Dumbbells or machine
Opening the arms backward in a bent-over or chest-supported position isolates the rear deltoids.
| Muscle | Role | Action |
|---|---|---|
| Posterior deltoid | Primary | Extends, horizontally abducts and laterally rotates the arm |
| Lateral deltoid | Secondary | Abducts the arm (raises it out to the side) |
| Middle trapezius | Secondary | Retracts the scapula (pulls the shoulder blades together) |
| Rhomboids | Secondary | Retract, elevate and downwardly rotate the scapula |
| Infraspinatus | Secondary | Laterally rotates the arm and stabilises the shoulder joint |
| Teres minor | Secondary | Laterally rotates the arm and stabilises the shoulder joint |
Primary muscles are the prime movers; secondary muscles assist or stabilise. The balance shifts with grip, stance, range of motion and technique.