4) Contour and shape should fit the user?s hand. Mouse arch should conform to the natural arch of the hand in a neutral position.Typical shapes are straight or kidney shaped.
5) Operating posture should encourage neutral positioning of the wrist, fingers and hand. Power position/vertical position is optimal and desired. Avoid over pronation if possible.
6) Button size should fit the dimensions of the user?s hand. The number and location of buttons should correspond to the predominant direction of flexion of the digits.
7) The mouse should be compatible with the user?s dominant handedness.
Here is a computer mouse that is designed, studied and approved by experts: "Approved ergonomic mouse of University of California at Berkeley: The Evoluent VerticalMouse 2 was evaluated in 2007 by a team at the health services clinic in UC Berkeley, including medical doctors, physical therapists, ergonomists, and administrators. It was found to be comfortable, easily adapted to, and promote a neutral wrist and forearm posture. It is the only vertical type mouse approved for staff use within UC Berkeley and for sale in the campus store." See ErgoRehab Blog for details.