Can a teensy slip of a muscle in the back of your knee cause chronic anterior knee pain? I got this question from a reader with a classic case of patellofemoral syndrome, one of the most common injuries affecting runners, but plenty of other people too:
The sports doctor I visited for the first time this morning immediately diagnosed me with “plantaris hypertonia” and did trigger point therapy on it. I have heard from others that this doctor offers this same diagnosis and treatment for almost any kind of knee pain. Have you heard of it?
So this doctor thinks that many or most people with knee pain have a tight plantaris muscle, which either causes pain indirectly via stress on the knee joint and/or directly because it’s inherently uncomfortable… but you won’t actually feel it on the back of the knee, because of pain referral (the neurological phenomenon of feeling pain away from its source).
The plantaris peaks out from behind the lateral head of gastrocnemius behind the knee & is even skinnier than this drawing makes it look. But it does have a crazy long tendon that descends all the way down the inside of the back of the leg to the inside of the heel.
Plantaris the variable
Plantaris is an odd, obscure, and tiny little muscle. It’s so slight that students mistake it for a nerve during cadaver dissections, giving it the nickname “freshman’s nerve” or “fool’s nerve.”
It has one of the longest tendons in the body, several times the length of the muscle. The muscle belly crosses the knee diagonally from the outside above to the inside below, and then the barely-there tendon descends all the way down the back of the calf to attach to the inside of the heel.
Maybe. Roughly. Like many small muscles, this one has seriously unpredictable anatomy, and it’s entirely missing in many of us (Simpson), roughly 10%. A 2021 paper asks with its title, “Is the plantaris muscle the most undefined human skeletal muscle?” (Kurtys). I think there are others that would give it a run for its money, but it is a weird one.
Plantaris the vestigial
What does plantaris do in terms of movement? Not much. Technically, it does the same pulling job as the big calf muscles, but it’s usually too small — even its larger versions — to make any serious contribution.
Does it do anything else? People seem to want to think so. “Researchers have published many reports on the potential clinical significance of the muscle belly and tendon,” write Kurtys et al.
One paper, trying to give plantaris purpose, notes that it “seems to have very important proprioceptive [position-sensing] role since it has very high density of muscle spindles” (Spina), citing Rosser). Really? A muscle that is missing in a significant percentage of the population has a “very important” sensory role? The plantaris-deprived don’t seem to have any trouble walking…
Sometimes a tiny muscle is just a tiny muscle.
No other tiny, variable, redundant skeletal muscle is known to play an “important” role in human physiology — and the plantaris probably doesn’t either. Its aggrandizement in the literature smacks mostly of publish-or-perish hyperbole: paper authors flattering their own subject matter with tantalizing speculation, the scientific version of clickbait. “Is the plantaris muscle what it seems to be? One researcher found out the hard way…”
Clinically important? Connecting the dots (for profit)
Plantaris has been blamed for a variety of troubles, like entrapment of the popliteal artery in the knee (okay, maybe) and Achilles tendinitis (bit of a reach there). And of course it can get injured like any anatomy (not that it would make much of a difference for rehab). But, returning to my reader’s question, in twenty years of writing about this topic, I have never heard of blaming the plantaris for pain in the patellofemoral joint.
Which is on the other side of the knee, to be clear.
I am all-too familiar with this type of diagnosis, however, and they are notoriously silly. They are particularly ridiculous when they are pitched as explanations for “all” otherwise unexplained pain in an area.
What all diagnoses like this have in common is making a big deal about “connecting the dots” between some seemingly unrelated minor cause and a physically remote clinical effect. In this case, the hypothetical connection is between the plantaris on the back of the knee, and the pain on the front of the knee.
This is a common simplistic way to think about musculoskeletal medicine, and it has often been taken to absurd extremes like this, fetishizing the a single piece of obscure anatomy (see also iliopsoas worship). These crank theories are the perpetual-motion-machines of musculoskeletal medicine, doomed to being perpetually obscure and unproven, well-intentioned but naive and deluded. Similar but more reasonable hypotheses about biomechanical and structural factors have been studied to death for decades with few useful results.
The extreme dot-connecting theories require a fragile chain of assumptions to be even technically true, let alone clinically important. Most are about as clinically sensible as citing astrological influences.
A more open-minded reaction: if plantaris really wanted to cause anterior knee pain driver, how would it do it?
Maybe sometimes there is a meaningful clinical relationship between plantaris and anterior knee pain. Maybe some anterior knee pain could be caused by referral from irritated structures in other parts of the knee, and a “trigger point” is one of the possibilities, and the plantaris specifically is one of the options.
Trigger points are themselves a badly abused concept: sore spots of unclear nature, but overconfidently treated by countless professionals as if we actually know what they are and what to do about them.
But the sore spots do exist, and so does pain referral. So… maybe.
Is this coming from “the big red books”?
My first thought when I saw this idea about plantaris was that it almost certainly came from “the big red books,” the hugely influential texts by Travell & Simons, a pair of hefty tomes that describe hundreds of hypothetical links between muscles and common painful problems — all of them largely anecdotal, almost entirely based on the authors’ clinical experience (admittedly vast). Almost any time that a muscle is blamed for causing a common painful problem, you can trace it to a claim made in those texts.
But the Big Red Books don’t mention plantaris at all — not one word. The only mention of patellar pain attribute it to quadriceps trigger points: “There is a relatively high probability that this symptom [quadriceps tendinitis] is actually caused by vastus lateralis trigger points.”
Popliteus, another small muscle behind and just below the knee, gets a whole chapter, but allegedly produces referral “primarily to the back of the knee joint” — not a word about anterior knee pain, and not a word about its very close neighbour plantaris.
So the idea didn’t come from the big red books, as the great majority of such ideas do, so apparently it’s just someone else’s pet theory.
It’s extremely unlikely that plantaris trigger points can explain any significant fraction of patellofemoral pain syndrome, let alone a majority of cases. It’s more likely that this little muscle has literally nothing to do with any kind of anterior knee pain at all.
This is an excerpt from my book about patellofemoral pain. Dozens of other sub-topics like this are covered in the book, and there’s a long free introduction.
The Complete Guide to Patellofemoral Pain Syndrome — An extremely detailed guide to rehab from patellofemoral pain syndrome (aka runner’s knee), with thorough reviews of every treatment option. (105,000 words, 425-min read)