Muscles of the Month: Tibialis Posterior and Peroneus (Fibularis) Longus
- 3 days ago
- 3 min read
Part 2 of the Foot & Ankle Series
Last month, we began our foot and ankle series with the calf muscles (gastrocnemius and soleus), focusing on their role in propulsion and power. This month, we shift our focus to two equally important — and often overlooked — muscles: Tibialis Posterior and Peroneus Longus. These muscles play a key role in supporting the arch, stabilizing the ankle, and controlling foot mechanics during walking and running.
The video demonstrates the exercises — the blog explains the “why” behind them so you can apply them more effectively.
🎥 Watch the Videos
In these videos, I demonstrate a few of my favorite exercises to strengthen these muscles and improve foot and ankle control.
Anatomy: Tibialis Posterior
The tibialis posterior lies deep in the back of the lower leg, underneath the calf muscles.
It originates from:
• the posterior tibia• the posterior fibula• the interosseous membrane
Its tendon travels down the inside of the ankle, passing behind the medial malleolus, and inserts primarily onto the:
• navicular bone• with additional attachments to the cuneiforms, cuboid, and bases of the metatarsals
Because of these broad attachments, the tibialis posterior plays a major role in supporting the medial arch of the foot.
Anatomy: Peroneus (Fibularis) Longus
The peroneus longus (fibularis longus) runs along the outer side of the lower leg.
It originates from:
• the head and upper portion of the fibula
Its tendon passes behind the lateral malleolus, then travels underneath the foot, inserting on the:
• base of the first metatarsal• medial cuneiform
This unique pathway allows it to help support the underside of the foot and transverse arch.
Function in Movement
These two muscles perform opposite actions:
Tibialis posterior → inversion and supination
Peroneus longus → eversion and pronation
But rather than opposing each other, they work together to create balanced, controlled foot mechanics.
The “Sling” That Supports Your Arch
A helpful way to understand their function is to think of the tibialis posterior and peroneus longus as forming a sling under the foot.
The tibialis posterior supports from the inside (medial side)
The peroneus longus supports from the outside and underneath
Both tendons effectively cross under the foot, creating support from opposite directions. Together, they help lift and stabilize the arch from both sides.
Why This Matters
When these muscles are working well:
✔ The arch is supported ✔ Foot mechanics are more efficient ✔ The ankle is more stable ✔ Force is transferred effectively during gait
When they are weak or poorly coordinated:
The arch may collapse (overpronation)
The foot may become less stable
Stress can increase at the ankle, knee, and hip
This is why strengthening these muscles can be particularly helpful for individuals with:
• flat feet or collapsed arches •overpronation •ankle instability •recurrent foot or lower limb injuries
Strengthening and Control
In Pilates and rehabilitation, exercises for these muscles often focus on:
✔ Controlled inversion and eversion ✔Arch support and activation ✔Integration with calf and foot muscles ✔Stability during standing and dynamic movement
Developing both strength and coordination helps the foot function as a stable yet adaptable foundation.
The Takeaway
The tibialis posterior and peroneus longus may sit on opposite sides of the leg, but together they create a balanced support system for the foot.
Strengthening them helps:
✨ support the arch✨ improve foot and ankle stability✨ enhance efficiency during walking and running✨ reduce strain throughout the kinetic chain
What’s Next?
What muscles or movements would you like to see featured next?Send me a message — your ideas always inspire future content.
Supporting strong movement from the ground up,
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