By volume, it seems to weigh more. That is, a pound of muscle occupies less space than a pound of fat.
Common sense tells us a pound of muscle and a pound of fat have to weigh the same, but they do differ in density. This means if you look at five pounds of muscle and five pounds of fat side by side, the fat takes up more volume, or space, than the muscle. Try this activity to discover which substance is denser than water: muscle or fat. If a substance is less dense than water, it will be buoyant, meaning it will be more likely to float. Density is determined by an object's mass and the amount of space it takes up (its volume). Muscle does not weigh more than fat, any more than lead weighs more than feathers. A pound is a pound is a pound. Where the misunderstanding often comes in is that muscle is much more dense than fat, so that, by volume, it seems to weigh more. That is, a pound of muscle occupies less space than a pound of fat. In addition, because a pound of muscle burns more fat than a pound of fat, even at rest, by increasing your lean muscle tissue mass, you're helping your body burn more calories. But back to your question. Within most of the human—and animal—body, whether muscle, fat, blood or bone, is a lot of water. This means our bodies really are close to the density of water. But this activity can also help explain why some animals—and people—are more buoyant than others. Muscular density is also referred to as muscle tone : In physiology, medicine, and anatomy, muscle tone (residual muscle tension or tonus) is the continuous and passive partial contraction of the muscles, or the muscle’s resistance to passive stretch during resting state. Muscle density develops as your muscles heal from the microscopic tears they develop during heavy lifting, but pushing yourself to the point of exhaustion can cause more significant muscle damage. If you feel that you can't continue with your lifting, stop. Don't risk injury to try and add another set to your workout.
http://www.everydayhealth.com/weight/busting-the-muscle-weighs-more-than-fat-myth.aspx http://www.scientificamerican.com/article/muscle-versus-fat/ http://www.weightwatchers.com/util/art/index_art.aspx?tabnum=1&art_id=8311 http://www.scientificamerican.com/article/muscle-versus-fat/ http://www.dangerandplay.com/2012/08/31/increasing-muscular-density/ http://healthyliving.azcentral.com/build-muscle-density-1728.html
why is muscle dense