Negative Mass after RRA

[Hans-Jochen Edenhart]

Hello. I was just beginning to think that i understood waht i am doing and why...
BUT... the intermediate example "The Strength of Simulation: Estimating Leg Muscle Forces in Stance and Swing, Part II: Leg Muscle Force Estimation in Stance" is not doing what i want it to do.
One of the exerises is (PArt B) to work with the RRA Tool.
In the summary after using the tool, he wants me to change the masses to negative masses. I hasve repeated the simulation really very often. always with the same results.
could anybody tell me what i am doing wrong or where i could search for a solution.
thanks alot
hajo