Ecorche Lesson 4:

The lower leg and foot

Summary

-Interosseus membrane

-Grouped deep flexors: Lateral

-Grouped deep flexors: Medial 

-Soleus

-Gastrocnemius

-Extensors of the Top of the Foot

-Grouped Extensors

-Tibialis Anterior

 -Muscles of the bottom of the foot

 

 -Fatty Pads of the sole of the foot


The Interosseous membrane

The Interosseous membrane (also called the middle tibiofibular ligament) is a sheet of connective tissue between the tibia and the fibula.  Broadly speaking, the muscles to the posterior of this sheet flex the foot (pointing the ankle and toes), whereas those to the anterior extend (pull back) the ankle and toes.  (A notable exception to this is the Peroneus longus that has its origin on the lateral fibula, with part of the bulk of the body of the muscle anterior to the plane of the tibia and fibula (This muscle is the top part of what we are grouping as the Lateral Deep Flexors below).  The tendon of this muscle does however hook behind the lateral maleolus (outer ankle bone - end of the fibula) thus exerting a flexion action).

Note that on your skeleton armature, the location of the interosseous membrane has been filled in with reinforcing material, so this is not something you can add.  The reinforcing acts as a partition in place of the membrane to divide the flexors on the back from the extensors on the front, as described above.


Cross section of the lower leg

In cross section we can see how much more bulk exists in the muscles toward the posterior of the leg, that are involved in pushing the body upwards against the force of gravity.

This difference in the mass of muscle on the front and back of the leg contributes to the S-curve sweep of the whole leg when viewed from the side (see the diagrams of the general pattern of the leg when viewed from the side)


Muscular attachments of the lower leg

Diagrams from Gray’s Anatomy: Muscle attachments on front and back of the tibia and fibula


Deep flexors group: Overview

-In the following two sections we will look at the lateral and medial components of this group

-Visualise this group overall simply as a broad mass that covers the posterior surface of the lower leg bones, and wraps forward to the side of the fibula at the top (the peroneus longus described above).  Tendons from muscles in this group produce notable surface markings on the foot.  It is then covered by the Soleus.

- The deep flexors group includes the Tibialis posterior, the peroneus longus and Peroneus brevis (also called the Fibularis longus and Fibularis brevis), the Flexor hallucis longus, Flexor digitorum longus

- In the diagram below, note on the left leg a gap has been left on the upper posterior surface of the Fibula, to indicate one attachement of the Soleus, which divides the Lateral from the Medial groups that we are discussing here. 

 

(Note: the Popliteus and Plantaris muscles are not included in this course since they are small and hidden under more superficial muscles)


Deep flexors group: Medial

-This group includes: Tibialis posterior, the Flexor hallucis longus, Flexor digitorum longus

 

Origins:

- The medial posterior surface of the fibula, 

- Posterior surface of the tibia, 

- Interosseus membrane, 

 

Insertions:

Medial side plantar (sole surface) of the toes and foot (for our purposes here, just wrap this tendon onto the plantar surface of the foot as shown.

 

Colour on your model:

Blue


Deep flexors group: Lateral

-This group includes the Peroneus longus and Peroneus brevis (also sometimes called the Fibularis longus and Fibularis brevis)

-The tendons of the Peroneus longus and brevis create notable surface forms above and below the lateral maleolus (outside ankle bone) as they descend to their insertion on the 5th metatarsal.

 

Origins

-Lateral fibula 

 

Insertions

The styloid process of the 5th metatarsal, and under the foot

 

Colour on your model:

Blue


Soleus

-Largest and most powerful muscle of the lower leg

- The illustration, at the start of this section, of the posterior of the lower leg bones from Gray’s anatomy, shows the posterior surface of the right Tibia and Fibula.  Note the “Popliteal Line”, which is the upper attachment of the Soleus. This slanting attachment shapes the form of the soleus to have its bulk higher on the lateral side than the medial side.

 

Origin:

Posterior surface of the upper end of the Fibula, Soleal line on the posterior tibia - Shown as the Popliteal line in the Gray’s diagram)    

 

Insertion:

Tendo calcaneus (the Achilles tendon)    

 

Colour on your model:

Red


Gastrocnemius 

(“calf muscle”)    

-Note the angle of the bottom of the calf muscle running diagonally downwards from the outside to the inside, following the form of the Soleus    

-The Gastrocnemius originates on the posterior surface of the Femur condyles.  A distinct part of the muscle in this area produces a mass at the back of the knee between the two lower ends of the hamstrings group.  There is a characteristic crease at the back of the knee at the lower end of this upper form of the Gastrocnemius, where the skin folds in a bent leg.  

