The Arnold press is a twisting variation of the basic dumbbell shoulder press. The starting position of the dumbbells during this press increases the range of motion of the exercise, but it doesn't change the target muscles. Like the basic shoulder press, the Arnold press targets the front and side shoulder muscles.
Hold the dumbbells in front of your chest with your palms facing you and your elbows tucked in close to your body. Exhale and press the dumbbells overhead, twisting your forearms so your palms face forward at the top of the movement. Pause for one count before lowering the dumbbells and twisting them back to the starting position.
The shoulder muscle has three heads, referred to collectively as the deltoids. The anterior deltoid sits on the front of the shoulder and is mainly responsible for shoulder flexion, lifting the upper arm to the front of the body. The lateral deltoid sits on the side of your shoulder and is mainly responsible for shoulder abduction, lifting your upper arm to the side. The posterior deltoid sits on the back of the shoulder and is mainly responsible for shoulder extension, pulling the upper arm down toward the side.
The anterior deltoid is the primary mover during the Arnold press. As you press the weight overhead, your upper arms move up and forward, which involves shoulder flexion and shoulder abduction. The anterior deltoid is involved in both shoulder movements, although it is mainly responsible for shoulder flexion.
Many other muscles are involved in the Arnold press as synergists, or secondary movers -- these muscles assist the primary movers. As the dumbbells move higher overhead and the forearms begin to rotate, the upper arms shift out to the sides, emphasizing shoulder abduction, which involves the lateral deltoids. The triceps, which are involved in elbow extension, also assist during the Arnold press. The lower and middle portion of the trapezius muscles activate to stabilize the scapula, or shoulder blades, during the exercise.
Based in Austin, Texas, Jolie Johnson has been in the fitness industry for over 12 years and has been writing fitness-related articles since 2008 for various websites. She received her Bachelor of Arts in English and philosophy from the University of Illinois.