A method of monitoring or assessing the motion of the limbs of a body when performing repetitive cyclic activity uses inertial measurement units secured to each of the limbs to be monitored. The operation of all of the IMUs is synchronized by way of a pulse from a computer. On performing the repetitive cyclic activity such as walking, running, trotting or galloping, the outputs of the IMUs are assembled and analyzed in realtime, to determine the relative phase of the limb movements, from which an assessment of the gait may be made.