For climbers, it's important to train the antagonist muscles to our primary movers, to prevent injury. For the fingers, these are the extensors (as you're always flexing your fingers when gripping holds). How can I best train them?
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The power web is a popular physiotherapy tool. You put your fingers through the holes and then just spread them out. Difficulty varies depending on the strength of the material (color-coded) and finger spacing.
Regarding training extensors to balance flexion, this article suggests that it is wrist extension that is a more important focus rather than finger extension in a non-rehabilitation setting. I don't know of any evidence that suggests that training finger extension reduces the rate of injury in climbers.
A cheap and effective way is to put your hand in a bucket full of rice while making a fist. Once you have done that, open your hand. The deeper you put your hand in the bucket, the harder it will be.
Alternatively, Metolius makes the Grip Saver which is a much more portable method and allows you to work both flexion and extension. This is what I use.
There is also the Gyro Exerciser. I'm not sure how well it works the extensors, but they say it does. I've only briefly used one of these, but I can tell you this, it does feel like it's working everything in your hand and arm.