Joint contact force analysis in going up stairs

[mohammad]

Hi evey one
We are going to evaluate the joint contact force of knee joint in going up and down stairs. Unfortunately we have only one force plate fixed on the ground. Can we put a table on it and do the test. I mean the table is one of the step of the stair. Could you please discuss if there is some errors associated with the outputs of the force plate

[Allan Carman]

Mohammad,

This is a good question. I was involved with a project which looked at stair ascent/descent with a box placed on a force plate, which was the middle step. I calculated the CoP for the force plate in the standard way and translated the CoP up the height of the box to apply the forces at the foot. I never tested this thoroughly, but visually appeared OK. If I was to do this again, I would check what effect horizontal forces applied on the box surface have on calculated CoP location.
My thought is that the key factor is whether or not the box is fixed to the force plate surface. If a box is placed on the plate surface then in the vertical direction it is only capable of produce a downward +ve force on the force plate. It is not capable of producing an upward or -ve force on the plate. Therefore, calculate CoP as normal. However, if the box is fixed/bolted to the force plate surface and can produce both +ve and -ve vertical forces on the force plate in response to forces applied at the box surface. Then you would have to consider the box as part of the force plate and change the plate calibration Zo value to include the distance to the box surface when calculating CoP.
Again my thoughts but not tested, someone may have looked at this in more depth than me.

Cheers
Allan Carman

[Edmund Cramp]

I don't know that anything was ever published but I remember discussing plans to create steps on a force plate at the Helen Hayes Hospital gait lab. Ramakrishnan and Mary Wootten planned to bolt steps onto the force plate, but this required returning the plate to the manufacturer (AMTI) because drilling a set of threaded holes in the plate (allowing the steps to be bolted to it) would have damaged the force plate sensors if it was done by users in the lab.
These days we offer the MTD (Mechanical Testing Device) that would allow force vectors to be applied to each of the steps and the physical vectors recorded in 3D so this might help verify that each step force plate configuration is correct.