Effects of inclination of 3 types of mouse and of their positioning on the work surface on the biomechanical stresses on the upper limb, on performance, and on the perception of the subjects

Outline of reasons and objectives
The computer is in common use at work. In 2010, in France, 52% of workers used a computer at work and 23% of them remained in front of a screen for 20 hours or more per week. If the computer is in common use, then so is the mouse, an essential input peripheral for most tasks. Various studies have highlighted a link between using the mouse and the appearance of pain, or indeed musculoskeletal disorders (MSDs) in the neck and upper limb. For example, Atkinson et al. (2004) reported that 45% of mouse users complained of musculoskeletal pain and of discomfort in the hand and wrist. According to Standard ISO 9241-410, the neutral position or the most relaxed position of the forearm corresponds to the hand in pronation at 30° relative to the vertical (60° relative to the horizontal). The conventional mouse requires full pronation (with the palm of the hand facing towards the work surface). In order to reduce the pronation of the forearm, compared with the pronation required with the conventional mouse, vertical mice having inclinations of in the range 60° to 70° relative to the horizontal and mice inclined at in the range 30° to 40° relative to the horizontal are now on the market. These new mice are the subject of frequent queries and requests for information from OSH specialists who are seeking solutions for employees suffering from pain or MSDs. In addition, the mouse is usually placed next to the keyboard. Certain researchers recommend placing the conventional mouse in front of the operator if the keyboard is little used. Vertical and inclined mice have been the subjects of few studies so far, and no such studies concern their location on the work surface.
The objective of this study is to compare the muscular and postural stresses, the performance and the perception during use of 3 computer mice (a conventional mouse, a vertical mouse, and an inclined mouse) at 3 different locations on the work surface (next to the keyboard (i.e. on one side of it), in front of the keyboard, and in a free position without a keyboard).

Approach
Eighteen right-handers participated in this laboratory study. Each of them performed 7 tasks (pointing, clicking with the index finger, clicking with the middle finger, double clicking with the index finger, clicking with the index finger and then the middle finger, dragging a target, and taking hold of the mouse again after typing on the keyboard) under all 9 conditions (3 mice X 3 locations). The activity of 9 muscles of the upper limbs was recorded as was the articular amplitudes of the index finger, of the wrist, and of the shoulder of the right upper limb. The performance was assessed through the mean time for performing each of the tasks and through the minimum number of clicks required for the tasks requiring clicking. The perception of the people was assessed by means of self-assessment scales; the questions concerned the difficulty of performing the tasks, and the comfort and ease of use of each of three mice at each of the three locations.

Main results
It appears from this study that:
The inclined mouse offers a good compromise between muscular and postural stresses, performance and perception of the people relative to the other two mice.
Placing the mouse in front of the keyboard reduces the stresses compared with when it is placed on one side of the keyboard. The performance and comfort are improved when the mouse is placed freely relative to the other two locations. Unfortunately such free placement requires the keyboard to be absent, which occurs rarely in everyday practice.
The action of taking hold of the mouse again after typing on the keyboard is the most demanding of the 7 tasks. It is therefore to be avoided whenever possible during the work activity.

Dissemination and added value
These results have made it possible to update the INRS documents relating to computer screen work, the contents of the 3 INRS training courses, and the courses for higher education run in two universities. They are being disseminated in a peer-reviewed international journal, in a journal for OSH specialists, and in an open-archive technical scientific note.