Fibularis Muscles (a.k.a. the Peroneals)

Human Movement Science & Functional Anatomy of the:

by Brent Brookbush MS, PES, CES, CSCS, ACSM H/FS

Insertion of Fibularis Longus - Image via http://www.scielo.cl/fbpe/img/ijmorphol/v24n4/fig19-02.jpg

What's in a name?

Before we discuss these muscles in detail, we should briefly discuss what to call them. The more modern name of these muscles is in reference to their origin on the fibula, just as the tibialis muscles are named in relation to the bone they cover. But, the name "peroneals" actually accomplished the same rationale approach to naming. It would appear that this bone was thought to resemble, and named after, the needle like part of the "clasp" that we still see used on some chains and other jewelry. Peroneus stems from the Greek root of this word, while fibula stems from the Latin root. As most of our anatomy is now based on latin roots it would seem rationale to stay consistent and call the muscles discussed in this article the fibularis longus, fibularis brevis, and the briefly mentioned fibularis tertius.

Fibularis Muscles (Peroneals) - http://classconnection.s3.amazonaws.com/694/flashcards/597694/jpg/lateral_compartment_leg_muscles1312171716591.jpg

Fibularis Muscles:

Fibularis Longus

Origin: Lateral condyle of the tibia, head and proximal 2/3 of the lateral surface of the fibula, intermuscular septa and adjacent deep fascia (11).
Insertion: Lateral side of the base of the first metatarsal and of the medial cuneiform bone (11).
- This muscles lies in the lateral compartment between anterior and posterior intermuscular septa, adjacent to the fibula along with the fibularis brevis. The belly of this muscle runs from roughly the fibular head to half way down the length of the fibula, beyond which the muscle continues as a long tendon that extends behind the lateral malleolus (held in place by the superior and inferior peroneal retinaculum) and courses underneath the trochlear process of the calcaneus and through the peroneal sulcus of the cuboid to its insertion.
- This muscle is relatively superficial and may be palpated on the lateral side of the lower leg while your partner lays in a side-lying position. First locate the fibular head and lateral malleolus, and then start moving into the space between these two point just anterior to the soleus. Having your partner evert their foot, and you will feel the muscle contract underneath your fingers. Often contracting the fibularis muscles will result in a depression between the tibialis anterior and soleus. You can follow the fibularis tendon down behind the the lateral malleolus and can palpate the fibularis brevis tendon to the 5th metatarsal head (12).
Nerve: Superficial fibular nerve - descending from the sciatic nerve via the lumbosacral plexus with nerves from roots L5, S1, and sometimes L4
Action:
- Primary evertor of the ankle and relatively weak plantar flexor
- Plantar flexion of 1st metatarsal (acting across the tarsometatarsal joint) joint contributing to forefoot pronation

Fibularis Tendons extending behind the lateral malleolus and inserting into the fifth metatarsal (fibularis brevis) and coursing underneath the foot (fibularis longus) - http://www.myrundoc.com/online-courses/online-courses/images/sce/17687_anatomyperonealtendons_2.jpg

Fibularis Brevis:

Origin: Distal 2/3 of the lateral surface of the fibula and adjacent intermuscular septa (11).
- This muscles lies in the lateral compartment between anterior and posterior intermuscular septa, adjacent to the fibula along with the fibularis longus
Insertion: Tuberosity at the base of the fifth metatarsal bone, lateral side (11).
- This muscles lies in the lateral compartment between anterior and posterior intermuscular septa, adjacent to the fibula along with the fibularis longus. The belly of the brevis is mostly covered by the fibularis longus and extends from roughly 1/3 down the length of the fibula to nearly the lateral malleolus. The muscles tendon continues behind the lateral malleolus (held in place by the superior and inferior peroneal retinaculum) underneath the trochlear process of the calcaneus extending to its insertion on the 5th metatarsal head.
- This muscle is deep to the fibularis longus and may be difficult to differentiate from the fibularis longus. The fibularis muscles may be as a group on the lateral side of the lower leg while your partner lays in a side-lying position. First locate the fibular head and lateral malleolus, and then start moving into the space between these two point just anterior to the soleus. Having your partner evert their foot, and you will feel the muscle contract underneath your fingers. Often contracting the fibularis muscles will result in a depression between the tibialis anterior and soleus. You may assume that muscle fibers palpated distal to the half way point between the fibular head and lateral malleolus are brevis fibers. You can follow the fibularis tendon down behind the the lateral malleolus and can palpate the fibularis brevis tendon to the 5th metatarsal head (12).
Nerve: Superficial Fibular Nerve, descending from the sciatic nerve via the lumbosacral plexus with nerves roots L5, S1, and sometimes L4 contributing to fibularis muscle innervation.
Action:
- Primary evertor of the ankle and relatively weak plantar flexor
- Dorsi flexion of 5th metatarsal (tarsometatarsal joint) joint contributing to forefoot pronation

