Regardless of how you opt to exert self treatment pressures on your muscles we've all experienced a myofascial trigger point.
Muscle knots. Landmines of painful, bound up muscle scattered across the full expanse of a muscle belly. Hot spots. Ouchies. Holy crap stop squeezing that.
Whatever you want to call them, a knot/trigger point is no fun. An abnormal but normal occurrence based around over/underuse of a muscle.
So what the heck is it?
Easy answer - It's an area of bound up muscle fibers. Like a tornado, it alters the whole system. It tacks everything down and drags everything towards it and alters your ability to optimally produce & absorb force through whatever muscle is in question.
Muscles have specific lines of pull (with a little variability) and their respective tendons are also designed to manage the same forces. When there are existing knots, they change the line of pull and over time can create irritation in both the muscle/tendon in question, as well as, other neighboring muscles/tissues.
So what causes it?
There are a number of theories as to why they occur. The best theory I align with is that unfamiliar eccentric activity is to blame.
An eccentric contraction is a lengthening contraction; force is still be generated but the muscle gets longer while controlling the weight. limb, movement, etc. through space.
The eccentric portion of the lift is when the most beneficial muscle damage occurs. Remember how sore you get after lifting heavy weights? It's not the up portion of the lift, it's you maintaining control over the weight so you don't get squished that creates all the microscopic damage that ultimately results in muscle growth/strength gains/better fitness.
So with all of those concepts covered, the precursor to a trigger point is the unfamiliar eccentric activity and is a really brilliant defense mechanism your brain employs to limit a specific range of motion with the goal of preventing muscular overload. Simply put your brain only wants you working WITHIN a range of motion it is comfortable with controlling. When you decide to venture beyond those limitations the brain sees as "safe" with a deeper squat, more weight, more speed, tempo, etc. the brain works to counter this through the formation of trigger points.
The fascinating thing about trigger points is how they can refer elsewhere in the body. For instance, I often see clients complaining of chronic knee pain. Their doctor has told them that they have "arthritis and early stages of degeneration" and now they're worried they can't be active anymore.
They're often some degree of active, have average body control/awareness AND are without any specific injuries. BUT. THEIR. KNEE. HURTS.
Often there are multiple knots lurking below the surface and scattered throughout the whole muscle. In the diagram below, common areas for knots are indicated by the 'X' and the red clusters are common referral patterns that present when the X's are stimulated (in this case pressed on with and index finger or thumb) and can vary considerably. You may be reading this thinking about the weird ache you get in your knee every time you foam roll a specific spot on your thigh. THAT'S A REFERRAL PATTERN!
The same goes for the hip muscles; when knotted up, can refer into the butt/hip, side of the thigh, as well as, into the outside aspect of the knee, calf and ankle.
Often what happens is that it's not just one problematic muscle and these referral patterns are superimposed on one another. Remember, we're a system of systems and your hip musculature coordinates work with a lot of other muscles, tissues, etc. The client's knee pain is the result of multiple areas being irritated, being stimulated simultaneously and the patterns end up overlapping.
The symptomatic knee is merely the victim in all of this and ultimately the hip and thigh MUSCLES (not the knee joint) were the culprits.