Why squeezing the desk you work on can give you a lean upper bod

Why squeezing the desk you work on can give you a lean upper bod: "Shoulder flexion:
Shoulder flexions firm and tone the fronts of your shoulders. Perform this exercise either sitting or standing. Try this move from different angles so that you will firm and tone all parts of the fronts of your shoulders.
If you are performing the exercise with your left arm, place your right hand on your left shoulder and feel the muscle flex. Be sure to breathe normally through the entire movement."

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