Ten-year secular changes in muscular fitness in English children.

Low muscle strength is independently associated with a poorer metabolic profile during adolescence (1) and with disease and all-cause mortality in adulthood (2). Muscular fitness, which includes maximal isometric strength, explosive strength and muscular endurance is also positively associated with skeletal health (1). Tomkinson (3) reported steady increases in measures of explosive lower body strength (vertical and long jump performance) in developed countries until the mid 1980s, followed by steady declines. There were declines in muscle strength in Swedish adolescents between 1956 and 1981 and Russian adolescents between the 1960s and 1980s (4). Later reports show increases in some components of muscular fitness (5,6), but the most recent studies in Canadian and Spanish children show declines in either handgrip strength (HG) (7) or both HG strength and standing long jump (SLJ) (8). Contemporary data showing trends in muscular fitness in English children are lacking. The aim of this study was to examine 10-year secular changes in measures of muscular fitness in English children. In 1998, after gaining parental consent, 309 children (51% boys) aged 10–10.9 years were recruited from a random sample of six schools in Chelmsford, Essex, UK. In 2008, 315 children (48% boys) aged 10–10.9 years from five schools matched for size and socioeconomic status were recruited. The University of Essex ethics committee approved the study. Duplicate measures of mass and stature were taken to the nearest 0.1 kg and 0.1 cm, respectively (Seca, Hamburg, Germany). Participants wore sports clothing (shorts and T-shirt) but no shoes. Unless specified, the following tests were performed according to standardized methods, with the protocols reported elsewhere (6,9,10). SLJ was used to measure lower body explosive strength (11). HG strength was taken as a measure of upper body maximal isometric strength (11). Two maximal isometric contractions of the dominant hand were performed using a portable dynamometer (Takei Corp Ltd., Tokyo, Japan), and the highest value achieved (kg) was recorded. Bent-arm hang was used to measure upper body strength endurance (11). A score of zero was recorded if the child could not hold the required position, using a pronated grip, for at least 1 sec. Otherwise, the time spent with their chin above the bar, was recorded in seconds. Trunk and hipflexor muscular endurance was measured by the maximum number of sit-ups performed in 30 sec (11), with knees flexed to 90 and feet held. HG, SLJ and sit-ups were performed in random order, while the bent-arm hang was undertaken last. Two-way (gender by year of testing) analysis of covariance (ANCOVA) was used to determine main effects and potential year-by-gender interactions while controlling for mass, stature and body mass index (BMI), using SPSS Version 16.0 for Windows (SPSS Inc.: an IBM company, Chicago, IL, USA). School was included as a random factor to correct for clustering. Bent-arm hang data were positively skewed, so a log transformation was performed on bent-arm (s) +1. The addition of 1 to each score was necessary as the bent-arm data included zero values. A chi-square test was used to compare the number of zero scores in the bent-arm hang between year of testing. Abbreviations BMI, body mass index; CRF, cardiorespiratory fitness; FFM, fat free mass; HG, handgrip strength; SLJ, standing long jump. Acta Pædiatrica ISSN 0803–5253