Proper scapular motion during arm elevation is achieved
by force couples provided by the upper trapezius (UT),
middle trapezius (MT), lower trapezius (LT), and serratus
anterior (SA). UT and SA act in scapular
upward rotation, UT in scapular elevation, and SA in
scapular protraction. The MT and LT resist the SA during
scapular protraction, and the LT resists the UT during
scapular elevation; as a result, the MT and LT maintain the
position of the scapula and build an axis of scapular upward
rotation. In addition, LT activity increases at 90 of arm
elevation and is important for scapular posterior tilt.