The Anus has two muscles: The external and internal anal sphincters.
The External Anal Sphincter
This muscle is under your control - it’s the last safety mechanism for continence.
The Internal Anal Sphincter
It’s not under your control.
Its natural state is closed but will open with a gentle stretch.
With mild pressure, it will open; however, it takes time.
If it’s pushed too hard too fast, it can be damaged.
Ways to open these muscles is gentle massage when you are feeling relaxed.
With time, patience and plenty of lube, massage will allow the muscle to relax and allow pain-free penetration. It is important to remember that it does take a little extra time, perhaps a few minutes before the muscle will get to full relaxation.
By taking your time and enjoying the sensations your muscles will gently relax allowing for improved ease with penetration.