While interviewing for jobs years ago, I met a therapist who primarily treated knee patients. He was looking for an experienced PTA to assist in this specialty. I ultimately declined the generous offer so I could explore other interests. I had difficulty understanding how an experienced therapist would only treat one body part. The human body interconnects, and yes, after knee surgery the muscles and structures are weakened but so are other parts of the body.
When I was an outpatient PTA, the policy was to treat one body part at a time. So although a patient might have upper-body weakness from a disability, we could not address that until the patient's back was better. There were exercises the patient was unable to do to mend his back because his UEs were too weak. I could understand that we simply didn't have the time to treat every body part that was weak and achy on every visit, but it seemed ridiculous not to instruct or send some HEPs home with the patient to strengthen the weakened area.
These and other experiences finally led directly or indirectly to where I am now, in a SNF. I have the freedom to treat the "whole person," provided I stick to the plan of care and address the patient's goals set by the PT. If I see the patient has difficulty with certain arm motions that will ultimately allow him to increase function and attain a goal, I can discuss the issue with the OT and PT. I don't have to wait until the patient's surgery site is at maximum improvement to progress the patient and I usually see greater improvement faster.
Most therapists will see patients with a variety of ailments and treat accordingly, but those who specialize in one area are either setting themselves up for a great niche market or limiting themselves professionally. I have yet to determine which.