Sprinters run with parachutes on and weight vests. I was in the ARMY for 6 yrs and in basic training, they do exactly what you are talking about. A strict exercise regiment of muscle burnout 3 times a week. This consists of exercises that don't really use weighted resistance. Push-ups, sit-ups, pull-ups and other exercises of that type-Do 3 sets of each until complete failure. Combine that with uphill sprints and weighted running or swimming and you will gain strength, lose weight, and gain minimal mass. I was shredded as hell when I came out of infantry basic!! I was also strong and agile.