Working out can result in muscle soreness. While muscle soreness may sound simple enough, it can actually happen for a wide array of reasons. Usually, muscle soreness is the result of lactic acid that builds up in the muscles during an intense workout. If you fail to stretch properly before and after a workout, this lactic acid stays in the muscles, and, soon after, causes muscle soreness. You also start to feel soreness in your muscles during a workout, you are actually causing your muscles to tear. As a result of these tears, you may experience muscle soreness.