The prone row exercise is effective for strengthening the muscles that pull your shoulder blades together, preventing Dowager's hump -- more commonly referred to as a hunched back. This exercise is easily done on a flat bench with dumbbells, especially when a seated row machine is unavailable. Prone rows are done lying flat on your abdomen. If you have large breasts or a large tummy, do one-arm dumbbell rows, because this exercise involves the same muscles.
The actions produced by contracting muscles may change depending on the position of your body and the number of joints involved. For instance, while the primary function of your triceps muscle is to extend your elbow, it is also slightly activated when you are in the position to do prone rows. The long head of the triceps muscle crosses the shoulder joint such that when the long head contracts, it assists in pulling your arm toward your body.
In a prone row, drawing the weight toward you until your upper arm is along the side of your rib cage is called shoulder extension. This aspect is performed by muscles that terminate on the upper end of your arm bone, crossing your shoulder joint. These include the latissimus dorsi, posterior head of your deltoid muscle, teres major and the long head of your triceps.
Squeezing your shoulder blades together is called scapular retraction. This movement is performed primarily by your rhomboid major, rhomboid minor and the middle fibers of your trapezius muscle. The rhomboid muscles extend in a diagonal manner from the last two neck or cervical spinal bones and the first four upper-back or thoracic spinal bones to the inner border of each shoulder blade. Once your upper arm is alongside your rib cage, these muscles become the more active muscles compared to the lats, deltoid, teres major and the long head of the triceps.
Four muscles press your arm bone or humerus into the shoulder joint cavity as your arm moves, and they are collectively referred to as the rotator cuff muscles. These muscles play an assisting role in moving your humerus. The three muscles engaged during a prone row include the teres minor, infraspinatus and supraspinatus muscles.