Ran into some issues with my skin mesh while trying to paint muscle weights. Not exactly sure what the issue is but it almost seems as though the muscle is trying to paint itself to the original bind pose.
Since the original model was not in perfect T-pose, I bound the skin to the rig and re-positioned the joints to an accurate T-pose so that it would be recognizable by Motion Builder and HumanIK. It looks like, while my solution was time-efficient, this may not have been a good workaround in the long run. I'll be sure to use Zbrush to re-pose the mesh into T-pose in the future.
This is just a theory though, still have yet to confirm the problem. At first I thought it might be a sticky bind distance issue, but the errors did not show any problems.
If anyone has any insight on this please let me know. Still learning the process for muscle. Thanks!