Isometric elbow extension in inner range - forearm in neutral

Stand with your upper arm by your side and your elbow bent to less than 90 degrees.
Your knuckles should be pointing outwards and your thumb should be on top.
Grasp your wrist from below with your other hand.
Keeping your upper arm by your side try to straighten your elbow whilst resisting the movement with your other hand so that no movement occurs.
Hold the tension, and then relax.