One that I believe has been under-rated is the role of posture, particularly in the neck, upper back, and shoulders. This area provides a stable platform from which the arms work. In computer work that platform is held fairly still for most working hours. If one's posture is erect, low muscle forces are required. But if people slump forward, or need to crane their necks over a badly set-up workstation, their muscles have to work hard and generate high forces to keep their heads from falling.
Muscles require adequate blood flow to maintain performance.
In static working postures, however, muscles just contract and hold and hold and hold. This reduces blood pumping and flow. Good blood flow is critical for the operating muscle to bring in nutrients and remove waste products. If blood supply and waste removal are reduced, acids build up around the muscle fibres and lead to the pain you feel when someone kindly presses a thumb into your shoulder muscles and gives them a rub. Also, there are postural muscles and movement muscles.