The Plantaris Tendon: A Hidden Cause of Achilles Pain
Achilles tendonitis is a common cause of ankle pain in active people, and excess activity is the main driver. But in some stubborn cases, another structure is involved: the plantaris tendon. A plantaris tendon injury can rub against the Achilles and keep it painful despite good rehab. So what is the plantaris tendon, and how does it cause Achilles pain?
What is the plantaris tendon?
The plantaris muscle starts at the knee and runs between the large calf muscles. Mid-calf, it becomes a thin tendon that courses down close to the inner Achilles and attaches to the inner surface of the heel bone. In some people, it runs especially close to the Achilles.
How does the plantaris tendon cause Achilles pain?
We’re not certain, but Dr Masci has researched this closely. The plantaris runs very close to the Achilles tendon in some people, and we believe this raises pressure between the two tendons, swelling the Achilles tendon. Dr Masci’s study showed the tension between the two tendons is greatest at the extremes of ankle movement.
How do we recognise plantaris tendon pain?
Dr Masci published the typical features of a plantaris tendon injury:
- Pinpoint pain on the inner (medial) Achilles — patients often point to it with one finger
- The tendon is most tender at exactly that point
- Ultrasound shows an enlarged plantaris and matching tendinosis on the inner Achilles
A special technique, Ultrasound Tissue Characterisation (UTC), combined with standard ultrasound, picks up over 90% of cases. Only a small proportion of people with Achilles pain have an enlarged plantaris.
How do we treat plantaris tendon pain?
Treatment resembles Achilles tendonitis, but with a key modification: Dr Masci’s research suggests that exercise should be adjusted to reduce pressure between the two tendons—avoiding heavy Achilles loading at the very top and bottom of a calf raise. An experienced therapist should guide this.
When rehab stalls, we add shockwave therapy, GTN patches, and tendon injections.
In persistent cases, Dr Masci injects hyaluronic acid between the two tendons under ultrasound guidance to reduce the pressure between them. After this injection, rest the tendon for a few days and limit walking; from day 4, increase walking gradually.
Some difficult cases need surgery to remove the plantaris tendon — usually under local anaesthetic. Dr Masci’s research shows plantaris surgery outcomes are good in the short and long term, with most patients returning to full activity. Surgery still carries risk, so it’s reserved for those who’ve failed other treatments, including the hyaluronic acid injection.
What about plantaris tendon rupture?
Sometimes the plantaris tendon ruptures, usually low in the calf near the Achilles — and it can be mistaken for an Achilles tendon rupture. Symptoms include sudden pain, swelling, and limping near the Achilles tendon; a mid-calf tear can mimic a calf muscle tear. Ultrasound or MRI confirms it. Treatment mirrors Achilles pain: a boot or heel raise for a few days, calf-loading exercises, and a progressive return to running.
Frequently asked questions about the plantaris tendon
How do I know if my Achilles pain is from the plantaris?
The classic sign is pinpoint tenderness on the inner Achilles that you can point to with one finger, confirmed by ultrasound showing an enlarged plantaris. Standard Achilles tendonitis is usually more diffuse.
Does everyone have a plantaris tendon?
No — around 10–20% of people are missing it entirely, with no ill effect. When present and enlarged, it occasionally drives Achilles pain.
Can plantaris pain be cured without surgery?
Usually, yes. Most cases settle with a modified exercise programme, shockwave, and — if needed — a hyaluronic acid injection. Surgery is a last resort with good outcomes when required.
Final word from Sport Doctor London about plantaris tendon injury
Consider the plantaris tendon in stubborn Achilles tendonitis, and in sudden pain near the Achilles or lower calf. The clue is pinpoint inner-Achilles tenderness. See an experienced sports doctor for a proper diagnosis — the treatment differs from standard Achilles tendonitis.
To book a one-stop Achilles assessment with Dr Masci in London, contact his team here or call +44 (0) 203 488 0350.
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