An Opportunistic Relay Protocol With Dynamic Scheduling in Wireless Body Area Sensor Network Due to varying on-body channel conditions, transmission between a sensor node and a body-worn coordinator is vulnerable to poor channel conditions caused by body shadowing. One possible solution to this is the use of relays where direct transmission to the hub is not possible. The two-hop relay mechanism proposed in IEEE 802.15.6 standard can be divided into channel assessment, relaying node election, and data relaying processes. However, as these three processes are initiated at different time intervals, simulations suggest that channel conditions actually vary between processes, which leads to data relaying failure. In order to reduce the possibility of data relaying failure, a relay mechanism with predefined relaying nodes are introduced and evaluated against the relay mechanism proposed in IEEE 802.15.6 standard. A predefined relaying node will be active during the data relaying process even if it is not elected. Simulations show that the proposed relay mechanism is able to achieve 50% reduction in data relaying failure rate, which in turn improves the packet delivery rate (PDR). The proposed relay mechanism is evaluated in a superframe structure. Simulation shows that with the presence of the predefined relaying node, the network lifetime is extended by 8%. To further improve the PDR, direct transmission in the relaying process is supported, and a dynamic scheduling algorithm is proposed to optimize slot allocation in the superframe for all nodes. The proposed relay protocol achieves 21% improvements in network lifetime and 14% improvements in PDR with decreasing transmission powers from -10 to -15 dBm.