Rear Delt Fly

Move · Resistance Training · Movement, Typed Supps

The rear shoulder is a small muscle sitting next to much larger ones, so the moment the weight gets heavy the back takes over.

Stage by stage

Set-up (Torso angle sets the line): A near-horizontal torso with arms hanging vertically keeps the load perpendicular to the humerus through the arc.
During the set (Transverse abduction): The posterior deltoid performs horizontal abduction; its small cross-section caps useful load well below what the back can move.
As load rises (Retractors substitute): Rhomboids, middle traps and lats finish the rep through scapular retraction; a brief end-range hold exposes the substitution.
Months later (Modest, proportional growth): Growth is real but numerically small because the head is small, showing up as shoulder balance rather than bulk.

Muscles trained

Primary: rear shoulders
Secondary: lower back, traps, upper back

Intensity

3.5 METs (Resistance (weight) training, multiple exercises, 8-15 repetitions at varied resistance).

Time to effect

Weeks: The rear of your shoulder starts to fatigue and burn distinctly from the front and side, a sign it's finally getting isolated attention.
Months: Modest but real growth in the rear delts, along with a more balanced-looking shoulder from the back, since this is a small muscle that's slow to grow compared with bigger pulling lifts.

Common myth

"Rear delt flies alone will fix rounded, forward shoulders." In reality: Rounded shoulders usually come from hours of daily posture and overall training imbalance, not just weak rear delts. Strengthening them helps, but it's one piece of a bigger picture that includes thoracic mobility and how much pulling volume balances your pushing volume.