'RSI' is repetitive strain injury. If you use a keyboard and mouse, repeating the same hand and arm movements over long periods can lead to aches and strains in the hands, wrists, arms, shoulders and back.
Occupational health specialists use other terms to describe these conditions, such as ‘upper limb disorders’ and ‘musculoskeletal disorders’. Soft tissues connecting muscles to bone (particularly the tendons), muscles themselves, and the associated nerve systems are all affected.