Updated: Jun 20
The plantaris tendon runs right next to the Achilles tendon and is thought to sometimes play a role in why the Achilles tendon gets injured. Other times plantaris tendonitis/tendinopathy may get mistaken for Achilles pain and can be the reason why your rehab isn't working. This is not a very common cause of Achilles pain but may be worth considering if yours isn’t getting better.
In this article we’ll look at:
I have also discussed plantaris tendonitis and its treatment in detail in this video:
Plantaris tendon anatomy
The plantaris consists of a small muscle belly and a long, thin tendon. The muscle belly sits behind the knee, somewhat towards the outer edge. The plantaris tendon crosses over the calf from the outside (lateral) to the inside (medial).
There’s quite a lot of difference between people with regards to the anatomy of their plantaris tendon. In some people it attaches directly onto the inner (medial) part of the Achilles tendon while in others it doesn’t attach but runs close to the Achilles tendon and attaches to the heel bone.
Researchers have actually found that the plantaris tendon can attach into nine different areas and between 7% and 20% of people don’t even have a plantaris. The thickness of the tendon also varies from as little as 2 mm to 6 mm.
What does the plantaris do?
We’re not entirely sure what the plantaris is meant to do. It is thought to help the calf muscles (gastrocnemius and soleus) with plantar flexion of the ankle (pointing the toes down or getting up onto your toes when standing) and also with flexing the knee. However, it’s not very strong and contributes only a little bit to these movements.
Recently, it’s been shown that the muscle part of the plantaris contains a large number of muscle spindles, which are little receptors that play an important role in proprioception or position sense. Because of this, researchers suspect that, rather than playing any large part in generating movement, the plantaris’ main function may be to help the brain to better control the lower leg and ankle.
How can the plantaris tendon cause pain?
Again, it is worth mentioning that plantaris pain is not very common. It is only a very small portion of patients with Achilles pain where the plantaris tendon may be the culprit.
The plantaris tendon can cause pain in a few different ways. Firstly, the plantaris tendon itself can develop a tendonitis or tendinopathy when it is overworked or overloaded. Secondly, it is thought that the plantaris can rub and compress against the Achilles tendon, causing the Achilles to become injured. It’s also not uncommon to find that someone has a combination of Achilles and plantaris tendonitis.
Research suggests that the plantaris tendon may be involved in some cases of insertional and mid-portion Achilles tendinopathy, but we’re not 100% sure yet if the plantaris tendon is the one that causes the trouble. It may just be that whatever activity you did injured both the Achilles and plantaris.
Ankle/foot motion may be significant
The load on the plantaris tendon increases the more your foot pronates (rolls in) and dorsiflexes. I explain this in more detail in the video. Increased pronation and dorsiflexion will also potentially increase the compression between the Achilles tendon and plantaris tendon.
Plantaris pain when running
Factors that can lead to plantaris tendon pain while running include the following:
Increasing your training volume or intensity too quickly.
Excessive foot pronation while you run, which can be caused by weakness around your ankle, running shoes that are too flexible, or weak gluteal muscles.
Plantaris pain when cycling
It’s not very common for cycling to be the main cause of your plantaris tendon pain. However, once the tendon is painful, cycling can aggravate it further due to the dorsiflexed position of your foot on the pedal. When your foot moves into that angle, it causes your plantaris tendon to stretch and to compress more against the Achilles tendon, and this can increase your pain if your tendon is already sore.
How to tell whether your plantaris tendon is causing your pain
There is no definitive way of telling whether your plantaris tendon is causing your pain. Instead, there are a combination of factors that can increase a clinician’s suspicion that this may be the case:
Your pain is on the medial (inner) part of the Achilles tendon. It will not be the plantaris if it is felt in any other part. However, medial pain in itself does not mean that you definitely have a plantaris injury.
The tender area is usually higher up in the tendon – about 6 cm above the heel bone or higher.
