Many people with neck and shoulder discomfort have a rounded upper back and are holding her shoulders and the improper position at their workstation. This can lead to muscle tightness in the upper back and neck, and chronic posture changes, leading to the hunchback posture.

 The 90 degree rule means that your forearm and upper arm form a 90° angle at the elbow joint. You’ll notice when you hold the angle at 90° the shoulders do not roll or extend forward.

Test this out for yourself.  While sitting there, extend your arms outward, and you’ll feel the tension on your upper back increase as your arms extend out.  Now retract them back towards you, and as they get closer to your body, it’s less and less strain on the upper back and shoulders.  This is where having your arms at 90 degrees saves your back.   

When the elbows are tucked back and the 90° angle is held it becomes much more difficult to round the shoulders, as the arms are not pulling the upper back forward and down. By having the 90° angle it can be a simple mental cue for you to make sure you’re not putting any excessive strain on the neck or upper back.