In my student days, our anatomy training included dissection practicals, working with donated bodies. It is an experience no student forgets. During one of those sessions, our tutor stopped us and pointed out a long, thin, pale strand running down the back of the calf. It looked for all the world like a nerve. It wasn't.
It was the tendon of the plantaris, a small muscle with a big story.
The Freshman's Nerve
The plantaris has a short muscle belly, only a few centimetres long, tucked behind the knee. From there, a very long, thin tendon runs down between the calf muscles to the heel, where it joins or runs alongside the Achilles tendon.
That tendon is so long and slender that first-year medical students regularly mistake it for a nerve, which has earned it the nickname "the freshman's nerve". At 30 to 45 cm, it is also the longest tendon in the human body.
Not Everyone Has One
Here is where my memory had played a trick on me. For years I thought only a small minority of people still had a plantaris. It is actually the other way round: it is missing in roughly one person in ten, and estimates range from about 7% to 12%. Occasionally, a person has two.
Whether you have one or not makes very little difference to how your legs work. Its job today is minimal: it helps, weakly, to point the foot and bend the knee, alongside the much larger calf muscles.
A Muscle From the Trees
So why do we have it at all? The plantaris is considered a vestigial muscle, a leftover from our evolutionary past. In other primates, it helps with gripping using the feet. When our ancestors came down from the trees and began walking upright, the foot stopped being a grasping tool, and the plantaris became largely redundant.
Why Surgeons Need to Know About It
A muscle that does almost nothing turns out to be very useful. Because losing it causes little or no loss of function, its long tendon is a favourite source of graft material. Surgeons can harvest it to reconstruct tendons elsewhere, for example in the hand.
But a surgeon planning to use it has to remember that it might not be there. A commentary in a heart surgery journal noted that the tendon is absent in about 9% of patients, and warned that needless surgical exploration to find it should be avoided. That is exactly why it was pointed out to us in that dissection room.
"Tennis Leg"
The plantaris has long been blamed for "tennis leg", a sudden, sharp pain in the calf during a quick push off the foot. More recent ultrasound studies suggest the usual culprit is actually a tear in the inner calf muscle, with a torn plantaris found only rarely. Either way, a sudden sharp calf pain during sport deserves a proper assessment, because calf pain can have other causes too.
The Bottom Line
The plantaris is a small muscle with a long tendon and a long history. Most of us have one, about one in ten of us don't, and hardly anyone would notice either way. It is a reminder that our legs still carry traces of our tree-climbing ancestors, and a useful spare part for surgeons.
Sources
- Plantaris muscle — Radiopaedia
- Editorial commentary on use of the plantaris tendon — Texas Heart Institute Journal, 2003
- Plantaris muscle — Wikipedia