Double bassist Rob Oxoby wrote in a great comment outlining an alternative exercise to the one described by doublebassblog.org contributor Phillip Serna.  Rob writes:

I was able to do this for about 45 seconds. However, it seems to me that this is the wrong kind of exercise to be building flexibility as it opens the risk for excessive strain.

Here are the exercises that I do. They focus on stretching the rest in building flexibility: The first is to hold your arms out to the side creating a straight line from one wrist to the other (i.e., arms held out to be in line with the shoulders). Move the hands so they face vertically (perpendicular to the wrists) and hold them there for three breaths. Then counterbalanced the stretch by making the fingers face downwards (again perpendicular to the wrists) and hold this for another three breaths. This not only flexes the wrist but also opens up muscles throughout the arm (in particular the armpit) which are important for rotation and movement of the whole arm (something I encourage when playing pizzicato or slapping).

The second stretch was suggested to me by yoga instructor. Sit on your knees with your hands on the floor, hands turned outward so that the fingers are pointing towards the knees. Your hands should be about 1 foot in front of your knees. Pushing your palms down and keeping your back straight, pull your hips back. This pulls back your whole torso and opens and stretches the wrists. Do this for a count of five breaths and then repeat the posture with the wrists stacked, palms up, fingers facing the knees. This reverses the stretch and builds/stretches muscles in the top of the forearm. This stretch has worked great for me and, when space and decorum permit, I try to do this before the start of each gig.

Read Phillip’s original arm exercise post here.