When your hands are fixed on a barbell while bench pressing there is a greater amount of shearing force on the shoulder joint as opposed to dumbbell bench pressing.
Another advantage to dumbbells while bench pressing is the additional range of motion. With dumbbells, you can bring your hands together increasing the pectoral contraction and range of motion.
Why Dumbbell Bench Press Is Better Than Barbell Bench Press for hypertrophy:
1- Less shearing force on the shoulder joint; and
2- Greater pectoral contraction and range of motion
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