All of those “muscle knots” that can cause or produce intense pain when pressed on and are what therapist will call a trigger point.
A bit more profound explanation? Muscles that are supplied by nerves that contains a trigger point can generate referral pain. Trigger points can appear in a restricted group of muscles and you will find it’s the most tender spot in that muscle. The secret would be to discover those spots with “knots” then simply use direct finger pressure to “deactivate” the trigger points. As a rule, where a therapist discovers a trigger point in your muscle, it has generally been there for a while, so it possibly could take a few massage sessions to make it disappear.
Typically the trigger points tend to be spots where your muscle is trying to keep things protected over a period of time. You can ask your therapist for different exercises or activities to reduce these trigger points from recurring.