Wireless Body Area Networks (WBANs) are one of the key technologies that support the development of digital health care, which has attracted increasing attention in recent years. Compared with general Wireless Sensor Networks (WSNs), WBANs have more stringent requirements on reliability and energy efficiency. Though WBANs are applied within limited transmission range, the on-body channel condition can be very challenging because of blocking or absorbing of signal. In this paper, we are looking into the design of Medium Access Control (MAC) protocols and propose an opportunistic scheduling scheme by applying heuristic scheduling and dynamic superframe length adjustment to improve the system performance. The simulations have been supplemented to show the advantages of the proposed solutions in outage rate performance, compared with existing solutions.