Which muscle is primary in shoulder abduction after initiation by supraspinatus?

Which muscle is primary in shoulder abduction after initiation by supraspinatus?

• A. Middle deltoid is primary after initiation by supraspinatus.
• B. Supraspinatus
• C. Anterior deltoid
• D. Infraspinatus

Answer: (A)

The main action here is that abduction of the arm is driven by different muscles at different phases. Supraspinatus starts the movement, providing the initial lift (roughly the first 0–15 degrees) and stabilizing the humeral head as the arm begins to rise. After that brief initiation, the middle fibers of the deltoid take over as the primary mover, powering the bulk of the abduction through the mid-range of motion. The anterior deltoid mainly aids flexion and internal rotation, while the infraspinatus primarilyExternal rotates and stabilizes the shoulder rather than driving abduction. So the middle deltoid is the best choice for being the primary abductor after supraspinatus has started the motion.

The main action here is that abduction of the arm is driven by different muscles at different phases. Supraspinatus starts the movement, providing the initial lift (roughly the first 0–15 degrees) and stabilizing the humeral head as the arm begins to rise. After that brief initiation, the middle fibers of the deltoid take over as the primary mover, powering the bulk of the abduction through the mid-range of motion. The anterior deltoid mainly aids flexion and internal rotation, while the infraspinatus primarilyExternal rotates and stabilizes the shoulder rather than driving abduction. So the middle deltoid is the best choice for being the primary abductor after supraspinatus has started the motion.