The human body is a very lossy dielectric, as it mostly is water. An antenna placed on a human arm will lose efficiency and might be detuned. Moreover, the radiation pattern of on-body antenna will be different from open-space antenna.

When designing wearable antennas, the right approach to simulation would be to 1) create an antenna concept in open space and 2) optimize the antenna's on-body performance. Note, that future measurements need to be conducted on-body as well.