Being physically fit, bottom line, is always helpful to piano playing. That’s why I frequent the gym every day .But I’ve found specific activities that more directly improve my endurance, and increase forearm and upper arm strength. (specifically my tube stretch routines that I demonstrate on video)
   The Gravitron is also one of my favorite machines, along with the arm pull-up housed over at the Y. (I stay away from hand weights because of the strain it puts on my wrists.)

PIANO PUSH-UPS demonstrated at a lesson with an adult student:

Go to 1:47 on the track

http://youtu.be/OSM-ANeX1Xk

 Yeol Eum Son, world class pianist does the jump rope, among other flexibility regimens as previously illustrated.

   And here she's applying these at a Tschaikovsky Piano Competition rehearsal: