This is the latest question I've answered on the Quora platform, where people ask the general public about anything and everything:
How do I prevent my shoulders from rounding while working with a keyboard and mouse if the recommendation is to use a keyboard and mouse while elbows are at about 90 degrees?
Your arms extend far enough that there's really no physical need to round your shoulders forward to use a keyboard and mouse.
To make this clear, try this experiment. Lie on your back, your shoulder blades will naturally be supported by the floor, and raise your arms towards the ceiling letting the weight of your arms push your shoulder blades into the floor. You can now move your arms about without your shoulders coming off the floor and get a sense of how much space you can interact with without having to round your shoulders forward. You'll soon realise you have more than enough peripersonal space (anywhere you can reach) to get your hands to a keyboard whilst allowing the shoulders to stay back.
The "trick" to keeping your shoulders back when upright isn't to hold them back, which is unsustainable, but to allow your chest and shoulders across the front to release into width. It's the chest muscles narrowing inwards across the front that cause the shoulders to round forward. The weight of the arms naturally allows the shoulder blades to hang down the back, if you keep your front released into width. Skeletally your arms are part of your back, the shoulder blades are obviously round the back, and your arms connect to the shoulder blades. Let your arms originate from your back, and stay back!
The reason the shoulders round forward is twofold. It's partly because we physically follow our mental intention, getting drawn forward by overly concentrating on the keyboard and mouse (and screen, which is why you poke your head forward at it). It requires a stronger sense of yourself in activity, and not allowing yourself to be bossed about by inanimate objects. Easier said than done, especially as it's probably an habitual response, so play the long game of changing your mental relationship with your computer set up. The other reason is that we tend to narrow across the front as a stress response, or when feeling emotionally vulnerable. It can be disconcerting to be open across the front whist in these emotional states, but your inner private life is yours, and by remaining physically open you can maintain that privacy rather than broadcast it.