Flat Dumbbell Fly

Move · Resistance Training · Movement, Typed Supps

Gravity pulls straight down, so the chest works hardest at the bottom and almost not at all at the top.

Stage by stage

Set-up (Elbow angle fixed): A small, unchanging elbow bend shortens the lever from hand to shoulder and keeps torque off the elbow joint.
At the bottom (Moment arm peaks): With the arms level the pec is longest and most loaded, which is where the whole stimulus of the movement sits.
Past level (Joint stress outruns stimulus): Below torso level the shoulder is loaded and lengthened with little active support, so stress rises faster than benefit.
Days later (Stretch-related soreness): The lengthened loaded position produces more next-day soreness than pressing, and is where the growth stimulus comes from.

Muscles trained

Primary: chest
Secondary: biceps, rotator cuff, shoulders

Intensity

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

Time to effect

Weeks: The deep stretch at the bottom tends to produce more next-day soreness than pressing work at a comparable effort, even at lighter loads.
Months: That lengthened, stretch-loaded position is increasingly linked in research to a meaningful hypertrophy stimulus, and pec size gains often track with how controlled the bottom position is, not just total volume.

Common myth

Keeping the arms perfectly straight makes it a "purer" isolation exercise. In reality: A locked-out elbow concentrates torque on the elbow joint and the biceps tendon rather than the chest. A slight, fixed bend in the elbow throughout — the standard cue in strength coaching — keeps tension on the pecs and takes leverage stress off the elbow.

In the glossary

Hypertrophy