In a normal population the distribution of body mass index (BMI) is su
ch that a certain proportion of the population is likely to be at low
values without necessarily being malnourished. However, although they
may have low BMIs without being malnourished, they could certainly be
physiologically and physically disadvantaged. An attempt is made to di
ssect out the probability of work capacity and physical activity being
influenced by changes occurring in the human body with diminishing BM
I. The conclusion reached is therefore that before physical activity i
s affected, the BMI would probably have to be 17 or less, although it
is possible that work capacity might be reduced before this level is r
eached. In any case, work requiring the use of the body mass - such as
carrying loads, digging or shovelling earth or coal, pulling or cycli
ng a rickshaw, stone splitting etc. - would impose a greater stress on
people of low BMI.