with a leg on each side of
Merriam-Webster Online Dictionary
astride (adverb)1.
with one leg on each side astride a horse - she rode astride, not sidesaddle
2.
with the legs stretched wide apart - standing astride
1.
on or above and with one leg on each side of - astride a horse
2.
placed or lying on both sides of - towns lying astride a river
3.
extending over or across - spanning bridging