Experts share the best trap exercises to look cut, prevent injuries in and out of the gym, and improve your posture.

Ask someone in the gym to point to their trapezius muscles, and they’ll probably gesture toward the small mounds nestled between their neck and shoulders—but that’s really just the tip of the iceberg.

Start in a standing position holding a dumbbell by each end. Be sure the shoulders are open and the muscles of the back are engaged. Press the dumbbell toward the floor as you perform a squat. In one ...