The fibularis tertius is a small but important muscle located in the lower leg. Often overlooked in training routines, it plays a key role in foot movement and ankle stability. Learning how to engage and strengthen this muscle can improve balance, support other muscles, and reduce injury risk — especially for runners, hikers, and athletes.

Where the Muscle Is Located

The fibularis tertius originates on the lower part of the fibula (the outer bone of the lower leg) and inserts on the top of the fifth metatarsal bone (outside edge of the foot). It’s situated on the front-lateral aspect of the shin and is part of the anterior compartment of the leg.

Its main function is foot eversion (turning the sole of the foot outward) and dorsiflexion (lifting the foot upward). These actions help with foot placement, shock absorption, and lateral stability during walking, running, and cutting movements.

Different Ways to Engage the Fibularis Tertius

1. Eversion and Dorsiflexion Drill
   • Sit on the ground with your legs extended.
   • Flex your foot upward and turn it outward simultaneously.
   • Hold for a few seconds, then relax and repeat.
2. Resistance Band Exercise
   • Anchor a light resistance band around the forefoot.
   • Pull the band medially (toward the inside), and use your foot to resist by pushing outward and upward.
   • Perform controlled repetitions to strengthen the muscle.
3. Balance and Mobility Work
   • Use balance boards or wobble pads to challenge ankle stability.
   • The fibularis tertius will engage automatically during these unstable surface exercises.
4. Toe Walks with Everted Foot
   • Walk on your toes with your feet slightly turned outward.
   • This subtly activates the fibularis tertius while also training coordination.

How Long to Hold a Flex for Muscle Growth

For isometric strength:

• Beginner: Hold for 5–10 seconds per rep, 2–3 sets
• Intermediate: Hold for 15 seconds per rep, 3–4 sets
• Advanced: Hold for 30 seconds per rep, 4–5 sets

Isometric holds should be done at maximum contraction without pain. These holds are especially useful for small muscles that are difficult to isolate with full dynamic movement.

Skill Levels and Progressions

• Beginner: Focus on awareness and basic engagement using seated flex and eversion drills.
• Intermediate: Add resistance bands and isometric holds.
• Advanced: Incorporate the muscle into functional balance training and agility drills.

How It Supports Other Muscles

• Tibialis anterior: Works alongside it during dorsiflexion, helping raise the foot and prevent tripping during gait.
• Peroneus longus and brevis: The fibularis tertius assists in eversion but from a different angle, providing balance between front and side leg muscles.
• Ankle stabilizers: Supports the lateral ankle and prevents rolling, especially during sudden lateral movement.

A strong fibularis tertius contributes to better ankle health, supports other lower leg muscles, and plays a protective role against sprains and instability.

Final Thought

Though small and often unnoticed, the fibularis tertius is vital for controlled foot movement and injury prevention. With focused engagement and consistent training, you can enhance its strength and contribute to overall lower leg function. Whether you’re walking a trail or sprinting on turf, this muscle quietly does its part — and now, you can give it the attention it deserves.