Wireless body area networks (WBANs) have emerged as a promising technology for health monitoring due to their high utility and important role in improving human health. WBANs consist of a number of small battery-operated biomedical sensor nodes placed on the body or implanted, which are used to monitor and transmit important parameters such as blood pressure, electrocardiogram (ECG), and electroencephalogram (EEG). WBANs have strict requirements on energy efficiency and reliability during data collection and transmission. The most appropriate layer to address these requirements is the MAC layer. Medium access control protocols play an essential role in controlling the operation of radio transceivers and significantly affect the power consumption of the whole network. In this paper, we present a comprehensive survey of the most relevant and recent MAC protocols developed for WBANs. We discuss design requirements of a good MAC protocol for WBANs. We further review the different channel access mechanisms for WBANs. Then, we investigate the existing designed MAC protocols for WBANs with a focus on their features along with their strengths and weaknesses. Finally, we summarize the results of this work and draw conclusions.