Several different factors have been proposed to contribute to the development of chronic low back pain (LBP). Specifically, researchers and clinicians have proposed that impairments of low back posture and movement, particularly during functional activities, are important to address during intervention. However, posture and movement are typically only measured in the clinical or laboratory setting. Observation of posture and movement in such settings is limited because people with LBP may not perform naturally when they are being observed, and observation in a single session does not provide information about the duration of postures or frequency of movements across the day. In this paper, we present a wireless body sensor cluster formed by up to seven sensors for monitoring spine movements both in absolute and relative coordinate systems. The Spinal Motion Monitor (SMM) system measures the magnitude and frequency of spine movements, and duration of spine postures in 3D, without impeding natural movement. The SMM node developed in this study is 3.0cm in diameter, and contains a 9-axis motion processor that records the raw inertial information of different spine regions. The system offers a standard Bluetooth Low Energy (BLE) protocol to communicate with mobile or fixed hosts. The SMM system has been validated in the laboratory by measuring lumbar spinal postures on a mechanical spine testing platform across a known range of angles. It is also validated by deploying on human subjects and comparing the data with a commercial camera-based motion capture (mocap) system.