In fact, centuries before Instagram and activewear, "doing yoga" didn't involve the body at all.
At its core, yoga is a practice for tapping into your mind and poses are just one way of doing that.
Yoga's philosophy is based on eight limbs; only one of them, asana, is about the poses. The rest: yama, niyama, pranayama, pratyahara, dharana, dhyana and samadhi, are best described as moral and ethical codes to live by.