Recently I have been trying to use the hand grip to train my palm to strengthen my hand. I find it works! I can actually hold the chords better and able to hold bar chord for longer period of time.
Bear in mind there is technique to train your gripping endurance. It is not just training hard using the hand grip alone, you must train together with guitar play too.
Training using hand grip alone may turn out making your hand less flexible, it just make your grip stronger; which is not enough.
How to -?
The proper way to do is, after playing guitar till your hand quite tired, and then continue with hand grip training. Or vice versa.
In conclusion, this way is actually training your palm’s muscles get used to the pattern of chord and have more strength to perform or sustain it, instead of just making your palm stronger alone.