The foot complex Three major articulations within the foot: Sub-talar, Mid- tarsals, MTPs. Sub-talar joint lies within the vertical weight bearing line of the lower extremity. It adds motion three- dimensionality to the foot. It also modifies mobility of mid-tarsal joints. The mid-tarsal joints or transverse tarsal joints (talo- navicular and calcaneo-cuboidal), attaches the hind to the forefoot. They contributes to the shock absorptive function of the foot. The MTPs or the toe-break, allows the foot to roll over, they provide a wide supportive area.

Motion of the sub-talar joint: Sub-talar joint axis passes from posterior-inferior-lateral aspect of the foot to anterior-superior-medial aspect of the foot which allows the foot to invert and evert. Eversion starts as part of loading response. Peak eversion (4-6 degrees) is reached at 14% GC. The motion reverses during terminal stance to inversion. Peak inversion is reached at 52% GC. During initial swing the foot drifts back to normal, followed by terminal inversion during the last 20% GC.

Muscle control: Invertors: according to leverage: tibialis posterior, tibilais anterior, flexor digitorum longus, flexor hallucis longus, soleus.

Tibialis anterior: ?? Soleus: ?? Tibialis posterior: variable activity among persons and among strides, but generally, there are two peaks of activity, at the end of LR and mid-terminal stance. FHL and FDL: both have a pattern of activity that is very close to that of GC and soleus. Evertors: anterior evertors: extensor digitorum longus and peroneus tertius. Posterior evertors: peroneus longus and brevis, and gastrocnemius.

EDL: ?? Gastrocnemius: ?? Peroneus longus and brevis: similar activity between 15 and 55% GC. Intrinsic muscles are included.

Functional interpretation of the sub-talar joint: Initiating floor contact with the heel introduces sub-talar joint eversion as a normal passive motion. This is due to the fact that the body of calcaneus is lateral to the longitudinal axis of tibia. The calcaneus everts, the talus loses its distal support and thus inverts, this with the tightly fitted rectangular shape of the ankle causes internal rotation of tibia. If the calcaneus is left to evert freely, a shock will result, thus controlled eversion is required, how?? Which muscles?? Tibialis anterior is already working during LR, why?? This is assisted by tibilais posterior.

Peak eversion is reached at 14%, by this time which muscles are controlling eversion? Tibialis posterior and soleus or gastrocnemius?? Do they contract primarily to control eversion or some other movement?

Towards the end of stance, the sub-talar joint inverts for reasons that will be explained later, the four main invertors are active during late stance, these are: soleus, tibialis posterior, FDL, and FHL. Why is soleus active during this time?? Mid-tarsal joints: Motion was observed but not measured. DF: flattening of the arch, PF: recovery of the arch. Flattening of the arch follows forefoot contact = early MST. Heel rise causes recovery of the arch = DF is reserved = in TST.

Functional interpretation of mid-tarsal joints: Mid-tarsal joints dorsiflexion serves the function of shock absorption. Mid-tarsal joints DF is evoked by forefoot floor contact, this absolutely follows LR.

Recovery of the arch after heel rise is needed for a very important function that is to be described later. Ankle-subtalar joint synergy is not included. Sub-talar- mid-tarsal joint synergy: Sub-talar joint controls mid-tarsal mobility by the relative alignment of the TN and CC joints. Sub-talar eversion = parallel TN and CC axes = shock absorption. Sub-talar inversion = TN and CC axes cross = very rigid lever = progression.

Motion of the MTPs: IC: 25 degrees of DF. LR: after forefoot contact, the MTPs PF until they reach neutral. MST: remain neutral. TST: DF to 21 degrees (Metatarsal rocker). PSW: DF to 55 degrees (to maintain contact with the floor). ISW: drop in PF to neutral. MSW and TSW: DF to 25 degrees.

Foot support patterns: Heel only support: calcaneograde (10% GC). Foot flat support: plantigrade (10 – 30% GC). Forefoot support: digigrade (30 – 60% GC).