During design for users in a particular age group, reference should be made to age-specific data where available, in order for design to take account of the age-related dimensions (absolute dimensions) and proportions (proportional dimensions) of the human body.
Human body dimensions vary according to the age of the individual. This is self-evident with regard to children compared to adults. An older person however also has different body dimensions and proportions than they themselves had when younger. The increase in the circumferential dimensions associated with the increase in body mass is generally known. Other dimensions also change however, such as the body height or height sitting (trunk length + head height). This may result in changes in proportion due to age. For example, the legs become longer in relative terms: this is because the height sitting decreases owing to decreasing straightness of the back (i.e. curvature of the spine), whilst the absolute length of the legs remains largely constant (see image below). The reasons are to be found in biological changes to the human body for example as a result of attrition, and reductions in the water content of tissue or in muscle strength.
