Wireless Body Sensor Networks (WBSNs) have recently gained a lot of research interests due to emerging new applications in health-care, sports and entertainment. As the design of effective network architectures is a key issue to achieve energy-efficient and low-latency communications between the on-body sensors, the 1-hop star and the 2-hop extended star architectures have recently been promoted by the IEEE 802.15.6 task group (TG6) as ideal candidates for WBSNs. In this paper, we experimentally investigate the topology dynamics and the performance of routing strategies in WBSNs using time-variant channel measurements at $2.45GHz$. We aim at better understanding the impact of human body shadowing on the performance of network and routing architectures, in terms of delivery ratio, latency and energy consumption. We finally provide guidelines and recommendations for the design and implementation of effective and practical WBSNs architectures.