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Dear all,

Could anyone point me to how the COM positions are calculated by the body kinematics routine of Opensim? Does it require only the masses of individual body segments and the joint kinematics? i.e., to compute the COM positions (through body kinematics analysis), it is sufficient to use only the inverse kinematics results and the model being used as inputs to the analyse tool. Is that correct?

Thank you very much in advance.

Dear Suparna,

The COM of each body is defined within the bodies definition in OpenSim, in the local body coordinate frame. This COM location is a defined attribute, not something which is calculated within OpenSim.

As such, given any simulation which has joint angles, as you would get from inverse kinematics for example, as the position of each body is known, and location of the COM is also known.

Hope that helps
Bryce

Bryce is correct that the COM is defined by the "mass_center" property for each body, and joint angles (from inverse kinematics) is all that is needed. The only other outputs of BodyKinematics to mention are the 3 "center_of_mass_{X,Y,Z}" outputs. These are calculated throughout the motion based on all the bodies, rather than defined in a body.