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Functional Anatomy and Muscle Action of the Foot


The miraculous actions of the feet are generally an overlooked topic within the fitness industry. The understanding of this adaptive lever system affects nearly all movement and musculoskeletal functions within the human body and every upright action of daily living. The changes within the fitness industry now demand a basic understanding of foot function to provide empirical rationale for the development of fitness programs.

The foot is comprised of 26 bones consisting of 14 phalanges (two in the great toe and three each in toes 2-5) and five metatarsals, all of which are in the forefoot; three cuneiform bones (medial, intermediate and lateral), cuboid and navicular, all in the mid-foot; and the talus and calcaneus of the rear foot, plus two sesmoid bones located under the first metatarsal head. These bony structures account for 33 joints. The density of these bones serve to absorb forces from body weight and gravity during the foot contact and the weight bearing phase of gait, and yet, they become a rigid lever as the body passes over the foot during the propulsion and heel off phase of gait (please see image below).

All movements of the foot and lower extremity evolve within a three dimensional environment. To fully appreciate the mechanical actions of the foot, ankle, leg and hip, a consistency of characteristics of human motion must be understood by the student of fitness and functional training. Universal to all motion is:

With these characteristics in mind, it is important to appreciate the loading mechanisms the body must do to achieve effective and efficient actions. During the initial phases of gait, the foot and ankle absorb forces to decelerate and control body weight and forces of gravity. As the foot hits the ground, a fulcrum is created between the calcaneus and the ground, causing the foot and ankle to go into plantar flexion. As the ground accepts the foot, the calcaneus turns outward, called calcaneal eversion. The chain reaction causes the tibia to internally rotate and the ankle to dorsiflex. When evaluating these three actions, it is immediately apparent the foot has loaded in all three planes of motion. Calcaneal eversion loads the foot in the frontal plane, tibial internal rotation loads the foot in the transverse plane, and ankle dorsiflexion causes the foot and ankle to load in the sagittal plane. This activity is absorbing in nature, and in turn, the knee and hip will absorb forces in three planes of motion and will be discussed in future writings about the lower extremity and hip.

As the body moves over the foot during the mid-stance through propulsion and heel off, the unloading phase transpires. During tri-plane unloading, the calcaneus inverts, the tibia externally rotates, and the ankle moves into plantar flexion. This concentric action creates force production. When force production occurs after tri-plane loading, which is eccentric in nature, motion becomes efficient, effective, and fluid. To begin to develop an understanding of tri-plane loading of the foot, leg, and hip, please refer to the table of joint actions. This was originally created by Gary Gray, P.T., and the author added the scapula and spine to the table. This table illustrates the actions of the joints of the foot, knee, hip, and spine as these structures work in each plane of motion during pronation (preloading phase) and supination (unloading phase). For instance, if we want to investigate what actions occur in the sagittal plane during pronation, we see the midtarsal dorsiflexes, ankle goes through plantar or dorsiflexion, the knee, hip, and spine flex, and the scapula elevates. During supination, these same structures perform the opposite action. The reader can review this table to analyze what each region does in each plane of motion during pronation and supination. In my opinion, this is the road map each student of functional training needs to study and apply in order to possess a true understanding of the chain reaction of movement and also for program development.

PRONATION   SUPINATION
Plane Plane
Joint Sagittal Frontal Transverse Sagittal Frontal Transverse
Scapula* elevation ABD-ADD int/ext rot   depression ABD-ADD int/ext rot
Spine* flex ABD-ADD int/ext rot extension ABD-ADD int/ext rot
Hip flex abduction int rot extension abduction ext rot
Knee flex abduction int rot extension adduction ext rot
Ankle PF-DF ADD-ABD PF-DF ABD-ADD
Subtalar   eversion abduction   inversion adduction
Midtarsal DF inversion abduction   PF eversion adduction
*Author's Note: Spine and Scapula have been added to the original grid.
Used by permission of Gary Gray, P.T.

