double legged support (12%) followed by an intermediate phase (38%) then another period of double legged support
In running there is no period of double legged support; indeed there is a float phase when neither foot is in contact with the ground
During Gait
centre of mass (anterior to S2) moves 5cm laterally
Shock absorption
Ground reaction forces during walking reach 1.5 times body weight. During running they are 3 times body weight
Shock absorption by the hindfoot, ankle & motors acting across them is very important to reduce stresses across the leg & back
Factors involved are
Static structures
Heel pad
Dynamic structures
Eccentric contractions of muscles e.g. tibialis anterior
Controlled descent of the forefoot after heel strike is the most important function of tibialis anterior
Absence of tib anterior function results in foot slap
Tibialis posterior
Contraction results in controlled hindfoot eversion
Gastrocsoleus
Controls forward motion of the tibia relative to the foot
Change in vectors – the axial load of impact is dissipated by partly converting it to a rotatory load, as the talus & tibia internally rotate on heel strike
Accommodation to surface irregularities
Achieved by the freedom of movement in the subtalar & transverse tarsal joints
Push off
At push off the flexible foot converts to a rigid one, to allow propulsion
How does this occur?
At foot strike the tibia internally rotates, the calcaneus assumes a valgus posture & the subtalar joint everts
As the pelvis rotates towards the stance foot the tibia externally rotates
This causes external rotation of the talus, in particular its head
This results in the orientation of the axes of the talonavicular joints & the calcaneocuboid joints changing from parallel to divergent
divergent axes lock the transverse tarsal joint & make the foot rigid
calcaneus assumes a varus posture & the subtalar joint inverts
Other factors
Tightening of the plantar fascia via the windlass effect: the fascia arises from the medial side of the calcaneus & as it tightens it pulls the calcaneus into varus
Contraction of Tibialis Posterior which holds the navicular medially on the talar head
Contraction of the peroneus longus which plantar flexes the first ray
Columns of the foot
medial column consists of the medial navicular, medial cuneiform & 1st MT
middle column consists of the lateral navicular, intermediate & lateral cuneiforms & the 2nd & 3rd MT
lateral column consists of the cuboid & the 4th & 5th MT
