The robustness of wearable UHF-band planar inverted-F antennas (PIFAs), with respect to body-antenna separation and human tissue dispersion, is addressed through numerical investigations. The main goal is gaining physical insights into the relationship between the grounded antenna performance and the distribution of the electric and magnetic energy densities in the antenna near-field region close to the ground plane border. A criterion for choosing a proper shape of the antenna ground plane is suggested, which can improve the antenna robustness with respect to the random variations of the body-antenna coupling scenario, but with a minimal impact on the antenna size.