DESIGN OF WORK SURFACES Work-space envelope is the three-dimensional space within which an individual works. Within this envelope, specific design decisions need to be made about various features of the workspace. Decisions include the location and design of benches, desks, tables, and control panels.

The following is a discussion of: The horizontal work-surface area, and Work-surface height (seated and standing).

The horizontal Work-Surface Area Workspace design should be performed with the shortest arm in mind and according to postural considerations and task requirements (Corlett and Clark, 1995). Ayoub, (1973), suggested an optimal work area approximately 25 X 25 cm located at 10 cm from the table edge with the body mid-line crossing at the middle of the area (see figure1).

FIGURE 1

The previous suggestion falls into the optimal area suggested by Squires, (1956), which takes into account the dynamic interaction of the movement of the forearm as the elbow is also moving (see figure 13-11 in your text book).

2. Work-Surface Height (seated and standing) Backaches?, neck aches?, shoulder pains? Are they a result of the height of the work-surface? There might be a difference between the work-surface height and the working height.

Work-surface height is simply the height of the upper surface of the table, bench, desk, counter, etc. measured from the floor. Working height depends on what the operator is working on. Arm posture and thigh clearance (if seated) should be considered in choosing work-surface height.

The arms should be relaxed at about 90° to provide comfort and help maintaining straight wrists. Three general principles are offered in your text book regarding seated work surfaces. Corlett and Clark, (1995), suggested providing adjustable chairs to achieve appropriate elbow height in relation to work-surface height if the work-surface height is fixed.

Sanders and McCormick, (1987), suggested that work-surface height should be at a level that permits the arms to hang in a reasonably relaxed position from the shoulder, with the forearm usually being near horizontal or sloping down slightly. The work-surface height should be at a level that does not require excessive flexing of the spine.

Pheasant, (2001), stated that the major determinant of a standing worker’s posture is the height at which manual activities are performed. Too high working levels will cause the shoulders and upper limbs to be raised. This will probably lead to fatigue and strain in the muscles of the shoulder region. If the task requires any downward application of force the upper limbs will be in a position of poor mechanical advantage for providing that force. A lower working level could overcome this problem.

If, however, the working level is too low the trunk, neck, and head will be inclined forwards with consequent postural stress for the spine and its muscles. Pheasant, (2001), added that it may be presumed that somewhere between a working level that is too high and one that is too low there may be found a suitable compromise at which neither the shoulders nor the back are subjected to excessive postural stress.

This limits the design in two directions to give a two-tailed constraint. Pheasant, (1990), suggested two options for such a situation. The first option is to provide adjustable workstation which could be set by the user according to his/her optimum dimensions.

The second option is to settle on a single overall compromise value which will maximise the number of users who are accommodated and minimise the inconvenience suffered by the reminder. Most authors consider elbow height and the type of work performed as the most important two critical features of working level.

Pheasant, (2001), recommended: A work level of 50-100 mm above elbow height for delicate manipulative tasks, A work level of 50-100 mm below elbow height for manipulative tasks involving a moderate degree of both force and precision, and A work level 100-250 mm below elbow height for heavy manipulative tasks.

Grandjean, (1988), recommended three different working levels for three types of work: 100-200 mm below elbow height for heavy work, 0-100 mm below elbow height for light work, and 0-100 mm above elbow height for precision work.