seated isometric forearm supination against manual resistance, holding contraction before releasing and repeating.
Sit resting the forearm on the thigh, bend the elbow and rotate the forearm such that thumb is pionting towards the ceiling. Place the other hand on the forearm just above the wrist. Try further rotating the forerm away from the body against the resistance of the other hand. Hold before releasing the reistance. repeat.
Nothing here is a prescription. Your physiotherapist decides which of these are yours, and when.
