I read a Frederic Delavier’s book « Strength Training Anatomy » and there is good stuff.
Standing or sitting with a dumbbell in a hand and your arm in a vertical position :
Inhale and flex your arm to bring the dumbbell behind your neck.
Back to the start position and exhale at the end of the movement.
The vertical position of the arm stretches the long head of the triceps to have a good contraction during work.
Note : It’s important to squeeze your abs to avoid over-arching your back.
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