Methods and devices are disclosed to reduce tissue trauma when a physician retracts a patient's tissues for surgery. In one part, methods and devices are disclosed for controlling retraction force and pace. In another part, methods and devices are disclosed for oscillating force when opening. In another part, methods and devices are disclosed for detecting trauma during retraction. In another part, methods and devices are disclosed for balancing forces on multiple retraction elements. In another part, methods and devices are disclosed for reducing forces in multiple dimensions. In another part, methods and devices are disclosed for engaging ribs. In another part, methods and devices are disclosed to compensate for deformation of a retractor under load. In another part, methods and devices are disclosed that combine these methods and devices. In another part, methods and devices are disclosed for controlling pressure inside inflatable devices used for deforming biological tissues.