I served everyday this week, 200-500 balls a day. Then today, I sprained my back while going for a DTL BH slice. It's the first time, aside from ankle irritation this summer, that I've injured myself. (Both times from serving). Now at home, I can't bend down without the left lower back hurting. How can I reach 1 million serves now... Only do 100 serves per day? I'm already 30. Haha. What exercises should I be doing to prevent this in the future? One legged dumbbell pickups?