Welcome to this tutorial on Flexibility Training guys!
As our hands are our tools of the trade as guitar players, keeping them in shape, keeping them strong and flexible should be a priority for us. This tutorial is geared towards that end, and will introduce you to some examples of flexibility, strength and coordination exercises I find useful to keep in shape.
One thing to keep in mind is the relationship between flexibility and strength. They are pretty much two sides of the same coin as far as guitar playing is concerned. If you are flexible, but don't have strength and stamina, your playing will suffer from lack of control and endurance. If you on the other hand, have lots of strength, but no flexibility, you will limit what you can accomplish with your playing.
Think of an Olympic gymnast for example. Those guys are incredibly strong, yet ultra-flexible. Same thing should apply to our hands. If you do not have strength in those stretched positions, not only will you lack control, but you will also run a greater risk of injuring yourself, due to your tendons and ligaments not being supported by strong muscles.
For this purpose, you should try playing all your regular legato exercises in stretched positions, as that will dramatically improve your strength, as well as flexibility. And remember, flexibility without strength and control is pretty useless!
ALSO, PLEASE MAKE SURE TO WARM UP PROPERLY BEFORE EMBARKING ON THESE EXERCISES!!!! You do not want to start stretching when cold. 5-10 minutes of legato exercises should be fine, or just opt to end your regular technique or playing sessions with these exercises, do not put them first thing on your schedule.
Another option is to use non-guitar warm-up exercises. These kind of gymanstics for your hands is very benefitial, provided you watch out for over-exerting yourself. And while you're at it, be sure to check out Sean Conklin's Health and Safety for Guitarists tutorial.