Arranging equipment on a desk A computer allows us to perform complicated tasks and is an indispensable device in the office. While working with a computer we usually use a mouse and keyboard. Calculations show that the average time we spend working with a computer mouse is 30 – 80 % of all the time we spend working with the computer. Long hours of using this device lead to maintaining an unnatural position of the upper limb, as well as overloading and static muscular activity. Therefore, it is very important to arrange a suitable position for the mouse in relation to the user and keyboard. There are several options to choose from and each has a different effect on the way the muscles and joints of the upper limb work. he most common combinations of the position of the mouse in relation to the keyboard are: · mouse on the right-hand side of the keyboard, · mouse situated centrally in relation to the user, · mouse above the keyboard, · mouse on the right-hand side of a keyboard that does not contain a numerical keypad. standard position / high position / central position / keyboard without numerical keypad on the side
Figure.1 Possible combinations of mouse and keyboard position in relation to the user. The most common position of the mouse among users is on the right-hand side of the keyboard; however, it is not the most favourable solution. As far as the biomechanics of the muscles and joints is concerned, the most beneficial option is when the mouse is situated centrally, in front of the user, or the option with the keyboard without numerical keys on the side. Both options cause a shift of the mouse to the left and, at the same time, a shift of the upper limb that operates the device. Such a position ensures the most natural position of the wrist joint and provides the largest area of support for the forearm during work. Sometimes, as a result of faulty construction of the desk, the mouse is situated above the level of the proper desk top. This position leads to an excessive palmar flexion of the wrist and results in overloading of this area. Moreover, overloading also occurs in the arm and forearm, which leads to an intensified action of the upper limb muscles.
The central position of the mouse in relation to the user has a drawback - the arrangement on the desk often limits the user; in this option there is often no room for the keyboard. However, this solution is the most favourable for people who use the mouse for the most of the time at work. Professions associated with using the computer involve various activities. A computer graphic designer usually works with the mouse, whereas a person creating databases mainly uses the keyboard. Working with these devices is associated with different positions of the upper limbs and different types of activity. Using the mouse leads to a position of the wrist and humeral joints which results in a considerable static load. Writing with the keyboard causes a dynamic load in the wrist joints as well as in the muscles of forearms, leading to the increased activity of these muscles. We should also pay attention to the fact that these apparently unimportant elements, because a long-lasting work in unfavourable conditions will always result in numerous ailments. References: 1. Dennerlein J., Johnson P.: Changes in upper extremity biomechanics across different mouse positions in a computer workstation. Ergonomics 49(14). p. 1456-1469. 2006.;
2. Dennerlein J., Johnson P.: Different computer tasks affect the exposure of the upper extremity to biomechanical risk factors. Ergonomics 49(1). p. 45-61. 2006.
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By Grzegorz Orlof from the help service www.aktywnaprzerwa.pl | 
