At the terminal phase of each gait cycle, the posterior compartment muscles contract concentrically to plantarflex the ankle joint. Additionally, the fibularis longus and fibularis brevis in the lateral compartment also contribute to plantarflexion torque.
During the push-off phase, the degree of muscle activation by plantarflexors is modulated based on walking speed and intensity. While these muscles are the primary force generators for propulsion, coordinated energy contributions from hip extensors (active in early stance) and hip flexors (active in late stance) are also essential for effective motion.
Magnitude of Plantarflexion Torque
In healthy adults, maximum isometric plantarflexion torque is significantly greater than that generated in any other movement direction at the ankle or foot.
This high torque capacity is critical for movements such as:
- Agile walking
- Running
- Jumping
- Climbing
Plantarflexion torque reaches its maximum when the ankle is fully dorsiflexed, meaning the plantarflexor muscles are stretched. Conversely, it is at its lowest when the ankle is fully plantarflexed.
Typically, dorsiflexion occurs as part of the preparatory phase for sprinting or jumping.
Stretch-Length Relationship and Two-Joint Muscle Function
A fascinating aspect of sprint acceleration and jump initiation is that strong dorsiflexion of the ankle also results in the gastrocnemius being lengthened by knee extension.
This biarticular muscle arrangement prevents excessive shortening of the gastrocnemius, allowing it to generate higher torque over a broader range of ankle motion.
Unlike the gastrocnemius, the soleus does not cross the knee joint, meaning that its length-tension relationship is unaffected by knee position.
- The soleus, with its dominance of slow-twitch fibers, is better suited for postural control, helping to stabilize minor tibial oscillations over the talus in standing posture.
- The gastrocnemius, with its higher proportion of fast-twitch fibers, is more specialized for explosive plantarflexion torque, making it ideal for jumping and sprinting, where powerful knee extension is required.
Torque Contribution of the Triceps Surae
Among all plantarflexor muscles, the gastrocnemius and soleus generate the strongest force, accounting for approximately 80% of total plantarflexion torque at the ankle.
This high torque capacity is due to:
- Large physiological cross-sectional area (PCSA) of the triceps surae
- Relatively long moment arm
The calcaneal tuberosity, where the Achilles tendon inserts, is positioned about 5.3 cm posterior to the ankle joint, providing a lever arm that is nearly twice as long as the average moment arm of other plantarflexor muscles.
This structural advantage significantly enhances the mechanical efficiency of the triceps surae, making it the primary powerhouse for plantarflexion-based propulsion in human locomotion.