Not only playing the guitar, if a person doing the same action for a long time, such as a long time fixed the posture to use the computer , and hold wrist to move the computer mouse, will cause muscle strain.
Avoid playing guitar for a long time leading to finger muscle and neck strain problems, we have to take rest and do some relaxation physical exercises for the hands, wrist , neck, shoulders and waist movement.
I suggest that you guys may go to run or swim, this is the best exercise, especially swimming, let water relax your whole body, and will not be like running, long-term may cause knee injury.