The subjects were either in groups that consisted only of people with ID (segregated) or had been placed in groups for people without such disabilites (mainstreamed). This is usually used to describe schools. Students with mental or physical disabilities may be segregated in schools or in classes that are designed just for them. The classes will consist solely of students with disabilities. Alternatively, they may be mainstreamed--placed in regular classes with students of normal abilities.
That study sounds like it is describing some sort of physical education/gym class, or perhaps a special program at a YMCA or something like that. Some of the students with ID did the strength training in groups that consisted only of students with ID. The others were placed in whatever regular strength training classes were available for the "mainstream" of students. The idea was to see if students with ID did better or worse when they were working out with people of normal abilities.