Examining the complex mechanics of foot movement, it is notable that all plantarflexor muscles, except for the fibularis longus and brevis, not only act at the talocrural joint but also contribute to supination (inversion) at the subtalar and transverse tarsal joints. This strong tendency toward inversion is particularly evident when considering the posterior compartment muscles' anatomical alignment relative to the subtalar joint.
An interesting observation is that the line of action of the Achilles tendon runs slightly medial to the subtalar joint’s axis of rotation, meaning that even the triceps surae (gastrocnemius and soleus) contribute a subtle but significant supination effect.
The primary supinator muscles of the foot include:
- Tibialis posterior (most significant contributor)
- Flexor hallucis longus
- Flexor digitorum longus
Beyond plantarflexion and supination, the flexor digitorum longus and flexor hallucis longus also control motion at the distal joints, particularly at the metatarsophalangeal (MTP) and interphalangeal joints.
Activation Patterns of Plantarflexor and Supinator Muscles During Gait
The activation of plantarflexor and supinator muscles is most pronounced during the stance phase, specifically from foot flat to toe-off.
Early Stance Phase
- Muscle activation begins immediately after the relaxation of the dorsiflexor muscles.
- From foot flat to just before heel-off, the plantarflexors eccentrically contract to control forward tibial motion over the talus.
Heel-Off to Toe-Off
- Muscle action shifts to concentric contraction, generating propulsive force for forward momentum and initial swing phase preparation.
- The flexor hallucis longus, flexor digitorum longus, and intrinsic foot muscles (lumbricals and interossei) ensure stabilization of the extended toes, allowing even distribution of plantar pressure across the forefoot.
Role of Tibialis Posterior in Controlling Pronation and Supination
During stance, the tibialis posterior, flexor hallucis longus, and flexor digitorum longus play a crucial role in:
- Resisting excessive pronation
- Facilitating controlled supination
Among these, tibialis posterior is the most effective in performing these functions.
Electromyography (EMG) Studies
- Research indicates that the tibialis posterior remains active throughout the entire stance phase, particularly from initial contact to heel-off, when it works eccentrically to control rearfoot pronation.
- As the foot fully contacts the ground, the tibialis posterior slows down excessive pronation, allowing for a gradual, controlled descent of the medial longitudinal arch.
This eccentric contraction enables the tibialis posterior to absorb impact loads efficiently.
Clinical Implications: Tibialis Posterior Dysfunction and Overpronation
Excessive and rapid pronation places excessive deceleration demands on the tibialis posterior, potentially leading to:
- Tendon overuse injuries
- Fatigue-induced dysfunction
- Medial tibial stress syndrome (shin splints)
The causal relationship between overpronation and tibialis posterior dysfunction is not always clear:
- Does excessive pronation lead to tibialis posterior dysfunction?
- Or does tibialis posterior dysfunction cause excessive pronation?
Regardless of the sequence, tibialis posterior weakness impairs the foot’s shock absorption mechanisms, leading to progressive arch collapse.
Tibialis Posterior Function in Mid to Late Stance Phase
- As stance progresses, the tibialis posterior actively contracts to induce rearfoot supination, restoring medial arch height.
- This contraction also assists in medial tibial rotation, aligning the talus and tibia for efficient force transmission.
Ultimately, the tibialis posterior's role in controlling foot motion is critical for arch integrity, shock absorption, and propulsion efficiency during gait.