Supination is rotation of the forearm and hand so that the palm faces forward or upward. When your shoulder is at a neutral position with the arm at your side, elbow at 90 degrees, palm supinated, the normal range of motion, according to the American Academy of Orthopaedic Surgeons is 81 degrees plus or minus 4 degrees. With the same form and the hand  facing downward (pronated), the normal range of motion of this movement is 75 degrees plus or minus 5.3 degrees.

Though these are accepted ranges of motion of a normal population there are those that have limits and do not fit these ranges. There are also men and women whose degrees of motion may differ from right to left hand. It is common in athletics to have wrist rotation affected by injuries or variable capabilities of other joints. When training pulling movements utilizing swivel handles addresses the functional wrist capabilities of each individual, which keeps constant stress on the targeted upper back muscles during exercise.

Pull up swivel handlesPull up swivel handles
Swivel Handles on the Pendulum Lat Combo Pull