The Seated DB External Rotation trains two very important muscles in the shoulder girdle: the infraspinatus, and the teres minor. These two muscles are the only muscles in the entire upper body that perform an external rotation of the humerus – the upper arm bone.
These muscles are small, and are outnumbered by literally every other upper body muscle that acts on the shoulder joint, as the rest of the muscles act to rotate the humerus internally. This resultant, built-in imbalance accounts for several shoulder problems. Performing external rotations is a requirement to keep the shoulder girdle healthy.
The external rotators also act as stabilizers for every upper body movement, and reinforce posture. They contribute, along with trapezius 3 and rhomboid strength, to postural improvement in people who have an overemphasized kyphosis – the traditional rounded back, slope shouldered posture that is so prevalent today.
Sit on a flat bench, and put one foot on the bench as shown. Place the same side elbow on the knee, between the natural divot made by the vastus medialis – teardrop quadriceps – and the kneecap.
As you inhale, lower the DB in an arc toward the floor, pivoting the elbow on the divot of the knee. Keep the chest up and the torso in neutral alignment during the movement. The forearm should travel in a line that is perpendicular to the ground at the top of the range of motion, above, and at the bottom of the range, below.
Once you reach the bottom of the movement, exhale and reverse the movement to return to the starting position.
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