Let's give the benefit of the doubt. A doctor will tell someone to avoid HIGH-IMPACT exercise if they have bad knees. For actual human sized people, that tends to boil down to no running or shit like stair steppers or step aerobics, which still leaves a WHOLE shit ton of exercise on the table (swimming, yoga, etc.) With all the extra weight fats are carrying, just breathing must be a strenuous workout, so walking to the mailbox is the equivalent of a human running a marathon!