Muscle Function

Of the 24 muscles that comprise the foot, 12 are extrinsic (superficial) and 12 intrinsic (deep, under the superficial muscles). To bring semblance of order when studying the foot musculature, the extrinsic muscles will be categorized into the anterior compartment, lateral compartment, superficial posterior and deep posterior compartments.

When studying anatomy and kinesiology from traditional textbooks, it is important to note that the descriptions typically are in open chain, single plane and isolated actions. The actions are concentric and propulsive in nature. However, muscle activity during human motion takes on a completely different motion. In human movement, muscles first eccentrically contract, stabilize and then concentrically contract. This process involves force reducing, a stabilizing moment, then force explosion. Eccentric contraction stores about three to nine times more energy than concentric contraction, therefore allowing a more efficient muscle performance. The student of movement must keep in mind that all muscles and joints function in a three dimensional environment and within a synergistic relationship with other adjacent muscles. Also, muscles are affected by other muscles in more distant parts of the body. As it relates to the foot, each compartment is dependent upon the muscles within that compartment for pure uncompensated action. Yet, other muscles are also functioning simultaneously during the gait cycle and have an impact upon foot action.

For consistency when describing the integrated actions during gait, all sequence of movements will start with the right foot at heel strike on the lateral calcaneus. The mid-stance is described when the forefoot is in full contact with the ground. Heel off is at the point when the calcaneus lifts off the ground and is the transition to the propulsive phase. Toe off occurs as the great toe leaves the ground during the final action of the propulsive phase.

Anterior Compartment

The anterior compartment consists of the anterior tibialis, extensor hallucis, extensor digitorum longus and peroneus tertius. Collectively, these muscles decelerate plantar flexion and pronation as the foot hits the ground during the contact phase of gait. Likewise, they accelerate dorsiflexion of the toes and foot in isolated actions.

Anterior Tibialis

The anterior tibialis (AT) originates from the upper two thirds for the lateral tibia and inserts at the medial cuneiform and first metatarsal head. In isolation, the AT shortens to concentrically cause dorsiflexion. However, during normal gait, the lateral calcaneus forms a pivot point or fulcrum with the ground. As the body moves forward, the ground reaction forces cause the foot to pivot forward, and gravity forces the foot into plantar flexion. At this point, the AT - by the nature of its attachments at the medial cuneiform and first metatarsal head - performs two essential roles: it supinates the foot so the fifth metatarsal hits the ground and decelerates plantar flexion and pronation as the foot gently descends to the ground. If the AT did not function during the heel strike phase of gait, the foot would slap or drop to the ground.

During mid-stance, the AT becomes relatively silent after the first 10% of the gait cycle and becomes very active again immediately after toe off of the same foot. Therefore, the anterior tibialis has two active cycles during gait: immediately after toe off it concentrically contracts to assist in toe clearance during the swing phase while maintaining a supinated position during heel strike and then it eccentrically loads to allow the deceleration of plantar flexion and pronation.

Extensor Digitorum Longus

The extensor digitorum longus (EDL) originates from the lateral tibial condyle, proximal three-quarters of the fibula and interosseus membrane. It inserts on the middle and distal phalanges of the 2-4 toes. Like its neighbor Anterior Tibialis, the EDL performs isolated action of ankle dorsiflexion. It also extends the 2-4 toes. Once again, human motion dictates an entirely different function during the gait cycle. Like all muscle function, the EDL decelerates, stabilizes and accelerates motion. The unique aspect of the EDL is a bi-phasic action during the gait cycle. At heel strike, the EDL are relatively silent. As the body moves over the foot into mid-stance, the foot begins to pronate. This action causes the EDL to eccentrically load as the tibia internally rotates during the pronation phase. As the EDL lengthens, tension is placed upon the distal phalanges, pulling them posteriorly and creating a rigidity of the lesser toes. This is crucial as, in this phase, the mid-tarsals also become rigid to form a stable foot, which is conducive for propulsion.

