Massage is the manual manipulation of soft body tissues. Rubbing, kneading, rolling, and pressing the tissue increases blood flow and warmth, relieves muscle tension, and promotes relaxation.
When you receive a massage, it allows the muscles to relax and fresh blood with oxygen and nutrients to flow back in
A knot is a contraction or spasm in a muscle. The most common cause is over-work. That means that you are over-working some of your muscles.
The knots form because the spasm keeps the muscle continuously "on". Normally, even when doing heavy lifting, no muscle is working continuously. Instead, as the body moves in normal activities, different muscles cycle on and off. The muscle spasm makes the muscle work continuously, around the clock.
After some amount of time the muscle overloads and forms these knots.