There are often muscles that we have in our anatomy that are small and perhaps incorrectly labelled insignificant when it comes to movement and function. However often these muscles can serve a different role in the body and in secondary factors of movement that are worth investigating when it comes to soft tissue injuries. The Plantaris is one such muscle. It lies behind the knee and at first investigation seems positively benign, with no primary motor capacity and unable to provide power in movement… even removal of the muscle does not render movement abnormal in function. So why do we have it?
The Plantaris muscle has it’s origin on the lateral supra-condylar ridge of the femur (outside edge of thigh bone) and then travels across the back of the knee deep to the gastrocnemius and inserts at the common tendon of the Achilles. In some people, this muscle does insert on the calcaneus bone of the ankle. It is a weak plantar flexor of the ankle (points the toes) and works mainly with the Gastrocnemius in helping the body to jump and stride in running. Oddly, it is was used by our primate ancestors as a means of grasping with the feet. Research is inconclusive as to the development of the Plantaris in foot mobility and function but it is worth noting that the Plantaris does have a role in determining the proprioception of the ankle. This is supported by the high density of nerve spindles found in the muscle and reveal that it may be a muscle that can help with stability and determining where the ankle and or foot is in motion.
The real advantage of the Plantaris is in it’s ability to be used for tendon grafts. The tendon that arises from the relatively short muscle is the longest tendon in the body. Often this tendon is utilised for surgery grafts to be used in rebuilding other structures in the body. It is often called the ‘freshman nerve’ as many new anatomy students often mis-identify this tendon as a nerve.
When this muscle is strained or torn it mimics the diagnosis of a calf tear. The condition is often referred to as ‘tennis leg’ and reveals pain in the medial compartment of the calf and down the lower leg. Clinically, in most cases, it is the fibres of the medial Gastrocnemius head that actually strains and creates this pain but the term has become associated with a rupture of the Plantaris. This is largely considered incorrect and the research determines that even with a rupture of the Plantaris, function in the calf can still be attributed via the gastrocnemius and other plantar flexors.
Perhaps the real advantage of the Plantaris is this function of proprioception of the ankle. It has been found that the Plantaris has very high amounts of proprioceptive receptor end organs or nerve cells that feeds information to the brain regarding the position of the foot. This could be quite useful in things such as cross country running where uneven terrain creates continuous adaption fo the ankle and the alignment of the feet or perhaps for sports use such as gymnastics or even in the dancers realm.
However, sadly - there is some conjecture as to whether the Plantaris is a sensory or vestigial (functionless in the course of evolution) muscle. Is it just a muscle that we as humans have stopped using because of the nature of our bipedal motion and that we no longer swing from trees? The science is inconclusive here and there is some grey areas but the concept of the Plantaris as a sensory muscle is yet to be proven as well. Comments fly between biomechanists and Orthopaedic surgeons as to whether there is any functional loss when the Plantaris is removed for surgical use elsewhere in the body. It would be very interesting to note that if we do remove it and it does have a small impact on our proprioception and what that would mean for people who are perhaps unstable or struggling with the ability to stand on one foot and find stability here. I would be very interested to keep an eye on this area of research and see what comes of the discussion.
So if you are told you are suffering from tennis leg, be aware of precisely what is going on with your lower limbs and plantar flexors. And when a surgeon says ‘we will just take this tendon here for reconstruction - it doesn’t matter it doesn’t do anything’ perhaps you can have more of an understanding of why and possibly question how much it doesn’t matter…
