Soft tissue injuries (STI) are when trauma or overuse occurs to muscles, tendons or ligaments. Most soft tissue injuries are the result of a sudden unexpected or uncontrolled movement like stepping awkwardly off a curb and rolling over your ankle.
However, soft tissue damage can also occur from excessive overuse or chronically fatigued structures, especially muscles and tendons. For example, if you were to do a long run when already fatigued (from a previous run or exercise), then it is possible to cause trauma or a strain to key running musculoskeletal structures like your calf muscles or achilles tendons, also see: “How to prevent running injuries”.