This is the exercise my physio gave me (I was told in the early stages to ‘help’ lift the weight with the other hand, and to lower it slowly on its own, until I had built up muscle strength). This, and forearm tendon stretching sorted my tennis elbow out, I had several months off work with it, but I found if I did this exercise first thing, it made a huge difference. It never bothers me now, but if I get a twinge some stretching does the trick.