EMS is a tool that physical therapists use to help people recover from surgery or rehabilitate injuries, Typically, EMS is used in situations where a person has muscular inhibition and weakness, due to pain, swelling, and immobilization, he says. For example, after anterior cruciate ligament (ACL) knee surgery, people tend to have weakened quads. EMS can "involuntarily activate" or wake up the quads, so that people don't totally lose their muscle mass or function.