It seems logical enough at first glance. If you work a specific muscle enough, you will “burn” off the fat that surrounds that muscle, resulting a leaner look for that area. It’s an idea that has long permeated the exercise world. Unfortunately, there’s not a shred of truth to it. Exercising a specific muscle does not result in local fat reduction in that area.
Physiologically speaking, fat is stored energy for the body. It’s not the only form, but its one of (if not the single) largest. Energy is stored as fat for a number of reasons, which are too numerous to go into here. What’s important is that the entire body’s fat stores are used to fuel the entire body’s energy expenditure. As a result, fat stores tend to rise and fall globally, rather than locally. The specific allocation of fat stores (where on the body you personally put it) is dependent on hormone profile. For instance, increased estrogen levels increase the storage of fat in the hips. This is why many people find that they have “stubborn” areas which tend to lose fat slower than other areas.
Practically, this means that all those crunches are not going to reduce the layer of fat covering your abs. The reps you put in on the hip adductor machine are not making your thighs any slimmer. To reduce body fat in any specific location, you have to reduce body fat all over.