In order to reach the main keys on a standard keyboard the hands must be rotated by bringing the thumbs to the centre so that both hands are level over the keys. This rotation, called pronation, is achieved largely by the pronator teres muscles in the proximal region of the forearm.
Pronator teres muscle.
There are two heads of the pronator teres, the humeral head and the carpal head. The main nerve to the hand and forearm, the median nerve, passes between these two heads, and beneath the bulk of the muscle mass. The ulnar artery, an important blood supply to the forearm and hand, passes through these muscles. Sustained pronation, besides straining the muscle with static load, puts the median nerve under pressure and may reduce blood flow to the forearm and hand. In addition the radio-ulnar articulation, a rotating joint, is placed under stress be being held in its extreme position for long periods.