Wireless sensor networks (WSNs) are developing into a promising solution for many applications, for example in healthcare. In many scenarios, there is some form of node mobility. The medium access control (MAC) mechanisms should support the expected kind of mobility in the network. Mobility is particularly complicating for contention free MAC protocols like TDMA-based protocols, because they dedicate unique slots to every node in a neighborhood. In scenarios such as body-area networking, some clusters of nodes move together, creating further challenges and opportunities. This paper proposes MCMAC (Mobile Cluster MAC), a TDMA-based MAC protocol to support mobile clusters in WSNs. The proposed protocol does not need adaptation time after movement of clusters. Several optimization mechanisms are proposed to decrease power consumption. Simulation results show that the optimizations decrease power consumption of nodes around 70% without increasing latency of data transmission compared to the non-optimized version.