Help with core and legs please?

Question

from 13 to 17 now i have been doing arms and upper body

never lower or core

i now know that most of the strength coems from the core and power builds from ground up

i have a swiss ball and have been doing stomach exercises and on weekdns i play tennis and run and do bleachers to get my leg muscle up

any other advice? other than weight lifting ex. squats

and how come when i do leg ups lying on the ground my quads get tired or the tendon or w/e connecting the quad to the hips/abs

Answer 1

swimming
jogging
crunches
dips