Use of knee height as a surrogate measure of height in older South Africans

Abstract

The study aimed to determine whether knee height would be a more appropriate surrogate measurement than armspan in determining height and body mass index (BMI) in a group of South African older people ( 60 years). A random sample of adults (older than 18 years) who attended selected clinics or who lived in selected old-age homes in the Western Cape volunteered to participate in the study. Subjects were divided into a study group of older people ( 60 years of age, N = 1 233) and a comparative group of younger adults (18 - 59 years, N = 1 038). Armspan, knee height, standing height and weight were measured using standardised techniques. The standing height measurements were significantly different between the two groups (p = 0.0001), with a mean for adults of 1.61 m (standard deviation (SD) 0.09) compared with that of older peole (1.57 m (SD 0.09)). Mean standing height decreased with age. Knee-height measurements were not significantly different between the two groups, but when used to calculate height, the adults were significantly taller (p = 0.0001), with a mean height of 1.67 m (SD 0.06) compared with that of the older people (1.59 m (SD 0.08)). Mean armspan also decreased with age, and derived standing height was significantly different (p = 0.0001) between the two groups, with adults being taller (1.67 m (SD 0.11)) than the older people (1.63 m (SD 0.11)). In this study group, the knee-height measurements were more closely related to the standing height than the armspan. The BMI calculated from armspan-derived height tended to classify the older people towards underweight. Knee-height measurement would appear to be a more accurate and appropriate method to determine height in older people in South Africa.