This happens to me a lot when I ride my bike ( I practice bike-trials ) because I pull on the handle-bar a lot when I'm on the rear wheel, after a couple of hours I can't open my hands properly and I need to rest. After pull-downs at the gym it happens to me as well although not as severe as after riding my bike ( I ride daily ).
video for reference on what my sport is: http://www.youtube.com/watch?v=S5BGbEJyFD0
video for reference on what my sport is: http://www.youtube.com/watch?v=S5BGbEJyFD0