Cross Section of Lower Leg with Septa labeled via Eanatomy - http://d3j7fudf8o8iuo.cloudfront.net/var/ezwebin_site/storage/images/media/images/e-anatomy/lower-limb-anatomy-illustrations-and-diagrams/human-anatomy-cross-sectional-diagram-leg-muscles-tendon-bone-en/2522735-1-eng-GB/human-anatomy-cross-sectional-diagram-leg-muscles-tendon-bone-en_medical512.jpg

Fibularis Tertius:

Although it bares the same name as the muscle above, that is where the similarities end. Due to its location in the anterior compartment (the fibularis muscles lie in the lateral compartment), its function as a dorsiflexor (the fibularis muscles are plantar flexors), and innervation from the deep fibular nerve (the fibularis muscles are innervated by the superficial fibular nerve) this muscle should likely be grouped (and will be discussed) with the extensor hallucis longus and extensor digitorum longus . It may actually be an outcropping of the extensor digitorum longus (12). Stranger still this muscle only exists in a relatively small portion of the population.

Note how the tendon of the EDL and Peroneus (fibularis) Tertius actually fuse superiorly in this cadaver - strange muscle - http://www.jortho.org/2010/7/2/e1/index.2.jpg

Integrated Function:

Stabilization: Ankle (tibiotalor and talocalcaneal joint), transverse tarsal and tarsometatarsal joints
Eccentrically Decelerates:
- Ankle inversion and dorsiflexion
- Forefoot supination
Synergists:
- The fibularis muscles may be considered synergists for plantar flexion (along with the gastrocnemius, soleus, posterior tibialis, flexor hallucis longus and flexor digitorum longus); however, the fibularis muscles contribute less than 5% of the total force generated (3). The fibularis muscles bare the closest functional relationship to the lateral gastrocnemius that also acts to evert and plantar flex the ankle.
- The fibularis muscles may also act synergistically with the pronators of the foot/ankle specifically the extensor digitorum longus, extensor digitorum brevis, and fibularis tertius (when present).

If you look closely you can see "Peroneus Longus" written in the "Peroneus Sulcus" of the cuboid - the path of the Fibularis Longus - Gray's Anatomy - http://www.bartleby.com/107/Images/small/image269.jpg

Arthrokinematics:

The fibularis muscles play a role in the arthrokinematics of various joints that affect ankle function. The most dramatic impact these muscles have is on the distal tibiofibular joint.
- Tibiofibular joint: The tendons of both the fibularis longus and brevis run inferiorly through a groove in the posterior aspect of the lateral malleoli before traversing to their insertions on the foot. Contraction of the fibularis muscles results in an anterior glide of the distal end of the fibula (the lateral malleolus), in essence, the fibularis muscles act as a bow string, pulling the distal fibular head forward toward the fibularis' insertions on the forefoot. As the fibularis muscles are prone to over-activity this often results in a decrease in distal tibiofibular joint motion and relative "fixedness" in an anterior position. Unfortunately, this also has a deleterious effect on motion of the talus during ankle motion. The distal fibula must be able to glide posteriorly with the talus during dorsiflexion. The inability of the talus to move posteriorly results in a bony block, and a decrease in dorsiflexion (this is a common scenario in lower leg dysfunction). Adding another layer of complexity, the fibula moves like a teeter-totter. A posterior glide of the distal fibula should be accompanied by an anterior glide of the proximal head of the fibula; however, just as the distal end may become "fixed" anteriorly the proximal head may become fixed posteriorly. This posterior bias is further contributed to by several commonly overactive structures, namely the biceps femoris and TFL/VL/ITB complex (Tensor Fasciae Latae , Vastus Lateralis , Iliotibial Band. Although there does not seem to be a great self-administered solution to the arthrokinematic dyskinesis described above, manual therapists may gain more dorsi-flexion by first releasing the fibularis muscles, PA glide of the proximal tib-fib joint, AP glide of the distal tib-fib, AP glide of the talus, release of other affected plantar flexors… and then your calve/stretch into dorsiflexion.
- Fibularis longus and the first metatarsal cuneiform joint (1st FCJ): The relationship between the fibularis and the mechanics of the first ray are complicated, involving1st FCJ, the 1st cuneonavicular joint, and the talonavicular joint. The fibularis muscles are likely to contribute to internal rotation of the first cuneiform. The tibialis anterior creates a compressive force, inferior glide, and external rotation of the medial cuneiform on the navicular. Often clients will present with rigidity and need for first ray mobilization. It appears likely that an overactive fibularis longus would contribute to rigidity of the 1st FCJ and a bias toward plantar flexion and medial rotation of the first ray, leading to pronation of the forefoot, and result in a relative superior shift of the medial cuneiform on the navicular. This would likely be the start of an accessory predictive model of postural dysfunction specific to "Foot/Ankle Dysfunction"
- Fibularis brevis and the 5th metatarsal-cuboid joint: Over-activity of this muscle will cause a posterior glide and a dorsi flexion bias of the 5th metatarsal. This contributes to forefoot pronation and may result in pain in the outside of the foot. Release and stretching of the fibularis muscles and caudal glides of this joint may be recommended; however, hyper-mobility of this joint is more often the cause of pain making mobilization contraindicated. If hyper-mobility is noted assess cuboid motion (often medially rotated and stiff) - hyper-mobility at one joint almost always indicates hypo-mobility at another.

