Title : Changes in cardiorespiratory fitness in 9 - 10 . 9 yr old children : SportsLinx 1998 - 2010 Running title : Changes in children ’ s fitness , 1998 - 2010

Purpose: 1. Assess changes in cardiorespiratory fitness (CRF) from 1998-2010, controlling for decimal age and body mass index (BMI). 2. Repeat the analysis in cohorts from 20052010, controlling for maturation, deprivation and BMI. Methods: 27,942 (n = 14,247 boys) 9-10.9yr old participants from one UK city were included in this serial cross-sectional study from 1998-1999 to 2009-2010. A deprivation score (IMD) was assigned to each participant on the basis of home postcode. Stature, sitting stature and body mass were estimated. BMI and somatic maturity were calculated. Performance on the 20m multi-stage shuttle runs test (20mSRT) was used to estimate of CRF (total shuttles). One-way ANCOVAs were completed to assess temporal trends in CRF, separately by sex. Model one assessed changes from 1998-2010 and included decimal age and body mass index as covariates. Model two assessed changes from 2005-2010 and included maturity, IMD and