Horizontal pull includes many variations of exercises, the movement consists of adducting the scapula towards midline (pulling shoulder blades together). The exercise targets the rhomboids, middle and lower trapezius and the latissimus dorsi and smaller stabilizer muscles, these muscles lie on the posterior rib cage and help keep the shoulder healthy. With our changing lifestyles we have more technology that brings us into a forward posture (computers, phones etc.), head and neck goes forward as well as the upper thoracic spine that creates a semi hunched over position. With this position it leads to neck strain, anterior neck tightness, can lead to headaches, shoulder issues and lower back issues. The levator can upwardly rotate the scapulas out of position creating a winging look. The importance of the muscles of the posterior thorax are to maintain proper position of the body in an upright position and when they do not perform their duties other muscles have to take over to maintain normal day life.
You can and probably have read somewhere that performing horizontal rows will cure the forward head posture and hunchback posture, it's not wrong but it's not entirely correct. Yes performing multiple variations of horizontal row will help influence posture but we need to go a step further to make sure that we are properly positioning posture. What I mean by that is you can perform those rowing exercises but if the thorax underneath is not in proper position then the posture will not be influenced. Respecting our asymmetrical human body the right oriented rib cage has a huge influence on posture so the first step to horizontal pulls is to make sure the rib cage is in a state of neutrality, then perform the exercise and that exercise is now a major postural corrector.
Muscles contract and relax and have tone and tension and they act on bones they attach to, so training appropriate muscles will influence posture of the bones. Changing position and posture of the bones has a chain effect influencing the rib cage/hips and thus influencing the whole system to change. The muscles that attach and act on the scapula that adduct it towards midline will be strengthened, adding more tension and will influence the shoulder blade. When changing shoulder blade position we are able to change the position and posture of the ribs underneath, if we engage muscles of the right back (lower trap, rhomboids) as we perform the row, the muscles produce right rotation through the trunk and visa versa for the left.
Horizontal training trains part of the gait cycle, moving the arm posteriorly with same side trunk rotation. This is one way to perform a horizontal row, another variation is to perform a row limiting trunk rotation so you are creating more scapular movement on the thorax, this is an aspect of the movement. We are still moving the arm posteriorly but we are mainly moving the scapula on the ribs to train muscles to act without movement underneath. Training muscles in multiple aspects enhance performance and teach the brain that the muscles can contract in various positions and postures and have the same influence.
Variations of rows to use in training programs are (a) dumbbell single arm row, (b) bent over row,(c) inverted rows, band or cable face pull, cable rows either single or double arm, to name a few. In your training program add multiple variations of rows in, to hit different angels and challenge the muscles in different biomechanical positions for optimal recruitment and benefit.