What Does The Fibularis Longus Muscle Have to Do With Knee Pain?

R Peroneal Longus (Peroneal Longus)

After 3 previous knee surgeries, and also seeing many patients (with knee pain) over the last 25 years I've learnt a lot about treating knee pain. So I was pleasantly surprised to learn more and hopefully get even better at treating knee pain.

Many of our patients come to our clinic with patellofemoral joint pain (PFP) or pain in the front/ side of the knee especially during loading activities like running, walking, squatting, jumping and going up and down stairs. Studies have shown that 23 percent of all adults will get PFP, and 50 percent of them will experience PFP that lasts up to 20 years!

I've written previously that the hip often affects the knee too. Well, a group of researchers found differences in fibularis longus (or peroneal longus) muscle size and even activation between individuals with PFP and healthy controls. This was done using ultrasound imaging in weight bearing (Jaffri et al, 2024).

Of the 60 subjects (30 healthy, 30 with PFP) investigated, there was a statistically significant result for the healthy group in the size (larger) and activation for the fibularis longus (or peroneal longus) muscle.  The group with knee pain had smaller fibularis muscle and it did not work (or switch on) as quickly as those without knee pain.

Note that besides its role as a evertor of the foot, the fibularis longus works together with abductor hallucis longus to stabilize the medial longitudinal arch after heel strike. Hence slower activation and smaller (weaker) of the peroneal longus muscle in a weight bearing position may contribute to arch flattening in PFP subjects. This add to faulty mechanics at the foot and ankle joints. 

This further explains the fact that the smaller (and weaker) the fibularis longus is, the higher likelihood of the foot pronating. This leads to knee valgus (knee facing inwards), which is a major biomechanical abnormality in individuals with PFP.

I am certain not every health professional reading this will agree since clinical treatment guidelines to treat PFP are still being developed as there is a lack of agreement about what actually causes PFP. Not everyone agrees that hip adduction, increased knee internal rotation, foot pronation etc causes PFP.

If you have persistent knee pain that does not seem to go away, please show the health professional treating you this article. Other than the hip, your fibularis longus muscle may help with your knee pain. And I am not talking about just strengthening it.

Better still, come see us at our clinics.

Reference

Jaffri A, Schwarting A and Beallow A (2024). Impairments In Peroneal Muscle Size And Activation In Individuals With Patellofemoral Pain In Weight-bearing Position. J Foot Ankle Res. 17: e12014. DOI: 10.1002/jfa2.12014

Fiblaris logus is part of the lateral line