Hi! Obviously I'm not Cathe, but I wanted to put in my two cents and hopefully if I'm wrong someone will correct me. The burning sensation you feel in your muscles when you lift weights is actualy a build-up of lactic acid, which is a by-product created as oxygen is used up. Since lifting is anaerobic exercise your blood does not have time to get enough oxygen to the muscles to take away the lactic acid and this creates the burning. The burning itself is not actually shaping the muscle, but when you feel the burn it means you are breaking the muscle down so that it can reform stronger. The fact that you can feel the burn even with no weights is because you are using the muscle repeatedly without letting it fully recover throughout the set. So I guess the bottom line is that you feel burning whether you are looking for strength gains, muscle sculpting or cosmetic affects. And then you tailor your weights according to the kind of gains you want, heavier weights and shorter sets (like the pyramids) are good for gaining strength and then lighter and longer sets (like muscular endurance and power hour) are better for endurance. After you stop lifting the burn goes away because the blood gets oxygen back to the muscle and clears the lactic acid. I hope that this kind of answers your question and I'll be interested to hear what Cathe or others have to say!
Angela