Plantaris – ‘The Monkey Muscle’ – Good trivia knowledge!

A client of mine who I’ve been treating for a medial gastroc (calf) tear came in last week saying his leg felt ‘different’. Not sore, just different. Questioning and clinical testing suggested he might have a DVT so I sent him off for a Doppler ultrasound as a precaution. To my surprise imaging showed he had a plantaris tendon rupture. Quite unusual. My client was interested in this diagnosis as he had never heard of this muscle. He was more interested when I told him it has been suggested this muscle does not exist in all people. This statement is still being debated in the literature with plantar is being present in 80-100% of limbs.

To Physio’s, this muscle seems to often be more problematic than useful…unless you’re a tree inhabiting primate. One of the main functions of Plantaris is to grasp objects with the feet. It is thought that the plantaris may have become vestigial as the foot evolved towards long-distance walking.

Plantaris is a small spindle shaped muscle which runs from the outside of the knee and crosses over between the two calf muscles and is sometimes invaginated within the inside portion of the Achilles’ tendon. Cadaverick studies have showed large anatomical variations between people. This muscle has a small muscle belly with a mean width of 1.5cm and length of 10cm and an elongated tendon. Although it crosses the knee and ankle and is active in weight bearing, it contributes less than 1% of flexion power. Instead it appears this structure has a greater involvement in proprioceptiom, giving the body feedback on where the limb is in space.

The current consensus is to treat ruptures of this tendon conservatively and focus rehabilitation on restoring proprioceptiom to the injured side. However it may be a case of ‘watch this space’ as we find out more about ‘the monkey muscle’ and it’s role in ankle and knee function.