SETUP: Grab a pair of relatively light dumbbells and curl them up to a 90 degree angle, with your elbows held tight to your sides.
REP: Keep your hands in a neutral-grip (palms facing in) and imagine you’re curling the weights towards your elbows. This will cause your wrists to flex up and down and you’ll feel your forearms light up. It’s the same exercise Sylvester Stallone used to build those momumental Rambo II forearms.
TIP: A good substitute for this exercise is a Barbell Roll Up (listed below), given the similar movement pattern.