Grip strength?

I have a question. I am starting a German Body Composition routine but I seem to have a problem. Whenever I do high reps, or high numbers of sets that involve gripping a dumbell or bar tightly (especially dumbbell presses) the muscle on the outside part of my hand tightens up and I can barely make a fist. My forearm also is tight but it is in the hand that high find the biggest problem.

Sometimes its gets so bad that I can’t finish my workout because I can’t hold the dumbbell any more. Anyone know why this is and how I can go about fixing the problem? I think my grip strength is kind of weak but am not sure how to strengthen it.

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