I provide you a quick note.
IEEE 802.15.4 is for wireless personal area networking (WPAN). As also pointed out in Marcus's answer, the IEEE standard for WBAN is 802.15.6 which uses three different PHY technologies (including a narrow-band PHY) with a sophisticated MAC protocol. But notice that the default PHY mode in this standard is UWB.
Due to the short distance, ground reflection has a considerable contribution. However, it depends on the Tx-RX orientation. For instance, if they are on opposite sides of the body, then there would be a rather strong ground reflection. About fading, there are issues that are far more complicated than only diffraction. To summarize, there are refractive, dispersive, and reflective effects caused by factors such as randomness associated with different body parts (tissue structure, fat, water density, etc.), metal parts in cloths, ornaments, metal implants, and many other effects caused by close proximity to the body. Most importantly moving body parts cause fading effects.
There are some channel models that are usually used in literature such as in-body channel model CM-3, or on-body channel model CM-4 which includes four different angles of Tx-Rx orientation. There are also channel models specific to locations such as hospital or work environments. By a quick search you may find Matlab implementation of such models easily. It is worth noting that because of all mentioned practical issues, WBAN channel modeling is a major task and an enormous body of research on this topic exists.