While we both have broad shoulders my Bokken workouts, have greatly reduced the size of my waist by strengthening my oblique, or side, abdominal muscles. Having these muscles exercised also prevents the likelihood of suffering from hernias as well.
I focus the body into 3 parts, each of which I make sure I pay equal attention to. There is the upper torso with the arms, shoulders, upper back, lats, and chest. There is the core section of the body which includes all of the abdominal muscles, the lower back and buttocks. Then there are the legs, which includes, well I guess it just includes the legs and occasionally the buttocks as well. Though the lower back should not be used with legs exercises for risk of injury.
Whether you do some sort of spot training where you isolate certain muscles or you go for more whole body type exercises depends on the specific kind of results you are looking for. It is important for both though that you ensure you are exercising your entire body to prevent certain injuries or deficiencies from affecting your results.
Failure to work the core muscle groups effectively is a source of back pain for many people. Using your back with your legs instead of with your abdominal muscles places an enormous amount of strain on the muscles of your lower back and will cause injury.