According to Dr. Jim Potvin, developer of Arm Force Field method:
We collected data with the trunk upright. However, we wanted to be able to apply it to all trunk postures. So, we estimated how much gravity would have contributed to the manual arm strengths and subtracted that effect. So, in essence, we are estimating the strengths in zero gravity. Then, depending on the posture and force direction, we (1) estimate the manual arm strength, (2) estimate any potential contribution of gravity and add or subtract that from the 0G MAS to get the MAS in a gravitational field.
For example, if you have your hand out in front of the shoulder with an upright trunk, gravity will increase the force your muscles can cause to push down but will subtract from how much your hand can push up. That is all accounted for.