Wireless body area networks (BANs) have attracted enormous attention due to the promising applications in healthcare systems. Energy saving is one of the major challenges in wireless BANs, because the sensors operating on or inside a human body are energy limited. As the applying cooperative communications offers energy saving, it is necessary to utilize the sensor devices jointly in BANs to form cooperative communication. In this paper, the energy consumption models of cooperative transmission strategies are built over in- and on-body wireless communication links for direct and relay transmission scenarios. In relay cooperative strategy, the implantable and wearable devices work together by using a cooperative multiple input multiple output (MIMO) technique. In direct cooperative strategy, two wearable devices operate as the cooperative MIMO. In these ways, the energy savings of both direct and relay transmissions are achieved during the data transmission in BANs. Moreover, the closed-form expression of end-to-end average bit error ratio (BER) is derived toward minimizing the required transmission power for relay transmission scenario. In the results, it is demonstrated that the significant energy savings of the proposed cooperative transmission strategies can be obtained compared with the existing approaches under the same conditions.