Patanjali says the asana must be alert and relaxed, explain.

When we practice there are two qualities an asana should have.
In sanskrit , sthira sukham asanam, we must not come into class
with a mind that we will conquer like a gym class. Instead we must find
 a steady balance, be awake in our movements but not force them. 
Sukha means comfortable and light we can bring this into our practise
by remaining joyful and soft.