if you hit someone while sprinting he will take the same knockback as if you hit him without sprinting. Behavior is the same regardless you hit a mob or a player.

Steps to reproduce.
1. Open any world
2. Hit a mob while sprinting
3. Hit a mob without sprinting
4. You will notice that knockback is the same in both situations

What is supposed to happen:
When you hit a mob while sprinting it's supposed to take more knockback than if you hit him while walking or standing still.