Two-Arm Dumbbell Preacher Curl
Prepare the appropriate weight taking account of your fitness level and training type. Grab the dumbbells using an underhand grip. Put your hands on the arm rest and let them lean against it from your shoulders. Your elbows are at shoulder width. Sit with your feet shoulder-width apart. To get into the starting position, hold the dumbbells with your arms outstretched on the rest and firm up your wrists.
From the starting position lift the dumbbells upwards up to your shoulders while flexing the elbows and contracting your biceps. Fix your elbows in a single position on the rest. Subsequently return to the starting position the same way. Exhale while moving the dumbbells upwards and inhale while returning to the starting position (down).
Avoid dorsal wrist flexing. Make sure your posture is correct, do not bend backwards and do not deepen lumbodorsal swayback. Avoid uncontrolled motion downwards. Slowly lower the dumbbells into extension and inhale.
Start with lighter weight to master the technique well.
This is a variation of EZ-bar preacher curls. Using two dumbbells makes the exercise somewhat less isolated/limited because of the need to balance the arm.
Preacher curls are one of the best examples of isolated exercise. As such they are not the best mass-gainers. The advantage of preacher curls is that the position of your body is forcing you to do the exercise precisely, without cheating and without moving your elbows.
The added value of preacher curls for the beginner is questionable. Advanced bodybuilders can do them for a better definition of their biceps muscles.
When lifting the dumbbells you may slightly turn the palm inside (so that they will be closer to facing each other). This will decrease the danger of wrist injury.