Author: Kevin B. Rosenbloom, C.Ped, Sports Biomechanist

The ankle joint is arguably one of the most complex and fascinating areas of study in the human body and plantar flexion is one of the movements seen from this area. The following is a summary that explores the range of motion, concise descriptions of the muscles contribution to the movement and explores briefly interesting research regarding the muscles involved with plantar flexion.

Introduction to Plantar Flexion

Plantar flexion is the inferior lowering of the mid- and forefoot while the tibia and fibula remain static, causing a downward bend at the ankle joint. The main muscle contributors to plantar flexion are the gastrocnemius, plantaris, soleus, tibialis posterior, flexor digitorum longus, flexor hallucis longus, fibularis longus and the fibularis brevis (Visible Body 2019). However, it is important to note that the Achilles (calcaneal) tendon is of vital importance to plantar flexion. Other muscles insert into the achilles tendon and only through this fibrous collagen structure are they able to contribute to this movement.

Range of Motion

Different sources list varying degrees of flexion, but when observed carefully, their results all fall within a specific range. Values from Quinn and Washington State DSHS, all fall within the results concluded by Roaas and Andersson. An acceptable range for plantar flexion has found that less flexible individuals can experience 10° and incredibly flexible individuals experience up to 55° of flexion. The mean in the study was estimated at approximately 39.6° (Roaas & Andersson 1982).

Posterior Compartment of the Leg

The plantar flexor muscles in the posterior compartment of the leg make up several layers of incredibly functional structures. The two heads of the gastrocnemius, along with the Achilles tendon, are the most superficial structures of the posterior leg. Followed by the plantaris and soleus beneath. Then from medial to lateral, the flexor digitorum longus, tibialis posterior and the flexor hallucis longus occupy the deepest layer of the posterior compartment. The origins, insertions and additional actions of each muscle can be viewed concisely below in the Muscle Overview portion of this article.

Previous research about the muscles of the posterior compartment have been numerous and a few have been selected to peak interest. The soleus muscle is said to to be predominately consisted of primarily Type I slow twitch fibers (Gollnick et al. 1974).

Research on the deepest layers on the posterior compartment revolves mostly around experimentation and resourcefulness in aiding with repair surgeries. When the tibialis posterior is induced with fatigue, it has been shown that the joint couplings in the surrounding structures can show noticeable variation (Ferber & Pohl 2011). The f. digitorum longus and f. hallucis longus are often used in transfer surgery to aid either the tibialis posterior via midsubstance navicular hole and/or the repair of the Achilles tendon through harvestation (Kitaoka 2002).

To avoid repetition, interesting research about the gastrocnemius and plantaris muscles in the more superficial layers of the posterior compartment can be reviewed on the knee flexion summary.

Lateral Compartment of the Leg

There are only two plantar flexor muscles in the lateral compartment of the leg. They are the fibularis longus and fibularis brevis and are two of the three peroneus muscles whose main bodies travel distally along the fibula. The fibularis longus is the most superficial of the peroneus muscles.

Research on these peroneus muscles have shown some interesting results. In the case of the fibularis longus, some clinicians have shown some concern of the dependency of consistent AFO wear, worrying that the function of other muscles may be hindered or even atrophy. A study of the f. longus muscle after long term ankle brace wear has shown that the brace does not diminish the muscle’s stretch capabilities in healthy individuals (Cordova & Ingersoll 2003).

Regarding f. brevis, a study has suggested that longitudinal splits in the fibularis brevis muscle can be one of the effects caused by f. longus mechanical compression combined with the looseness of the superior peroneal retinaculum at a shallow fibular groove (Sobel et al. 1992).

Muscle Overview - Plantar Flexors

Figure 1. Sketch of ankle plantar flexors (right), posterior view. a. Superficial layers of leg muscles. b. Deeper layers of leg muscles.

Figure 2. Sketch of ankle plantar flexor insertions (right), inferior view.

Gastrocnemius [1]

Achilles tendon [2]

Plantaris [3]

Soleus [4]

Tibialis posterior [5]

Flexor digitorum longus [6]

Flexor hallucis longus [7]

Fibularis longus [8]

Fibularis brevis [9]

References & Works Cited

Barclay, T. 2018. “Anatomy Explorer,” innerbody.com. Accessed 19 Mar 2019. https://www.innerbody.com/anatomy/muscular/leg-foot.

Cordova, M. L., Ingersoll, C. D. 2003. “Peroneus longus stretch reflex amplitude increases after ankle brace application,” British Journal of Sports Medicine 37: 258-262. https://bjsm.bmj.com/content/bjsports/37/3/258.full.pdf

Ferber, R., Pohl, M. B. 2011. “Changes in joint coupling and variability during walking following tibialis posterior muscle fatigue,” Journal of Foot and Ankle Research, 4:6. https://doi.org/10.1186/1757-1146-4-6.

Gollnick, P. D., Sjödin, B., Karlsson, J., Jansson, E., Saltin, B. 1974. “Human soleus muscle: A comparison of fiber composition and enzyme activities with other leg muscles,” Pflügers Archiv: European Journal of Physiology, 348; 3: 247-255. https://link.springer.com/article/10.1007%2FBF00587415.

Gray, H. 1918. “The Muscles and Fasciæ of the Lower Extremity,” Anatomy of the Human Body, 20th Ed. Lead & Febiger. Philadelphia & New York, USA. 482-490.

Kitaoka, H. B. 2002. Master Techniques in Orthopaedic Surgery: The Foot and Ankle 2nd Ed. Lippincott Williams & Wilkins. Philadelphia, USA. 284-289, 326-334, 377-382.

Quinn, E. 2019. “Generally Accepted Values for Normal Range of Motion (ROM) in Joints,” verywellhealth.com. Accessed 19 Mar 2019. https://www.verywellhealth.com/what-is-normal-range-of-motion-in-a-joint-3120361.

Roaas, A., Andersson, G. B. J., 1982. “Normal Range of Motion of the Hip, Knee and Ankle Joints in Male Subjects, 30-40 Years of Age,” Acta Orthopaedica Scandinavica, 53:2, 205-208. https://www.tandfonline.com/doi/abs/10.3109/17453678208992202.

Sobel, M., Geppert, M. J., Olson, E. J., Bohne, W. H. O., Arnoczky, S. P. 1992. “The Dynamics of Peroneus Brevis Tendon Splits: A Proposed Mechanism, Technique of Diagnosis, and Classification of Injury,” Foot & Ankle International 13; 7: 413-422. https://doi.org/10.1177%2F107110079201300710.

Visible Body. 2019. “Muscle Premium,” VisibleBody.com. Purchasable Application. Accessed 21 Feb 2019.

Washington State DSHS. 2014. “Range of Joint Motion Evaluation Chart,” Washington State Department of Social & Health Services. Accessed 20 Mar 2019. https://www.dshs.wa.gov/sites/default/files/FSA/forms/pdf/13-585a.pdf.

Kevin B. Rosenbloom, C.Ped, Sports Biomechanist

Kevin B. Rosenbloom, founder and president of KevinRoot Medical, is a renowned certified pedorthist and sports biomechanist practicing in Santa Monica, CA. With his continuing research on the historical development of foot and ankle pathologies, comparative evolution of lower extremities and the modern environmental impacts on ambulation, he provides advanced biomechanical solutions for his patients and clients.