Flat fingers are something that can seriously hold a string player back. By keeping fingers curved, the left hand moves more efficiently, giving more speed and helping create a strong tone.
Here is a trick I learned from one of my teachers, Dr. Elizabeth Morrow. I hope it helps keep your left hand strong!
Neil Fong Gilfillan teaches cello in Frisco, Texas. With Rachel Samson on viola and violin, they operate Chili Dog Strings, the only music studio in Frisco named after a dog.