Energy management is a perennial issue in the design of wireless sensor networks that require long-term sustainability. Radio transmissions being the main energy consuming tasks, adaptive transmission power and relaying protocols represent promising solutions to increase network lifetime. In this work, a novel MAC protocol leveraging emerging wake up receivers is proposed for multihop wireless sensor networks. It uses energy-based back-off mechanism to allow nodes to choose the best transmission schemes (relay or not) and corresponding transmission powers. The proposed approach has been implemented on a real hardware platform. Performance evaluation combining analytical models and microbenchmarks demonstrates a potential lifetime gain up to 1.7 when two relays are used.