Obtaining Accelerations used in Joint Reactions Analysis

[Ryan Byrne]

Hi all,

I have done some searching on the Forum, but haven't quite found an answer I am looking for. I was hoping someone could help guide me on how to obtain body accelerations used during Joint Reactions Analysis step (which utilizes results from Static Optimization). I am working in Matlab through the API, OpenSim 3.3.

I am working on a lumbar spine model. When a lumbar joint is changed from 3DOF (no translation) to 6DOF (translations), the accelerations reported by BodyKinematics and PointKinematics become much larger. Obviously I expect some difference, however, the magnitude of differences even after filtering of the kinematic data seem large (e.g. accelerations ranging from 0-20,000 with translations as opposed to 0-500). The differences in joint reaction forces aren't too different between the 2 cases either, which makes me skeptical of these acceleration values. I've read that accelerations from PointKinematics and BodyKinematics are calculated based on F=ma and not based on kinematic derivatives; however I'm unsure of whether these accelerations are the ones being used in Joint Reactions Analysis.

My main goal is to just obtain the accurate accelerations of bodies within a joint which Joint Reactions Analysis utilizes on the right hand side of F = ma. I'm hoping that these values are stored somewhere that I can access? I've read some about using computeStateVariableDerivatives(), however the translational motion at the joint is not a prescribed coordinate - instead they are incorporated using a moving joint center via SpatialTransforms. This makes me think that only the rotational accelerations will be quantified by computing the derivatives of the state variables.

Any help would be appreciated,

Ryan