I think that if you are noticing changes, they are probably due to the exercise, not the four lbs you lost. I know that when I started exercising, I saw improvements very quickly, both in terms of performance and appearance.
I've heard that it is difficult to gain much if any muscle mass while losing weight. I believe that, but my muscles started looking more buff (and felt firmer to the touch) within a few weeks. Maybe they were just storing more glycogen (and water) so that they appeared larger. Whatever -- I'll take it, along with the speed up in weight loss that came along with the exercise

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