Scapular retraction is movement of the scapula (shoulder blade) backwards and inwards towards the spine. It is mainly produced by the rhomboids (M/m) and medial / lower trapezius in this isolated movement:
Keeping these muscles toned is important in order to support functionally healthy placement of the scapula in static postures and to have good shoulder girdle strength. This strength is balanced with those muscles that pull the scapula forward, which can be overly tight and weak with extended slouching.
A simple isolation exercise to strengthen these muscles of scapular retraction involves squeezing the muscles backwards against resistance. Such resistance can be accomplished by using simple arm weights, or by using an added weight or resistance device, such as free weights (or even a soup can), strengthening bands, pulleys, etc.
Any of these variations of this exercise will be more effective in isolating these muscles if you bend your torso forward 90′, which prevents primary loading of supporting muscles.
Here’s a video which shows such an exercise (credit: WellCor):
If you have ANY neck problems or discomfort, support the head with a towel and relax it (do NOT lift up head or tuck the neck). Remember to breathe in a relaxed manner during the exercise: breathe IN through the nose during the contraction, then EXHALE through the mouth when you relax the contraction.
If you experience any discomfort during or following the exercise, alert your therapist or doctor immediately. Good luck!