A 3-year longitudinal analysis of changes in fitness, physical activity, fatness and screen time.
OBJECTIVE
To analyse whether changes in physical activity index (PAI), screen time (ST: television, computer) and body mass index (BMI) made a contribution to longitudinal changes in fitness of children and adolescents. Additionally, we analysed the interaction between baseline fitness level and changes in fitness.


METHODS
This is a 3-year longitudinal study of 345 high school students aged 11-19 years. Students performed curl-ups, push-ups and 20-m shuttle run tests from Fitnessgram. PA and ST were evaluated using a standard questionnaire. Standardized scores of fitness tests were summed. Changes over time were calculated as Delta(1) (2007 minus 2006), Delta(2) (2008 minus 2007) and Delta(3) (2008 minus 2006).


RESULTS
Changes in PAI were positively and independently associated with changes in fitness in Delta(1), Delta(2) and Delta(3). Changes in BMI were negatively associated with changes in fitness in Delta(3). Participants highly fit at baseline were those who showed positive changes in PAI over Delta(3), decreased changes in ST and had the lowest increase in BMI over 3 years compared with those low-fit at baseline.


CONCLUSIONS
Changes in BMI were associated with changes in fitness over 3 years. However, changes in PAI were the best predictor for changes in fitness in each year and over the 3 years of evaluation in youth.
Aires
L
L
Research Centre in Physical Activity, Health and Leisure Time, Faculty of Sports, University of Porto, Porto, Portugal. luisa.aires@gmail.com
Andersen
L B
LB
Mendonça
D
D
Martins
C
C
Silva
G
G
Mota
J
J
D000293:Adolescent; D000294:Adolescent Behavior; D015992:Body Mass Index; D002648:Child; D002652:Child Behavior; D003201:Computers / Q000706:statistics & numerical data*; D015444:Exercise; D005260:Female; D006801:Humans; D016014:Linear Models; D008137:Longitudinal Studies; D008297:Male; D009043:Motor Activity; D010809:Physical Fitness; D011795:Surveys and Questionnaires; D013690:Television / Q000706:statistics & numerical data*; D013997:Time Factors; D055815:Young Adult