Be careful! There are NO MUSCLES in the fingers. All movement is through tendons attached to muscles in the forearm. YES stretching before and during playing is important!!!!!! It limbers up the tendons and their fulcrum points. Soreness in the back of the hand is from those tendons which OPEN the grip fighting against the ones which ultimately get "trigger finger" or stenosing tenosynovitis. There is tons of literature on that condition.PaulS said:The cramps and soreness are like tj said from muscle fatigue.
I will eventually require injections or surgery to combat trigger finger, caused, I find out now, by not being careful to warm up before playing, and by playing mandolin, which scrunches up my hand unnaturally.
GET INTO GOOD HABITS NOW before it gets out of control!!