A monitoring system has biomechanical sensors, physiological sensors and a controller which receive sensory inputs from the sensors to provide output signals for the output device, and it detects from the sensory inputs risk of a syncopal event The bio-mechanical sensors include sensors arranged to allow the processor to detect a user postures and posture transitions. The processor operates a finite state machine, in which there is a state corresponding to each of a plurality of user physical postures and to each of a plurality of transitions between said postures, and the processor determines a relevant state depending on the sensory inputs. A device output may be muscle stimulation to prevent syncope, and there are stimulation permissions associated with the finite state machine states.