Biomechanics

From "Donatelli - The Biomechanics of the foot and ankle - 2nd ed. CPR"

Vocabulary:

Calcaneus:

inversion = varus; eversion = valgus

Subtalar joint (Calcaneus - Talus):

Axis: Antero-medially, through talus neck to postero-lateral aspect of calcaneus

Pronation (eversion) = dorsiflexion + abduction + eversion (pronation)

Supination (inversion) = plantar flexion + adduction + inversion (supination)

Forefoot (Tarso - MT joint = Lisfranc):

Varus (supination) = clockwise rotation of 1st metatarsal round 5th metatarsal

Valgus (pronation) = COUNTER clockwise rotation of 1st metatarsal round 5th metatarsal

Gait

At heel strike, pronation (absorption compressive force, adjust to uneven ground, maintain equilibrium)

Eccentric action of Tibialis anterior + Toes extensors -> to control plantar flexion

At forefoot contact,

Eccentric action of supinators (Tibialis posterior, flexor hallucis longus and digitorum longus, soleus) -> to control pronation

During midstance,

Same muscles control eccentrically forward toppling of tibia on talus

Subtalar joint in neutral position

At push-off,

Supination (from mid-stance to toe off) -> to establish a rigid lever for extrinsic foot muscles (plantar flexors)

Windlass mechanism = Tension of plantar fascia (aponeurosis)

Plantar fascia

From postero-medial calcaneal tuberosity to proximal phalanges

Tension during push-off because of phalangeal flexion

Medial origin -> tension -> Inversion of calcaneus + Supination of subtalat joint

Standing position

High medial arch (Pes cavus) = external rotation of tibia + supination at subtalar and talar joint + pronation of forefoot

Low medial arch (Flat foot) = internal rotation of tibia + pronation at subtalar and talar joint + supination of forefoot

Concurrent motions:

- Internal rotation of tibia + pronation subtalar joint

- External rotation of tibia + supination subtalar joint

Orthotics

Rearfoot posting -> modify subtalar joint position from heel strike to flat foot

Brings calcaneus back to neutral position:

- Varus (Inversion) = lateral posting to correct inversion

- Valgus (eversion) = medial positng to correct eversion

As well, forefoot posting brings the ground to the metatarsals, reducing need for rearfoot compensation

- Medial wedge = forefoot varus = No need for rearfoot pronation to compensate

- Lateral wedge = forefoot valgus = No need for rearfoot inversion to compensate