Isn't it true that some people may work at weights for a reasonable amount of time and never show a difference in their muscle definition while others develop muscles quite rapidly? Can you tell me if this is so, why it is so, and is there any thing that those people who don't show muscles can do to "get muscles"?

Yes, it does appear that some people build muscle more easily than others. At Tufts, we are trying to figure out the physiologic factors that are responsible for this difference. While we still have a long way to go to fully understand the mechanisms, we do believe that natural hormone levels are largely responsible. Some women just naturally have more anabolic (building up) hormones and some women don’t. Another big issue here (that most women don’t want to hear about) is that some people have a larger layer of fat tissue over top of the muscles, and even though their muscles are becoming more defined, you can't see it because they are hidden! When this is the case, I suggest adding some extra cardiovascular exercise as well as limiting fat and “empty” calories in the diet. Even when muscular definition is not apparent, a progressive strength-training program is building muscle and bone and keeping you strong and healthy!