 

Origin:

superior to articular surfaces of lateral condyle of femur and medial condyle of femur    

 

Insertion:

tendo calcaneus (achilles tendon) into mid-posterior calcaneus    

 

Colour on your diagram:

Green


Extensors of the top of the foot

The Extensor digitorum brevis and the Extensor hallucis brevis 

-We need to put these on before going further with the Extensors of the lower leg

-These small muscles lie on the top of the foot with tendons inserting on the big toe (Extensor hallucis brevis) and the other toes (Extensor digitorum brevis).  For our purposes here, the most important form that results from these muscles is the volume nestled in beside the end of the fibula, on top of the Cuboid bone, bordered on the lower side by flexor tendons running to the 5th metatarsal.  The tendons of the Extensor digitorum longus and Extnensor hallucis longus (in the next group to add) run over this mass at an angle.


Grouped Extensors

This group includes the Extensor hallucis longus, Extensor digitorum longus, and the Peroneus (or fibularis) tertius (note that the tendon of the Peroneus tertius has not been included in this course because it is not usually visible in the living figure). 

-These muscles lie in the anterior cavity between the Tibia and Fibula    

-Note the general convexity of the lateral side of the lower leg (as seen from various frontal views), sweeping outwards in counterbalance to the sweep of the quadriceps and innner knee mass towards the medial side, bordered by the sartorius muscle.

-Note how the muscle spirals around to front of the ankle as it descends, tapering gently all the while

- The tendons of the Extensor digitorum longus and Extensor hallucis longus and run across the top of the foot, out to the tops of the toes

 

Origin:

Fibula, interosseus membrane and the inferior surface of the lateral condyle of the tibia

 

Insertion:

The tendons of the fibularis are hooked behind the fibula maleolus and wrap under the sole of the foot.    

The extensor digitorum longus provides a tendon to each of the smaller toes

The extensor hallucis longus provide the large tendon that runs to the big toe across the top of the foot

 

Colour on your model:

Yellow



Tibialis Anterior

-Muscle next to the "shin" (the tibia), with its volume projecting just slightly in front of the most anterior edge of the tibia.  See the diagram above showing the cross section of the lower leg.

-Its tendon is the most obvious when the foot is extended (ie toes curled towards the shin)

-The tendon of this muscle projects further anteriorly than the tendons of the other extensors, which means that the form of the ankle, when seen in cross section slopes back towards the outside, the opposite diagonal angle to the slope of the shin.

Origin:

body of tibia    

Insertion:

medial cuneiform and first metatarsal bones of the foot (middle of inside of foot)    

Colour on your model:

Purple


Muscles of the sole of the Foot

We will be leaving out several muscles of the foot for the purpose of keeping this course to essentials of the observed form. The following are added in simplified form to achieve the main forms of the foot:

Abductor hallucis

Flexor digitorum brevis

Abductor digiti minimi

Notice from the diagram how these three muscles seem to “fan” or spiral across the sole of the foot, with the axel of this spiral at the calcaneus (heel bone).  The Abductor hallucis, on the big toe side, is set more deeply into the underside of the foot than the others, while the Abductor digiti minimi, on the opposite side is most shallow, keeping the form of the arch of the foot that the bones of the foot establishes.


Fatty pads of the sole of the foot and toes

-The bones, muscles and other tissue of the foot are protected by three fatty pads, that also contibrute to the form of the foot: The oval heel pad, the pad of the joint of the big toe , and the tear drop shaped pad under the outside toes (the four small toes)  

-Notice how the pad and joint of the big toe projects medially a little beyond the general line of the inner foot, thus counterbalancing the space under the arch of the foot on the medial side

- Fatty pads also protect the sole of each of the toes

- The big toe will often turn slightly inwards towards the smaller toes, and when combined with the joint of the big toe can be seen as a mass that counter balances the arc of the outside of the foot and toes


A simplified conception of the foot

1. Foot Arch (Tarsus group plus metatarsals)    

    -Top of arch articulates with the tibia and fibula at the ankle

    -The shorter side of the arch, the calacaneus,  forms the lever that the Calcaneal tendon (Achilles tendon) attaches to.

    -Abductor hallucis, Flexor digitorum brevis, and Abductor digiti minimi (discussed above) fill out the bottom of the mass of the arch

 

2. Heel pad

 

3. Small toes paddle

    -Sweeps around on the lateral side of the arch from the attachment of the peroneus brevis on the fifth         (most lateral) metatarsal

 

4. Big toe joint

    -The deepest mass in the ball of the foot, it projects medially a little beyond the general line of the             inner foot, thus counterbalancing the space under the arch of the foot on the medial side

 

5. Big toe     


Observed form of the foot: Lateral, anterior aspect


Observed form of the foot: Lateral, posterior aspect


Observed form of the foot: Medial, anterior aspect


Observed form of the foot: Medial, posterior aspect


Observed overall leg and foot structure: Anterior


Observed overall leg and foot structure: Posterior


Observed overall leg and foot structure: Lateral aspect, straight knee


Observed overall leg and foot structure: Lateral aspect, bent knee


Observed overall leg and foot structure: Medial aspect, straight knee