Results There was consistent evidence for a positive association between MF and physical activity .
For both MF components , there was support for objective ly measured physical activity , particularly for activity of vigorous intensity .
Sports participation was also consistently linked with both MF components , whereas the association with active transport was inconsistent .
For both MF components , associations with sedentary behaviors were inconsistent irrespective of measurement method , and the association between MF and sleep was considered uncertain .
Conclusions The available evidence supports a link between MF and physical activity , particularly for vigorous intensity activity and organized sports participation .
Background Muscular fitness ( MF ) is considered a powerful marker of health , but the extent to which common health behaviors are associated with MF during childhood and adolescence is currently unclear .
Objective We conducted a systematic review of studies investigating associations between MF ( i.e. , strength/power , local muscular endurance ) and physical activity , sedentary behaviors , and sleep in children and adolescents .
Secular and longitudinal physical activity changes in population‐based samples of children and adolescents
The aims of this study were to investigate whether  physical activity ( PA ) and sedentary time ( ST )  in 9- and 15-year-olds differed between 2005 - 2006 and 2011 - 2012 ( secular change ) , and to investigate changes in PA and ST from age 9 to 15 ( longitudinal change ) . In 2005 - 2006 , we invited nationally representative sample s of Norwegian 9- ( n=1470 ) and 15-year-olds ( n=1348 ) to participate . In 2011 - 2012 , we invited a new nationally representative sample of 9-year-olds ( n=1945 ) , whereas 15-year-olds ( n=1759 ) were invited to participate either based on previous participation in 2005 - 2006 or from a r and om sample of schools . We assessed PA and ST objective ly using accelerometers . In 2011 - 2012 , both 9- and 15-year-olds spent more time sedentary ( ≥35.7 min/d , P<.001 ) and less time in light PA ( ≥35.2 min/d , P<.001 ) compared to their peers in 2005 - 2006 . Nine-year-old girls also spent less time in moderate-to-vigorous PA ( MVPA ) ( 4.2 min/d , P=.041 ) . In both age groups , the proportion accumulating an average of 60 min/d of MVPA did not differ between the two cohorts . From age 9 to 15 , girls and boys decreased their time spent in LPA ( ≥106.7 min/d , P<.001 ) and in MVPA ( ≥20.8 min/d , P<.001 ) . During the same period , ST increased by a mean of > 2 h/d ( P<.011 ) . We observed an adverse secular change in PA from 2005 - 2006 to 2011 - 2012 among 9- and 15-year-olds , and a large decline in PA in the participants followed longitudinally from age 9 to 15 years
Results There was consistent evidence for a positive association between MF and physical activity .
For both MF components , there was support for objective ly measured physical activity , particularly for activity of vigorous intensity .
Sports participation was also consistently linked with both MF components , whereas the association with active transport was inconsistent .
For both MF components , associations with sedentary behaviors were inconsistent irrespective of measurement method , and the association between MF and sleep was considered uncertain .
Conclusions The available evidence supports a link between MF and physical activity , particularly for vigorous intensity activity and organized sports participation .
Background Muscular fitness ( MF ) is considered a powerful marker of health , but the extent to which common health behaviors are associated with MF during childhood and adolescence is currently unclear .
Objective We conducted a systematic review of studies investigating associations between MF ( i.e. , strength/power , local muscular endurance ) and physical activity , sedentary behaviors , and sleep in children and adolescents .