Isokinetic dynamometer gravity correction

[lucy smith]

Hi All,

I am using a Biodex system 3 dynamometer and Bipac but unfortunately they do not communicate when it comes to gravity correction (although the limb is weighed on the biomechanics software this does not get applied to the biopac output). Does anybody know how to correct for gravity during isokinetic dynamometer, is it simple? I have seen a recommendation by Andersen et al (https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2849864/) but must admit I am not entirely sure of how to apply these methods.

any help would be appreciated, thank you in advance.

[Ton van den Bogert]

The moment due to gravity is (theoretically) c*sin(theta) where theta is the angle of the limb (the line from joint center to center of mass CM) relative to vertical. You can determine the constant c from mass properties of the limb (c = m*g*(CM_distance), or measure it as Andersen proposed.

If you are not interested in the passive joint moments, and you want to subtract the passive moments, you can simply do a passive trial where you ask the subject to relax. Subtract the passive trial results from the active trial result. This automatically subtracts the gravity effect also, without needing a model.

Ton van den Bogert

[lucy smith]

Hi Ton,

this is very useful thanks so much. Could I just ask you to clarify the difference between passive joint moments and passive moments? Sorry to be a pain!

[Ton van den Bogert]

Sorry for using ambiguous terminology. I should have said "if you are not interested in passive joint moments". If you would be interested in those, you will have to measure them (with subject relaxed) and subtract the gravity effect.

If you're not interested in the passive joint moments, you can take advantage of the fact that the passive trial measures the sum of passive joint moment and the gravity effect. Since both need to be subtracted from the active trial, you can just subtract the passive dynamometer data from the active dynamometer data. Then you don't need a model for the gravity effect.

Ton