Contractions within the agonist muscles cause increasing motion in a specific direction as well as stopping or slowing down the motion in the same direction.
Imagine that you are doing a barbell biceps curl. Your biceps muscles are the agonist helping you move the barbell. Now imagine that you are carrying a bag of groceries from your car to your kitchen. Your biceps are the agonist that help you hold the grocery bag to your chest and lower it gently to the kitchen counter without breaking the bottles inside.
We often think of agonist as the prime movers and we forget the other two roles they play.
