To armrest or not to armrest? That is the question. Here are the principles. The positions are critical. Too high and your shoulders will be raised and therefore tensed. You will also have too much pressure on the forearms and the especially vulnerable ulnar nerve (funny “bone”) at the elbow. Insufficiently cushioned armrests are particularly evil on the latter point.
Too low and you will be drawn into a leaning posture, collapsing the thoracic outlet in the neck and shoulders, putting undue pressure on the nerves and arteries feeding the arms, wrists, hands, etc. This also closes the rib cage around the lungs, reducing your capacity to draw precious oxygen that the blood needs to carry to your muscles for all of that continuous contracting (the same logic for the monitor being at eye level).
Too far apart and your shoulders will overwork (imperceptibly) because of the extension of the elbows, and your wrists will be deviated (bent on the horizontal plane toward the pinkies) when you type.
Too close and they may interfere with relaxed typing and with being able to place your hands in your lap (which you should do as often as possible, palms facing, to let the whole system relax).
Thin armrests actually require extra muscular effort to keep your arms on them. I find with my clients that this is largely a matter of personal preference. With or without, know how to use or not use them. And as always, make sure everything else is in place. Keyboard low enough (or chair high enough, with footrest only if needed), monitor in FRONT of you with the top roughly at eye level, don’t reach as a habit, lighten up on keying and clicking pressure, stand up often, breathe, drink water, sleep, etc., etc., etc.