Muscle Soreness
When you use muscles you have not used for a while or try a new exercise or training technique, it is normal to feel a dull ache of soreness in the muscles that were trained. This pain is caused by microscopic tears in the fibers of the connective tissues in your body--the ligaments that connect bones to other bones, and the tendons that connect muscles to bones.

This microtrauma may sound harmful but is in fact the natural response of your muscles when they experience work. This is the primary reason it is so important that you get enough rest between specific muscle workouts. Each time you work out with weights, you cause this "damage"--these tiny tears in your muscles; they need ample resting time to rebuild and become even stronger, bigger, and more firm.

To read the rest of this article: