One arm, overhand grip, forearms on fire.
Keep your wrist firm and straight, no bending back as you lift.
Flipping to an overhand grip changes who runs the curl. The biceps still flexes the elbow, but the palm-down position pulls the forearm muscles into heavy duty, which is why your lower arm burns before anything else. One dumbbell at a time keeps the wrist honest and the reps strict. It earns its place for building forearm size and grip strength that standard curls quietly neglect.
The overhand grip puts the biceps at a mechanical disadvantage and shifts work to smaller forearm muscles. Everyone drops significant weight here, so lighten the dumbbell and chase clean reps.
The weight is overpowering your wrist extensors. Keep your knuckles pointing forward, hold the wrist flat and firm, and drop the load until it stays that way for a full set.
Both hit the forearms, but differently. The neutral hammer grip is stronger and lets you load the brachioradialis heavier, while the full palm-down reverse curl works the wrist extensors harder. Rotate them.
