Plank

This is my by far the most beloved exercise. I don't know why but I do it after every tennis game, match or open session. I pay attention to my flat posture during the entire exercise. Now, I can hold like this upto 2 minutes, for which they say after 1,5 min doesn't have any more benefits and again for which I disagree. Nevertheless I usually leave it at 1,4.

This exercise is for mainly rectus abdominis, internal/external oblique, erector spinae muscles. By the way the photo is not mine but close... ;) That was for the curios.