During the swing phase, the EDL and the AT extend the toes for clearance from the ground. The angular pull along the 2-4 toes causes the lateral foot to pronate slightly and prevents excessive supination.

The synergistic relationship of muscles is extremely evident in the EDL and Extensor Digitorum Brevis (EDB), as the tendons of these muscles conjoin around the proximal phalanges. This junction also attaches with the interosseus and lumbricales of the deep layers of the foot musculature (please see images below). This union provides the synergistic effort to cause the rigidness of the foot through the supination and propulsive phase. At the toe off phase of gait, the dorsiflexed toes are stabilized posteriorly against the metatarsal heads to form the rigid propulsive foot. Therefore, the two actions of the EDL and EDB are to stabilize and form a rigid foot during propulsion and assist in ankle dorsiflexion by assisting the tibia with forward motion over the foot.

Extensor Hallucis Longus

The extensor hallucis longus (EHL) originates at the middle half of the medial aspect of the fibula and nearby interosseus membrane and inserts on the distal phalanx of the great toe. Isolated motion extends the great toe, but the integrated function decelerates plantar flexion of the great toe and assists in clearance during the swing phase of gait. Additionally, the EHL controls excessive foot eversion and helps to stabilize the 1st metatarsophalangeal joint during mid-stance. Late in stance phase, the EHL becomes taut by posteriorly pulling the phalanx and creating a rigid beam in preparation for propulsion.

Peroneus Tertius

Peroneus Tertius is the small, weak muscle of the anterior compartment, originating on the distal third of the anterior fibula and inserting at the base of the 5th metatarsal. Isolated function suggests this muscle dorsiflexes and everts the foot. Yet, the integrated functional action of the tertius decelerates plantar flexion and excessive supination during the contact phase of gait by providing an eversion influence during the supination (calcaneal inversion) phase.

Posterior Compartment

Six muscles comprise the posterior compartment of the foot: the gastrocnemius, soleus, plantaris, posterior tibialis, flexor digitorum longus and the flexor hallucis longus. To further classify this group, the gastrocnemius, soleus and plantaris are considered the superficial muscles, while the posterior tibialis, flexor digitorum longus and flexor hallucis longus are the deep posterior compartment group. Collectively, these muscles decelerate dorsiflexion as the tibia moves over the foot and accelerate supination from mid-stance through heel off. Additionally, the posterior compartment muscles influence knee function. Likewise, they accelerate plantar flexion of the ankle in isolated actions.

Gastrocnemius

The gastrocnemius originates on the medial and lateral femoral condyles and inserts on the Achilles tendon. Its isolated action is plantar flexion of the ankle and contributes to knee extension. This muscle has an extremely important tri-plane function during the gait cycle. As the trunk moves over the foot, tension in the gastrocnemius increases to maintain knee flexion through late contact phase and during the early mid-stance phase.

As the hip moves further over the foot, the gastrocnemius gains eccentric tension and pulls the femoral condyles posteriorly, thus assisting the soleus and posterior tibialis with knee extension. Additionally, during the mid-stance phase, the foot pronates, causing the tibia to internally rotate and the femur to follow. The attachments at the origin of the gastrocnemius assist to decelerate the femoral internal rotation at the knee. When the hip moves further forward over and anterior to the foot, the eccentrically stored energy in the gastrocnemius assists in the propulsive stage to allow heel off, subtalar joint supination, tibial and femoral external rotation. The forces generated allow the heel to rise, the knee to flex and a smooth acceleration of the trunk over the foot. Due to the extensive action of the muscle, it is active from the foot contact phase through the propulsive action at heel off.

Soleus

The soleus originates on the posterior surface of the head and upper shaft of the fibula and the soleal line of the tibia. It inserts on the calcaneus via the Achilles tendon. Its isolated function is plantar flexion, but the tri-plane action is prevalent at the onset of the contact phase of the gait cycle. Soleus contracts early in the stance phase through the mid-stance to decelerate the tibia during dorsiflexion by slowing forward tibial motion. As the femur and trunk continue to move over the foot, the soleus assists the other posterior compartment muscles to extend the knee due to the posterior force resulting from the lengthening under tension. The soleus also serves to stabilize the lateral foot on the ground during the stance phase.