The pain usually increases when you load the ankle in dorsiflexion, e.g., do heel drops over the side of a step or strong calf stretches.
Ultrasound and MRI scans can help to diagnose this. However, even when using scans, it’s not always possible to clearly tell whether the plantaris tendon or the Achilles tendon is causing your main pain. In the end this doesn’t really matter too much. Scans can't tell you about the severity of your injury or what exercises will definitely work. If you’re seeing a skilled physiotherapist, they shouldn’t care too much about what your scans show. Instead, they should be adapting your rehab exercises in response to the symptoms you report. If an exercise that they’ve prescribed is causing an increase in your pain, you should report back to them so that they can adjust it.
Plantaris tendon pain treatment
Load management is always the first place to start. If you want your tendon to recover, you have to temporarily reduce the loads that you place on it. This will likely mean that you have to reduce your exercise volume or intensity for a while.
For the plantaris tendon, strategies that reduce the amount of pronation (rolling in) at the ankle can also help to off-load it. Things to consider include:
taping your ankle to reduce pronation;
you may benefit from orthotics that support the arch and reduce pronation;
using more supportive running shoes.
This taping technique may be useful:
Temporarily reducing the dorsiflexion angle in the ankle may allow it to calm down more quickly. This can be done by:
not running up hills;
placing a heel lift in your shoe;
avoiding calf stretches;
doing your calf/Achilles heel raise strength training to floor level rather than over the side of a step.
Strength training is an important part of the treatment regime for any injured tendon. For the plantaris tendon, there are three components to consider:
1. Achilles and plantaris tendon loading through heel raise exercises (going up on your toes). These should be done only to floor level (avoiding excessive dorsiflexion) at the start to limit the compression between the plantaris and Achilles tendons. Only once the tendon has fully calmed down and built a good base strength should these exercises be progressed to over a step.
2. Weakness in the muscles that control your hips (mainly the glute max and glute med) can also increase the amount of pronation at the ankle and may contribute to overloading the plantaris tendon.
3. Exercises that strengthen the muscles that control ankle/foot pronation may also be useful. The main ones to consider are the tibialis posterior and tibialis anterior muscles.
Not a lot of research has been done with regards to injections for plantaris pain, but there are some experts who feel that high volume injections may be of use. During this procedure, a large volume of sterile water is injected and used to separate the plantaris tendon from any surrounding soft tissue (usually the Achilles tendon) that it may have attached itself to. This is a useful procedure to try before considering surgery.
Surgery, where they either scrape or remove the plantaris tendon, has been shown to be very effective for both short- and long-term pain relief, so it’s something to consider if your pain doesn’t want to settle. However, please ensure that you’ve tried the correct rehab exercises first.
Your exercises should be prescribed and adapted according to how your symptoms react to them and they should be progressed from easy, low intensity to high intensity exercises over several months. If you’ve been given exercises that cause you pain or they have not been made harder for several weeks, then you’re likely not using an optimal programme. This is why it’s important to feed back to your physio and consult with them on a regular basis so that they can fine tune your programme.
Need more help with your Achilles injury? You’re welcome to consult one of the team at TMA online via video call for an assessment of your injury and a tailored treatment plan.
About the Author
Alfredson, H., et al. (2021). "Ultrasound and surgical inspection of plantaris tendon involvement in chronic painful insertional Achilles tendinopathy: a case series." BMJ Open Sport & Exercise Medicine 7(1): e000979.
Masci, L., et al. (2015). "Achilles tendinopathy—do plantaris tendon removal and Achilles tendon scraping improve tendon structure? A prospective study using ultrasound tissue characterisation." BMJ Open Sport & Exercise Medicine 1(1): e000005.
Ruergård, A., et al. (2019). "Results of minimally invasive Achilles tendon scraping and plantaris tendon removal in patients with chronic midportion Achilles tendinopathy: a longer-term follow-up study." SAGE open medicine 7: 2050312118822642.