The biceps, forearms and the triceps, are relatively small muscles. As a result, they tend to be easier to train directly, yet it is more difficult to be completely immersed in the muscle doing the work. Doing a bicep curl is easy, all you have to do is flex the biceps and not move your elbow. However, it is more difficult to feel the entirety of the muscle contracting and lengthening throughout the whole movement. Therefore, at first, watching yourself train these muscles might help your form, rather than relying entirely on the feel of the motion.