Stand facing a wall, about arm's length away.
Place your forearms flat against the wall with elbows at shoulder height, forming a goalpost position (elbows bent 90 degrees).
Press your forearms firmly into the wall and protract your shoulder blades (push them apart and forward around your ribcage). You should feel your serratus anterior engage.
While maintaining pressure into the wall, slowly slide your forearms upward, extending your arms overhead.
Keep protraction throughout — do not let your shoulder blades pinch together.
Slowly slide back down to the starting position, maintaining wall contact and protraction.
Repeat for 10-15 repetitions.