A formulation of reflex control is presented which has been developed and used in the control of a prototype leg of the adaptive suspension vehicle to implement reflex actions. In particular, the concept of reflex control has been demonstrated experimentally in high-speed contact and foot slippage. A constraint analysis of the leg-environment interaction is found to be especially useful in deriving the invitation and completion conditions. To reduce the total processing required, only simple Boolean conditions are generally checked on the sensor variables. Also, because of the often critical nature of such action, reflex control is executed at the lowest level of control without high-level intervention to achieve maximum response speed; the high-level motion planner is informed of such action so that appropriate steps can be taken when reflex control is relinquished.>