Changes in body size, shape and composition during adolescence have a great effect on fitness testing scores. Relative muscle mass increases from 42 to 54 per cent of body mass in boys aged five to 17. In girls, it increases from 40 to 45 per cent of body mass between five and 13 and then, in relative terms, declines after age 13, because of an increase in fat accumulation. Coupled with the fact that more mature boys are also able to recruit more motor units than less mature boys, the problems with strength tests during adolescence are readily apparent.

Differences in body shape become noticeable between the sexes during puberty. Boys gain about twice as much in shoulder breadth compared to hip breadth, whereas there is only a small difference in the amount gained in these dimensions in girls. As small differences in shoulder breadth can result in large differences in upper trunk muscle, this partially explains why strength differences between boys and girls, and mature and less mature boys, in the upper body are much greater than in the legs. When greater upper body muscle mass is combined with the greater leverage of longer arms, the generally better performance of more mature boys in rowing and throwing tests becomes predictable.

Young girls have more body fat than boys, but adolescent girls’ body fat increases while boys’ declines. So in tests which require moving body mass (eg shuttle runs) mature girls are penalised for carrying excess fat mass.

As aerobic fitness depends on muscle mass for oxygen consumption and optimum return of blood to the heart, and more mature boys have the added advantage of a marked increase in haemoglobin concentration, the limitations of assessing and interpreting aerobic fitness through shuttle running can be put into perspective.

Until puberty the legs account for 66 per cent of the increase in height.

Growth in leg length stops earlier than growth in trunk length and trunk growth makes a greater contribution to increase in height during adolescence. The sit-and-reach test is a popular test of flexibility, but the changing ratio of trunk lengthleg length questions the interpretation of flexibility scores. Changes in flexibility are further confounded by the relative broadening of girls’ hips, which allows for a greater range in hip and lower limb flexibility. The relative broadening of the hips in mature girls provides greater stability and may account for their generally better performance in balance tests.

Is it surprising that Professor Roy Shephard, a world-leading exercise scientist, said that performance tests may merely be a complicated method of identifying tall or fat pupils?

Professor Neil Armstrong is the director of the Children’s Health and Exercise Research Centre, Exeter University. www.tes.co.ukgetactive