Inertia reference frame

[Nirav Maniar]

Hi all,

I am interested in calculating the angular momentum of a particular segment manually, which requires one to multiply the angular velocity vector by the inertia tensor. From what I have read, the inertia values for any given body within the .osim file are expressed about the body's centre of mass.

I am seeking clarification that the inertial values within the .osim file for any given body are represented in the body's local reference frame? If so, would I be correct in assuming that these inertia tensor values therefore will not change depending the orientation of the segment (for the sake of any momentum calculations), since they are expressed in the local coordinate system about the rigid segment's own centre of mass?

EXAMPLE:

Assuming that:

1) The pelvis inertial properties given in the .osim file were:
inertia_xx = 0.1028
inertia_yy = 0.0871
inertia_zz = 0.0579
inertia_xy = 0
inertia_xz = 0
inertia_yz = 0

2) The angular velocity (in the local reference frame about its own centre of mass, obtained from bodykinematics analysis) is 100 radians/sec in all three axes at time point 1 (T1) and 200 radians/sec in all three axes at time point 2 (T2).

Would the angular momentum (about the x, y and z axis) simply be:
T1 = 10.28, 8.71 and 5.79 kg.m^2/s
T2 = 20.56, 17.42 and 11.58 kg.m^2/s

In other words, would the angular momentum at any given time point simply be the product of the inertia tensor (constant) and the angular velocity (time-varying), with ONLY the angular velocity vector changing with respect to time?

Cheers,
Nirav