The standing asanas build the foundation of the Asthanga series. They are practiced every day. . On a sticky mat it doesn’t matter if the legs pull apart from each other or if they move together. The sticky mat prevents the feet from sliding. On the grass it’s crucial to engage the leg muscles and to take care that the feet rather pull to each other than apart from each other. This gives stability to the asana. That’s the goal to be grounded and to stand stable.
It’s possible to engage the leg muscles without any inner direction. The legs and feet can move slightly together or apart from each other. One can feel a huge difference even though nothing might be seen.
Even when learning side split. The legs move together when they are in position.
The inner body work is important.
Correct muscle work helps to perform an asana savely. To engage the muscles also supports stretching.
Practicing on soft grass, in fresh air is a joy. I avoided sitting asanas. I didn’t like to spoil my clothes.