Pushing your keyboard and mouse deep onto the desk creates a hidden reach that pulls your shoulder blades forward all day. Even a few extra inches of forward reach forces the muscles around your shoulder joint and upper back to stay under constant low-grade tension just to hold your arm out in space. Because you are focused on typing or clicking, that quiet muscular effort goes completely unnoticed until stiffness sets in.
This habit is especially common among people who rest their forearms heavily on the desk to type, or who place a paper notebook, tablet, or coffee mug between themselves and their keyboard. Over time, the pectoral muscles across the front of your chest tighten while the rhomboids and lower trapezius muscles between your shoulder blades become overstretched and fatigued, leading to a dull, burning ache along your upper spine.
Reaching outward to the side for a mouse creates an additional layer of strain. When your mouse sits far outside shoulder width, your arm rotates outward for hours, irritating the rotator cuff and tightening the side of your neck on your dominant hand.
The fix is immediate and costs nothing: bring your keyboard and mouse close enough that your upper arms hang naturally beside your torso. Your elbows should stay near your ribs rather than drifting forward toward the monitor, and your wrists should float lightly or rest gently without stretching.
If you use a wide full-size keyboard with a number pad you rarely touch, consider switching to a compact tenkeyless layout. A narrower keyboard lets your mouse sit much closer to your body's centerline, reducing outward shoulder rotation and relieving strain on your dominant arm. Keep frequently used items like your water bottle or notebook to the side rather than in front of your keyboard, and verify every few hours that your elbows are still resting comfortably near your ribs.