View Full Version : force plate issue

10-10-2005, 11:38 PM
Dear All,

typical force platform data are coordinates of application point, force vector in three directions and so called free torque Tz that has only one vertical component. I apology for perhaps lack of my physics understanding, but I could not find clear explanation why is biomechanics of normal human gait such that it not possible during the normal gait to have torque also in two other horizontal directions Tx and Ty. Some people say it is absolutely impossible to have Ty and Tx, others say those two component are neglibagle. Seems to me 'absolutely impossible' and 'neglibagle are two quite distinct statements. If we can apply with the foot force in all three directions, even in situation where the foot remains still on the platform, why is it then not possible to have torque in three directions too. What is the physical/biomechanical 'guidance' that determines whether there will be torque in horizontal directions or not?

Besides some people emphasize 'free' torque? Does word 'free' implicate this torque does not depend on actual point/axis of rotation?

Last question is concerned with cases where Ty and Tx do normally exists and particular platform construction. For example Kistler platform normally outputs data from eight channels which are originally provided by four force sensors. Data from those either channels are the combined to ultimately give ground reaction force in all three directions, cop and torque in only vertical direction. However, if I am not mistaken, based on particular derivation of equations, it seems to be necessary to assume Tx=Ty=0. In other words, in cases where we cannot assume Tx=Ty=0 usual eight channel data would not allow us to compute Tx and Ty as well. Under those circumstances are there any additional force (torque) sensors needed then on platform or what?

Thank you for your time.

Tomislav Pribanic, M.Sc., EE
Department for Electronic Systems and Information Processing
Faculty of Electrical Engineering and Computing
3 Unska, 10000 Zagreb, Croatia
tel. ..385 1 612 98 67, fax. ..385 1 612 96 52
E-mail : tomislav.pribanic@fer.hr