Upon heel lift, the soleus exerts its unloading forces to decelerate tibial internal rotation and assists the foot into plantar flexion and supination. This helps to cause the foot to invert and the tibia to externally rotate.

Posterior Tibialis

The often forgotten yet extremely important member of the posterior compartment is the posterior tibialis (PT). This muscle originates at the interosseus membrane, posteromedial fibula and posterolateral tibia. It inserts at the tuberosity of the navicular, medial cuneiform, metatarsals 2-4. Isolated function is supination of the foot; however, this does not closely resemble the importance in maintaining foot control of pronation.

Working with the gastrocnemius and soleus, the PT decelerates tibial forward motion and ankle dorsiflexion. When the forefoot begins to strike the ground, the posterior tibialis eccentrically decelerates subtalar joint pronation and internal tibial rotation, continuing the action until the heel lifts off the ground. The tendon of the PT passes behind the leg, medial malleolus and connects to the navicular and cuneiforms. Due to its alignment, as the tibia moves over the foot and prior to the heel lifting off of the ground, the PT tendon creates tension posteriorly to synergistically work with the other foot musculature to posteriorly compress the mid tarsals, causing foot rigidity and a propulsive lever. As the hip continues to move forward, the PT strongly supinates the subtalar joint, ankle plantar flexion and concomitantly tibial external rotation.

Plantaris

A thin and often overlooked muscle is the plantaris, which originates on the lateral femoral epicondyle and inserts by the calcaneal tendon of the calcaneus. Traditionally, this muscle has been described as a concentric ankle plantar flexor. Functionally, the plantaris decelerates dorsiflexion of the ankle during the contact phase through the mid-stance and helps to stabilize the tibio-fibular joint during motion.

Flexor Digitorum Longus

The actions of this stance phase muscle, flexor digitorum longus (FDL), starts at heel strike and lasts throughout the propulsive phase of gait. The FDL originates at the lower two thirds of the posterior tibia and inserts on the base of the 2-4 phalanges. It crosses the medial aspect of the ankle and then splits to the four tendons of the phalanges. Isolated actions of the FDL are plantar flexion and inversion of the foot. The integrated action of the FDL is to assist in deceleration of subtalar joint pronation, tibial internal rotation and reduction of the forward momentum of the tibia as it moves over the ankle and foot, resulting in ankle dorsiflexion. Immediately prior to the heel lifting off of the ground, the tension created by the location of the attachments causes the FDL to synergistically pull the phalanges posteriorly toward the mid-tarsals and assist to create a rigid lever so the foot is stable during the propulsion phase. As the heel lifts off and the foot goes through the swing phase, the FDL works to invert and supinate the foot as it prepares to strike the ground again.

Flexor Hallucis Longus

Originating on the distal two-thirds of the fibula and interosseus membrane, the flexor hallucis longus (FHL) runs underneath the plantar surface of the foot and medially to the distal phalanx. Isolated muscle function dictates this muscle inverts the foot and flexes the great toe. However, an integrated motion of the muscle assists in deceleration of foot eversion, hallux extension and helps to stabilize the great toe on the ground as it dorsiflexes. Additionally, the FDL pulls the phalange posteriorly to give the foot and first ray rigidity and stability during the heel off phase of gait. As the tibia further moves over the foot, the FDL assists to slow the motion, and as the heel rises, the FDL assists to then supinate the subtalar joint and tibia external rotation.

Lateral Compartment

The lateral compartment is comprised of two peroneal muscles: longus and brevis. Collectively, these muscles stabilize the foot on the ground, especially during mid-stance phase.

