This paper studies the coordinated motion control of a hydraulic arm driven by single-rod hydraulic actuators (a scaled down version of an industrial backhoe/excavator arm). Compared to conventional robot manipulators driven by electrical motors, hydraulic arms have a richer nonlinear dynamics and strong couplings among various joints (or hydraulic cylinders). This paper presents a physical model based adaptive robust controller (ARC) to explicitly take into account the strong coupling among various hydraulic cylinders (or joints). In addition, an observer is employed to avoid the need of acceleration feedback for ARC backstepping design. Theoretically, the resulting controller is able to take into account not only the effect of parametric uncertainties coming from the payload and various hydraulic parameters but also the effect of uncertain nonlinearities. Furthermore, the proposed ARC controller guarantees a prescribed output tracking transient performance and final tracking accuracy while achieving asymptotic output tracking in the presence of parametric uncertainties. Simulation and experimental results are presented to illustrate the proposed control algorithm.