A system includes a support, a pendular element, an elastic restraint, an image sensor, and a processing system. The elastic restraint couples the pendular element to the support and applies to the pendular element a restraining force that opposes movement of the pendular element in at least a first direction. The image sensor captures images of the pendular element. The processing system determines one or more movement measures describing movement of the support from comparisons of ones of the images captured by the image sensor. In another aspect, a pendular element is elastically coupled to a support. A restraining force that opposes movement of the pendular element in at least a first direction is applied to the pendular element. Images of the pendular element are captured. One or more movement measures describing movement of the support are determined from comparisons of ones of the captured images.