stretching the back

[k11rh]

Last week I had worked out my back as normal with shrugs, bent over rows, and pull ups. I then had a baseball game a couple of days later.
Long story short, I began to have back spasms during the game and had to come out. I know the spasms probably happend as a result of not stretching my back properly. Does anyone know of any good warm-up back stretching exercises?

Thanks
K11rh