We have developed adaptive coordinated medium access control (AC-MAC), a contention-based medium access control protocol for wireless sensor networks. To handle the load variations in some real-time sensor applications, ACMAC introduces the adaptive duty cycle scheme within the framework of sensor-MAC (S-MAC). The novelty of our protocol is that it improves latency and throughput under a wide range of traffic loads while remaining as energy-efficient as S-MAC. We illustrate such optimized trade-offs of AC-MAC via extensive simulations performed over wireless sensor networks. Our simulation results show that AC-MAC is as energy-efficient as S-MAC while its latency and throughput are always trying to follow the classic IEEE 802.11 MAC (no duty cycle), which outperform the S-MAC (fixed duty cycle), specially under the heavy load.