The knots you detect in your muscle, which may feel as small as a marble or even as large as a golf ball, are called myofascial trigger points. The fascia is the thin layer of connective tissue that surrounds the muscle.

When your muscle gets damaged, even just a little, it can cause inflammation in the bands of muscle and the layer above. A clump of inflamed tissue is a myofascial trigger point. The little lumps are typically tender to the touch and can limit your range of motion or lead to pain during various movements. Muscle knots don't show up on medical imaging scans, and researchers are still trying to figure out the exact physiological mechanisms within the muscle that cause this reaction.

Myofascial trigger points tend to develop when a muscle is irritated by a new or more-strenuous-than-usual repetitive motion. For example, you may develop knots in the muscles you stressed the most during a particularly intense day of exercise. They can also crop up if you introduce a new movement pattern to your daily workout.

You don't need to be a gym rat, though, to be familiar with muscle knots. For instance, if you are consistently hunched over a computer all day, you may notice knots developing in your upper back and shoulders. Most people wouldn't consider sitting at a desk strenuous, but holding one position for hours at a time places stress on your muscles.