The peroneus longus originates at the upper two thirds of the lateral surface of the fibula and inserts at the medial cuneiform and base of the 1st metatarsal. The peroneus brevis originates at the lower two thirds of the lateral surface of the fibula and inserts at the 5th metatarsal tuberosity.

The peroneus tertius originates on the distal one third of the anterior fibula and inserts at the base of the 5th metatarsal.

Isolated function of the longus and brevis is plantar flexion and eversion. However, integrated actions of the muscles are critical for foot stability. The tendons of these muscles run posterior to the malleolus and on the plantar surface of the foot. As the longus is a stance phase muscle, it contracts at mid-stance and into the propulsive phase of gait to heel off. These tendons serve as a strong pulley to stabilize the first ray on the ground during the stance phase. Concomitantly, this muscle acts as a strong pronator of the foot and ensures the 1st ray stability during this phase. During the propulsive phase and prior to heel off, the angle of the peroneus longus tendon pulls the 1st ray laterally to assist in tarsal stability as the foot transforms into a rigid lever for propulsion.

The brevis tendon, being attached to the base of the 5th metatarsal, acts to pull upward on the lateral aspect of the foot to assist in keeping the foot firmly on the ground during the pronated stance phase of gait. It becomes recruited late in the stance phase and is active during the first half of propulsion.

Intrinsic Muscles of the Foot

The majority of the deep foot muscles are on the plantar surface of the foot to protect the bony and ligamentous structures that are inherent there. The actions of the majority of these muscles are to provide stability, especially during the mid to late stance phase of gait. Therefore, a general description will be given as there is not a great dynamic reaction within these muscles.

The muscles of the intrinsic foot consist of the flexor digitorum brevis, abductor hallucis, abductor digiti minimi, quadratus plantae, lumbricales, adductor hallucis, flexor digiti brevis, dorsal interossei, plantar interossei, extensor digitorum brevis and extensor hallucis brevis. These muscles work synergistically to provide stability during the tri-plane loading of the foot during all phases of gait. Secondly, by nature of the loads placed upon the foot, the joints undergo transition from a loosely packed unit to absorb forces to a tightly packed lever during the propulsion phase. All these muscles stabilize the joints and assist to control the tri-plane pull upon the joints as well as the posterior shear the foot withstands. These muscles provide the necessary “webbing” among the joints to allow the foot to work as a synchronous unit.

Summary

To obtain efficient and effective movement, the body must undergo certain actions that have profound impact upon the foot, and likewise, the foot also affects the body’s motion. There is an influence from the top down and the bottom up. During the gait cycle, the foot must pass through the loading (deceleration) phase, which happens as the calcaneus strikes the ground, forcing the heel to evert as the foot begins its plantar flexion moment. When the calcaneus moves through this frontal plane action, the subtalar joint will abduct, causing the mid-foot to “dive” medially and “flatten” out. As the body moves over the foot and further deceleration transpires, the forefoot (metatarsals and phalanges) will abduct in relation to the mid-foot. This lengthens the musculature of the foot to decelerate and stabilize this action. The response at the ankle is to dorsiflex, and the tibia internally rotates. As the body moves further over the foot and immediately prior to heel off, the lengthening of the muscle tissue stores energy, but equally important, a posterior directed force causes the bones of the forefoot and mid-foot to shift toward the calcaneus to create a rigid, stable lever to allow the foot to go from an absorbing mechanism to a propelling cantilever. When the foot goes through the acceleration phase, the calcaneus inverts, the ankle plantar flexes, and the tibia externally rotates to create a forceful push, thereby commencing into the next gait cycle.

Many interesting issues arise when these actions are compensated due to the myriad of foot abnormalities such as a high arched foot (cavus foot), excessively flat foot (planus foot), hammer toes, neuropathy or numerous other issues. Frankly, those conditions are out of the realm of the scope of this article. If students of fitness, training and human motion begin their journey to understand the nuances and complexities of the foot, it will greatly enhance their understanding of movement and program design.

References:

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