In IEEE 802.15.4 wireless personal area networks (WPANs), reliable data transmission is highly valued for health and medical information. The channel access in IEEE 802.15.4 employs Carrier Sense Multiple Access with Collision Avoidance (CSMA). In IEEE 802.15.4 WPANs, health and medical information are expected to transmit reliably to a single central controller (i.e., coordinator). However, due to CSMA/CA, the important data that informs the patient's emergency situation of the coordinator could not be transmitted. It is because CSMA/CA impairs network efficiency. Therefore, a more effective frame transmission scheme should be proposed to provide reliable frame transmission and solve contention problems for Wireless Body Area Networks (WBANs). In this paper, based on IEEE 802.15.4, we propose a Backoff Counter Reservation (BCR) scheme. We propose that the next backoff value is added to the MAC payload of each transmitted data frame in order to the coordinator could know their future backoff duration information. If the data frame does not arrive at the predicted transmission slot, a guaranteed time slot (GTS) is allocated to the sensor in the next superframe. In this scheme, a sensor can reserve a backoff counter for next packet transmission. An analytic model and a simulation model are developed to evaluate the performance of IEEE 802.15.4 and BCR. The simulation results demonstrate that the proposed scheme significantly improves throughput, delay and collision probability. Our proposed scheme significantly outperforms IEEE 802.15.4 backoff scheme.