Core Subsystems:

This muscle is part of the Deep Longitudinal Subsystem (DLS), playing an important role in proprioception and optimal deceleration of ground reaction force. The behavior of this muscle in postural dysfunction further supports this assertion as the fibularis muscles often become over-active, along with the other muscles that comprise DLS, during Lower Leg Dysfunction. Often, an increase in DLS activity is accompanied by inhibition of the Posterior Oblique Subsystem.
- Deep Longitudinal Subsystem
- Posterior Oblique Subsystem

Fibularis Longus and Brevis - RVU Anatomy - http://www.rvuanatomy.com/uploads/1/3/4/5/13457421/lc2b.gif

Fascial Integration:

My Fascial Hypothesis: Large fascial sheaths not only play a role in the transmission of mechanical force, but may also play a role in dictating the function of muscular synergies. This is likely caused by reducing or increasing tone of invested musculature via reflex arcs formed between mechanoreceptors imbedded in the connective tissue and the attached musculature. In this way my view of fascia differs slightly from noted expert on the subject Tom Myers. I think of these large fascial sheaths (specifically the thoracolumbar fascia, iliotibial band, and abdominal fascial sheath) as natures "mother board." A place for mechanical information to be communicated to the nervous system for more efficient recruitment of the muscular system. Despite having a slightly different philosophy it does not change the fact that fascia plays an important communicative role in the human body and we have Tom Myers to thank for his work.

Iliotibial Band and Lateral Inter-muscular Septum – The iliotibial band (and lateral intermuscular septum which invests in to the ITB) creates a complicated network that invests in several structures

Lateral retinaculum and the patellar tendon
The lateral collateral ligament at the knee
Fibular head
Anterior tibiofibular ligament
Biceps femoris tendon

The potential communicating synergies that may arise from this fascial network are staggering and could be the subject of an article unto themselves. In relation to the fibularis muscles there may be a fascial continuity between the Iliotibial band, the fibular head, and the fibularis muscles, or the fibularis muscles may only have a mechanical relationship to this fascial structure via the proximal and distal tib-fib joints.

To date, the most useful idea to come from my personal consideration of the relationships arising from the iliotibial band is the muscle synergy that would result in tibial external rotation. The iliotibial band may act as a communicating medium for these muscle, resulting in an increase in muscle activity and force generation from all of the muscles in the group when tension is increased in the ITB. But, back to the fibularis muscles, the external rotation force at the tibia also causes a posterior glide of the proximal fibular head, resulting in an anterior glide of the distal fibula, which could result in adaptive shortening of the fibularis muscles and a disruption in ankle mechanics (Sound familiar? This is what was discussed under "arthrokinematics," but in reverse).

So we have two potential relationships between the fibularis muscles and the ITB; either fascial continuity between the ITB, fibular head, and the fibularis muscles, or a mechanical relationship via the proximal and distal tib-fib joints.

Iliotibial Band crossing Knee - http://www.magicalrobot.org/BeingHuman/wp-content/uploads/2010/03/body_worlds_knee.bmp

Fascial Integration and the Foot:

There is undoubtedly a fascial network on the plantar surface of the foot that influences the kinematics of the foot/ankle complex. Although I would like to say I have an expert's level of understanding, the kinematics and interaction between muscles, fascia and joints is incredibly complex. I simply have not had enough practice applying human movement science concepts to refine and clarify my understanding of this body segment.

