Weak grip, tennis/golfers elbow are fairly common. Your hand and lower arm muscles need strength and ‘balance of strength. The bones twist, turn and rotate in several directions and modern lifestyle wears out certain angles.
Here in one simple exercise is a strength builder and tendonitis fix if you take time out at least once or twice a week to perform these. You can use any item which has a comfortable handle and is weighted to the opposite end. An adjustable dumbbell with weights take off one end works well but a hammer or frying pan will do. We favour the 4 lb mini sledge and simply adjust grip in or outwards to vary the weight.
The key to this exercise…like all exercise, is to do it! Just a few revolutions once of twice a week is worth the investment. Go slow, make sure you support your entire forearm. Don’t swing the weight or over twist. To activate all the small deep musculature you need to ‘control’ every portion of the motion.
Be well, be strong,
Andrew and Tierney
wefit.ca 
I use to have really sore fore arms during all of curling season
I did this exercise a few times a week and no longer feel any dicomfort