This paper introduces a slope-based measure of the stability of a pose of an underconstrained cable robot. In order to define this measure, the set of twists that the end-effector can instantaneously undergo is found. The measure is then derived using an energy-based approach to generalize the definition of slope for mixed-dimensional twists. In order to more easily understand and calculate the measure an intermediate space is introduced. An example is worked out for a sample manipulator and the stability measure is used to define a measure of overall manipulator stability.