Biomechanics of the foot
Analysis of the stride sequence

The gait cycle corresponds to all the movements performed between two successive contacts of the same foot on the ground. It is classically divided into three major phases:

1. Tardigrade phase: attack or initial contact phase

• Duration: ~10 hours (1 lecture, 3 lab sessions) over the course
• Main support: calcaneus (heel bone)
• Joint position: ankle in dorsiflexion, knee slightly flexed

Biomechanical objective: initiate the damping of the initial impact and start the absorption phase.

The heel contacts the ground, triggering a vertical transmission of impact forces. The hamstrings, quadriceps, and tibialis anterior muscles activate in synergy to control the foot's descent and stabilize the talocrural joint.

Please note: An overly brutal heel strike or poor shock absorption can cause heel pain (aponeurositis, plantar fasciitis, heel pain), or problems extending up to the knee or hip.

2. Plantigrade phase: Midstance / Mid-support

• Duration: ~40 hours (1 week of lectures, 3 weeks of lab work)
• Main support: midfoot
• Key structures: Chopard's joints (talonavicular and calcaneocuboid), medial arch

Biomechanical objective: stabilize single-leg stance and redistribute loads.

The foot progressively shifts from a shock-absorbing role to that of a stabilizer. The midtarsal joints allow the foot to adapt to uneven surfaces, while the posterior tibialis, long fibularis, and short fibularis muscles ensure dynamic arch support.

Typical dysfunctions:

• Hyperpronation: Excessive collapse of the medial arch of the foot
• Functional flatfoot: impaired muscle control
• Mid-tarsal surcharge: diffuse midfoot pain

3. Digitigrade Phase: Propulsion Phase (Terminal Stance & Pre-swing)

• Duration: ~50 hours (1 week of theory, 3 weeks of practice)
• Main support: forefoot and first ray (hallux)
• Key structures: metatarsophalangeal joint of the hallux, triceps surae, Achilles tendon

Biomechanical objective: generate the propulsive force allowing the transfer of the center of mass.

The foot becomes rigid to maximize mechanical efficiency. Terminal supination locks the hindfoot, and tension in the triceps surae via the Achilles tendon releases elastic energy essential for propulsion. The big toe acts as a pivot lever through the extension of its MTP joint.

Common problems:

• Metatarsalgias (pain under the metatarsal heads)
• Hallux limitus or rigidus: loss of big toe mobility
• Morton's neuroma: nerve compression between the metatarsals