I believe you are talking about trills. In any case, for highly repetitive motions, avoiding any tension is key. One warning right away: do not practice these types of motions for extended periods. A few seconds every now and then is good to start out with. Also, do not practice until your arm hurts, as suggested before. You do not need strength at all. You need coordination as prometheus already alluded to. Also, do not play as fast as possible. Here, I would say, starting slow is the better approach (it often isn't). Once you can do a trill evenly and tension-free at slow speed, go faster.
So, first play trills slowly, focussing on the movements involved, getting rid of all tension in fingers, hand, and forearm. Vary the fingers you are using. There is no rule that says which fingers to use. Find your own preferred fingering. Use forearm rotation to minimize finger movements. Using too much finger movements is what wears you out and causes fatigue and tension. Never try to play trills with fingers 3 and 4 or fingers 4 and 5 in a way that imagines complete independence of these fingers. It's anatomically impossible, so better not try to do it, unless you want to risk injury.
Hope that helps.