A "knot" in muscle tissue is really a small (or large) muscle cramp or spasm where the muscle fibers are not able to relax as they normally should be able to do. Heat, or a hot bath or shower can helps to begin the process of rubbing it out — most effectively done with deep tissue or Thai massage.
Sitting or standing in the same position too long can create knots, which are really due to uninterrupted flexing of the muscle. It’s a good idea to get such knots fixed lest they turn into a “trigger point” which in turn can cause pain elsewhere in the body. Knots can press against nerves or pull bones out of place. If you have inexplicable pain somewhere in your body, it might be interesting to observe whether or not you also have any knotted muscles in your back, shoulders or neck. If you do have them, that might be the key to ending the pain though there is certainly no guarantee.
As an example, the nerves in your leg have to start in your spinal column. If you have a knot in your lower back that is pressing on such a nerve, the result can be that your feet or legs hurt.
As a note, muscle cramps (knots) can have other causes other than overexertion — for example, a lack of potassium (which regulates water retention in muscle cells) can create muscle cramps. This is not medical advice, but some people swear that taking some potassium gluconate can vanquish most muscle cramps and also fix things such as "lower back pain" and "neck pain" which are really just awful muscle cramps caused by a deficiency of potassium. Proper nutrition always makes massage even more effective and enjoyable, of course.
Some people wake up at night with cramps in their toes or feet. A few tablets of potassium gluconate will usually resolve such a problem for several months. This is good to know since, unless you happen to be married to a massage therapist, when you awaken in the middle of the night with a painful charley horse you are usually on your own. Try to immediately straighten out the limb or muscle — though hard to do, as soon as you get the limb straight, the cramp usually stops. Then try a few potassium gluconate tablets. Each tablet is only 3% of your daily value but if you have any question, check with a doctor first.
It’s also really relaxing to have someone else take charge of your body and work it over. Never underestimate the power of human touch and the effect it creates in your mood and well being when you allow yourself the “luxury” of being pampered.
Any good massage will work out any knots that you have. And getting massage therapy on a regular basis is the best way to guarantee that your life and health are not being affected negatively by a situation that is so easily fixed.