The only relationships I can be sure of is the interaction between the fascial sling created by the fibularis longus and tibialis anterior, and the importance of balance in length and activity between these muscles for maintaining the medial longitudinal arch during gait. This is also related to the arthrokinematics of the first ray discussed above. (see the first image)

Note the sling created and the close proximity of the insertions of the tibialis anterior and fibularis longus on the plantar aspect of the first metatarsal-cuneiform joint. - http://www.dartmouth.edu/~humananatomy/figures/chapter_17/17-8_files/IMAGE001.JPG

Behavior in Postural Dysfunction:

The fibularis muscles have a propensity toward adaptive shortening and over-activity. It should be mentioned that this may happen unilaterally, and lead to a functional leg length discrepancy, or an asymmetrical weight shift during dynamic assessment. We find altered length and activity in lower leg dysfunction. For more information on a predictive model of postural dysfunction/impairment of the lower leg click on the link below:

Lower Leg Dysfunction (LLD)

The fibularis muscles are short and overactive in the majority of individuals with lower leg dysfunction and should be considered during flexibility training - Release and Stretch. Despite these muscles being indicated as weak in individuals with a history of ankle sprains it has been my practice to rarely if ever specifically target the fibularis muscles with "strength" or activation exercise. In practice it would seem that release and stretching of these muscles returns optimal length tension relationships and resolves stability and strength issues related to this muscle.

Practically, modifications to calf stretches are sufficient for lengthening. Attempting to stretch the fibularis muscles alone by inverting the ankle complex will only result in unwanted lengthening of the lateral ligementous structures of the ankle.

Clinical Implications:

Achilles Tendonitis
Plantar Fasciitis
Peroneal Tendonitis
Pes Planus (Flat feet)
Shin Splints
MTSS (Medial Tibial Stress Syndrome)
Ankle impingement
Peroneal Nerve Entrapment
Transverse tarsal and tarsometarsal joint dysfunction
Cuboid Subluxation

Signs of Altered Length/Tension and Tone:

Overhead Squat:
- Feet Flatten: Short/Over-active
- Feet Turn-out: Short/Over-active
- Knees Bow-in: Short/Over-active
- Knees Bow-out: Short/Over-active
- Asymmetrical Weight Shift: Short/Over-active on side of dysfunction
Goniometric Assessment
Dorsi Flexion: < 20° (Active Assisted Range of Motion)
Inversion: Goniometry of ankle inversion would seem be ideal for testing the length of these muscles, but I find that measuring inversion with a traditional goniometer to be so unreliable (poor inter- and intra-tester reliability) that it is essentially useless. Further, it is likely that without paired dorsiflexion and inversion the limiting structures of inversion are the lateral ligamentous structures of the ankle.
Fibularis Longus and Fibularis Brevis Trigger Points:
- Palpation results in tenderness (trigger points or tender points) and may result in radiating symptoms along the lateral leg, and lateral side and dorsum of the foot. Based on the theoritical model of trigger point development (8) it would seem likely that "trigger points" are dysfunction at the "motor point" of a muscle. You may find the motor point map created by this research study helpful in directing patients/clients to the appropriate location - Motor Point Map

www.triggerpoints.net

**Specific Techniques for Fibularis Extensibility:

Fibularis (Peroneal) Self-Administered Release:

Static Calf Stretch with Modifications for Fibularis Extensibility:

Slant Board Calf Stretch with Modification for Fibularis Extensibility:

Active Calf Stretch with Modifications for Fibularis Extensibility:

Dynamic Calf Stretch with Modifications for Fibularis Extensibility:

Manual Calf Stretch with Modification for Fibularis Exensibility:

Bibliography:

Phillip Page, Clare Frank , Robert Lardner , Assessment and Treatment of Muscle Imbalance: The Janda Approach © 2010 Benchmark Physical Therapy, Inc., Clare C. Frank, and Robert Lardner
Donald A. Neumann, “Kinesiology of the Musculoskeletal System: Foundations of Rehabilitation – 2nd Edition” © 2012 Mosby, Inc.
Michael A. Clark, Scott C. Lucett, NASM Essentials of Personal Training: 4th Edition, © 2011 Lippincott Williams and Wilkins
David G. Simons, Janet Travell, Lois S. Simons, Travell & Simmons’ Myofascial Pain and Dysfunction, The Trigger Point Manual, Volume 1. Upper Half of Body: Second Edition,© 1999 Williams and Wilkens
Cynthia C. Norkin, D. Joyce White, Measurement of Joint Motion: A Guide to Goniometry – Third Edition. © 2003 by F.A. Davis Company
Cynthia C. Norkin, Pamela K. Levangie, Joint Structure and Function: A Comprehensive Analysis: Fifth Edition © 2011 F.A. Davis Company
Florence Peterson Kendall, Elizabeth Kendall McCreary, Patricia Geise Provance, Mary McIntyre Rodgers, William Anthony Romani_, Muscles: Testing and Function with Posture and Pain: Fifth Edition © 2005 Lippincott Williams & Wilkins_

Questions, comments, and criticisms are welcome and encouraged.

Fibularis Muscles (a.k.a. the Peroneals)

Human Movement Science & Functional Anatomy of the:

Integrated Function:

Core Subsystems:

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