The researchers studied 469 preschool students enrolled in urban, state-funded programs serving disadvantaged youth. Included were 275 children, mostly African American, from a Midwestern city and 194 children, mostly Hispanic, from a southwestern city.
The children were evaluated using a standardized test of motor skills. They participated in tests of locomotor skills which included running, jumping, hopping, leaping, sliding and galloping. They were also evaluated on object control skills through tests of throwing, catching, kicking, striking, dribbling and rolling.They found 86% of this group were below the 30th percentile in basic skills like running, catching, throwing, and kicking. Imagine kids who don't know how to run for lack of experience at it. What would that look like? Does it mean boys running and throwing like girls? And girls throwing like kids with cerebral palsy? The girls in this study were averaged the 11th percentile in object control, meaning using a ball or a bat.
Assuming this is reliable and valid (and all I've read about it is the link above), it means these kids don't go to parks, or the beach, or probably even outside. It has to be just lack of experience doing stuff. It's not like you need actual instruction in running.
And of course this is probably one factor in the observation that poor people are fatter than rich people. They don't get as much normal exercise as other kids. So it ends up as just another way poor people have it worse than the rest of us.