fix these problems.

> >is Ax =3D the sum of moment of force about the Z axis for plates one

> >and two devided by the sum of the vertical forces for plates one and

> >two.

> >

> >is Ax =3D [ Mz(1) + Mz(2) ] / [ Fy(1) + Fy(2) ]

>

Naaah..... determine the effective point of application for each plate. =

Multiply the coordinates (in global coordinates) by the vertical force =

on that plate. Do this for the second plate. you now have two 'moments'. =

Add them up, and divide by the subjects weight (or the sum of the forces =

on the two plates).

ie, Zeff=3D((COPz1 * fy1)+(COPz2 * fy2))/(fy1 + fy2)

where Zeff is the effective z coordinate of the centre of pressure =

(or application), COPz1,COPz2 are the centres of pressure for each =

plate, and fy1,fy2 are the vert. reaction forces for each plate. The =

COPz's are calculated in the normal way.=20

Your kinematics needs to know where the force plate centres are. If =

you only measured one side of the body, you need to correct the =

measurements to get rid of the asymetry, so things like total body =

centre of gravity line up (assuming that you are using or 'simulating' =

3D measurements.

=20

> P.s.

> (The coordinate system is the ISB system which is reactionary and

> follows the left hand rule. Positive Y is vertically normal to the

> force plate and positive X coincides with the direction the subjects

> are facing)

>=20

It follows the left or right hand rule depending on which way you =

establish Z+. If z+ is to the subjects right, then the right hand rule =

applies. The XYZ axes seem silly until you realize that many labs =

started with a 2D system, often digitized from video or film. Then =

height is Y, forwards is X+. When you can afford 3D, the obvious =

coordinate system you'd choose to remain compatible with your old data, =

is the ISB system. Maybe it's not mathematically kosher, but it's =

practical.

>=20

-Paul

--------------------------------------------------------------------

Paul J Guy work phone:519-885-1211 ext 6371 =20

paul@gaitlab1.uwaterloo.ca home/FAX/:519-576-3090

pguy@healthy.uwaterloo.ca 64 Mt.Hope St.,Kitchener,Ontario,Canada

In your letter (some parts deleted):

>=20

> I have what may be a very simple question, but the answer to which is

> very important to my studies. My PhD studies partly involve the

> evaluation of Bi-lateral symmetry of the lower extremities during

> vertical jumps. The results are being evaluated at the joint moment

> level and, therefore, I require the determination of the point of

> application of force.

>=20

> I have access to two platforms which are located parallel to each

> other ( and seperated by 5cm in the Z direction). With my subjects

> facing in a positive X direction the point of application of force in

> the X direction for each plate is equal to the moment of force about

> the Z axis (Mz) divided by the total vertical force (Fy):

>=20

> Ax =3D Mz / Fy (where moments are taken about the plate surface).

>=20

...Make sure also to remove any components in that moment that could =

be caused by shear force and the effective depth of the gauges. Not all =

force plates have gauges at zero height. Torgue about the vertical axis =

can cause a similiar problem if the mechanical and 'electrical' centres =

aren't coincident. Tables of correction values should be supplied by the =

manufacturer to fix these problems.

> >is Ax =3D the sum of moment of force about the Z axis for plates one

> >and two devided by the sum of the vertical forces for plates one and

> >two.

> >

> >is Ax =3D [ Mz(1) + Mz(2) ] / [ Fy(1) + Fy(2) ]

>

Naaah..... determine the effective point of application for each plate. =

Multiply the coordinates (in global coordinates) by the vertical force =

on that plate. Do this for the second plate. you now have two 'moments'. =

Add them up, and divide by the subjects weight (or the sum of the forces =

on the two plates).

ie, Zeff=3D((COPz1 * fy1)+(COPz2 * fy2))/(fy1 + fy2)

where Zeff is the effective z coordinate of the centre of pressure =

(or application), COPz1,COPz2 are the centres of pressure for each =

plate, and fy1,fy2 are the vert. reaction forces for each plate. The =

COPz's are calculated in the normal way.=20

Your kinematics needs to know where the force plate centres are. If =

you only measured one side of the body, you need to correct the =

measurements to get rid of the asymetry, so things like total body =

centre of gravity line up (assuming that you are using or 'simulating' =

3D measurements.

=20

> P.s.

> (The coordinate system is the ISB system which is reactionary and

> follows the left hand rule. Positive Y is vertically normal to the

> force plate and positive X coincides with the direction the subjects

> are facing)

>=20

It follows the left or right hand rule depending on which way you =

establish Z+. If z+ is to the subjects right, then the right hand rule =

applies. The XYZ axes seem silly until you realize that many labs =

started with a 2D system, often digitized from video or film. Then =

height is Y, forwards is X+. When you can afford 3D, the obvious =

coordinate system you'd choose to remain compatible with your old data, =

is the ISB system. Maybe it's not mathematically kosher, but it's =

practical.

>=20

-Paul

--------------------------------------------------------------------

Paul J Guy work phone:519-885-1211 ext 6371 =20

paul@gaitlab1.uwaterloo.ca home/FAX/:519-576-3090

pguy@healthy.uwaterloo.ca 64 Mt.Hope St.,Kitchener,Ontario,Canada