Define Who the Backrest Must Fit
Start with the intended users rather than an assumed average body shape. Headrest-free office chairs without adjustable back height have to be assessed at the backrest position provided. An assigned desk allows a detailed trial with its regular user, while shared seating requires observations from several colleagues. Include differences in torso proportions and working habits where relevant. Ask each person to sit naturally against the backrest and describe the contact they experience. Record individual results, including any need to move away from the backrest, so that a positive response from one person does not conceal another person's fitting concern.
Read Shape and Position Together
Backrest height alone does not describe how a chair meets the body. Examine its shape, width and relationship with the seat, using confirmed dimensions where available and a trial to understand their practical effect. Because vertical repositioning is unavailable, check the contact achieved without asking the person to adapt their posture simply to match the backrest. Office chairs should also be evaluated while the user handles normal desk tasks. A still photograph or a brief seated impression cannot show every relevant interaction between the person, the backrest and the workstation they will use during their ordinary working routine.

