Plantaris

What it is

A small muscle with a short belly high in the back of the knee and a very long, thin tendon running the length of the calf to the heel. The belly is a few centimetres; the tendon can be thirty or more.

It is absent on one or both sides in a meaningful minority of people — reported absence rates vary considerably between anatomical studies, which is itself worth noting rather than picking a number to quote. Its absence causes no problem whatsoever.

It is sometimes called the freshman's nerve, because students dissecting a leg for the first time mistake its slender tendon for a nerve.

What it does

Weakly assists plantarflexion of the ankle and flexion of the knee. Its cross-section is so small that its mechanical contribution is close to negligible.

There is a reasonable suggestion that it serves more as a sensory organ than a motor one — it is unusually rich in muscle spindles, the receptors that report muscle length — and that its job may be feeding information about calf position rather than producing force. That interpretation is plausible and not settled.

What loads it

Nothing meaningfully or specifically. It works alongside the calf and contributes very little.

What is commonly got wrong about it

"Tennis leg" as a plantaris rupture. Sudden pain in the calf during a push-off was long attributed to this tendon snapping. Imaging has shown that most such injuries are actually tears of the medial head of gastrocnemius, and the plantaris explanation is now regarded as largely historical.

That every human has one. A substantial number do not.

That it needs training. Nothing about it is trainable in any way you could observe.