The subject of physical activity (PA) is of particular importance to those involved in the study and promotion of positive health in youth (1). In children, PA is considered a prerequisite for healthy growth and a positive predictor of psychosocial adjustment and well-being (2). To optimize the health benefits associated with regular involvement in PA, children should be encouraged to be (and remain) active from an early age. However, evidence from the past 40 years suggests that children have become increasingly less active in many domains, including leisure-time activities, physical education, and active transport (3).

To better understand and promote PA in children, researchers and practitioners have studied the influence of various psycho-social variables and/or the characteristics associated with the physical environment. Although there is little doubt that factors such as motivation, lifestyle, family and peer support, and the built environment contribute to variation in PA in youth, it has been argued that PA has a biological basis (4-5), and that a true understanding of PA resides within the interactions of various biological, psychosocial, and physical factors (5). Physical activity is, after all, a biological process tdhat exists within a cultural setting in which various meanings and customs are ascribed to it (6).

Biological maturation has been identified as a potential contributor to involvement in PA, and may be particularly pertinent to those studying PA in youth. Maturation refers to progress towards the adult state and ca be viewed in terms of timing and tempo. There is good reason to believe that maturation may contribute to variance in PA in youth. First, the decline in PA with age is considered one of the most robust findings in PA epidemiology, with individuals become less active as they progress towards the mature state (i.e., adulthood) (5). Second, the physical and functional characteristics associated with early maturation in males and late maturation in females are considered more conducive to successful involvement in most forms of physical activity (6).

Although there is compelling evidence to suggest that biological maturation contributes to sex differences in PA among same age cohorts (7-10), evidence that variation in maturity status (i.e., timing of maturity) accounts for individual differences in PA within sex is limited. A recent review of literature pertaining to maturity associated variance in PA (11) noted that relations between biological maturity status and PA, within sex, were unclear, with only a limited number of having been published (N=9). Further, the available studies were largely restricted to females and had produced equivocal results. Whereas a few studies had documented an inverse relation between maturity status and PA in females, others had observed a positive, or no, relation between the constructs. Relations between biological maturity and PA are equally unclear in males. The inconsistent nature of these results has been attributed to a number of methodological and analytical limitations including a reliance on cross sectional data, modest sample sizes, variation in the measurement of PA and maturation status, and a general failure to consider, or control for maturity associated changes in body composition (11).

With the aforementioned limitations in mind, the purpose of the proposed study is to examine the independent and interactive effects of biological maturity status and body composition (i.e., relative fat mass) upon changes in PA and fitness in British youth. The Khamis-Roche (KR) method for estimating percentage of predicted adult stature attained at measurement will be used as an estimate of biological maturity status. The KR method has been used as an estimate of biological maturity status in US youth 4-17 years of age and has been validated against an established marker of biological maturity status (12). The median error bound (median absolute deviation) between actual and predicted mature height is 2.2 cm in males and 1.7 cm in females (13). The KR method has also been employed in studies of activity level (10), perceptions of physical and social competence in youth soccer players (14), and injury risk in youth football (15-16). Whole Body Dual X-Ray Absorptiometry (DEXA) will be used to determine body composition and daily PA will be measured objectively by the Actigraph accelerometer. Methods of analysis will included descriptive statistics, Pearson product moment correlations, and hierarchical multiple linear regressions. Linear growth modelling, using procedures described by Singer (17), will be used to examine the independent and interactive effects of biological maturity status and body composition upon changes in physical activity and fitness from the age of 11 to 13 years. Separate analyses will be conducted for males and females.