The Loading Phase of Gait and Pronation: Joint Mechanics and Clinical Implications

Joint Mechanics During Pronation

Ankle Joint

The ankle joint plays a key role in the loading phase, allowing for controlled plantar flexion during initial contact, terminal stance to toe off and dorsiflexion at .

• Sagittal Plane: In the sagittal plane, the ankle begins the loading phase dorsiflexion at heel strike, and remains dorsiflexed as the foot makes full contact with the ground (Inman, 1976). This movement is controlled by the gastrocnemius and soleus muscles, which eccentrically contract to manage the rate of dorsiflexion.

• Frontal Plane: In the frontal plane, the subtalar joint, part of the ankle complex, begins to evert, which is a key component of pronation. Eversion allows the medial side of the foot to lower, accommodating uneven surfaces and promoting shock absorption. Subtalar eversion is particularly important because it allows the foot to unlock and become more flexible during the loading phase, a mechanism crucial for distributing forces through the lower limb (Wright et al., 2011).

• Transverse Plane: In the transverse plane, the tibia internally rotates during the loading phase, which corresponds with the internal rotation of the talus due to the heel internally rotating. This movement further facilitates pronation, enabling the foot to adapt to the ground. Internal tibial rotation is an essential part of the pronation process, as it helps to align the lower limb for effective force transmission through the joints (Perry & Davids, 2010).

Rearfoot (Subtalar Joint)

The subtalar joint, located between the talus and calcaneus, is the primary site of foot pronation during the loading phase.

• Sagittal Plane: In the sagittal plane, the rearfoot begins in dorsiflexion and quickly moves into plantar flexed position as the foot flattens during loading (Inman, 1976). The rear foot’s plantar flexion is less pronounced than at the ankle but still contributes to the overall ‘pronatory’ motion.

• Frontal Plane: Rear foot eversion is a significant feature of the loading phase, as the calcaneus rolls inward (everts) in response to ground contact. This eversion helps to unlock the foot, allowing the midfoot and forefoot to adapt to the ground and distribute forces evenly. The subtalar joint plays a pivotal role in this motion, as it controls the degree of eversion and ensures smooth progression through the stance phase (Wright et al., 2011).

• Transverse Plane: As the calcaneus everts and plantar flexes creates the internal rotation, the talus simultaneously rotates internally. This talar motion facilitates medial rotation of the tibia, a key aspect of pronation that allows for the transfer of forces from the foot to the knee and hip (Inman, 1976).

Forefoot and Toes

The forefoot and toes are involved later in the loading phase, particularly as weight shifts toward the metatarsals and toes.

• Sagittal Plane: The forefoot appose the rear foot and moves into dorsiflexion from a relative plantar flexed position as the foot flattens. The toes oppose the forefoot during this phase, preparing to assist with propulsion later in the stance phase.

• Frontal Plane: The forefoot undergoes pronation, as the medial arch lowers to accommodate ground contact requiring an inversion motion. This allows for even pressure distribution across the metatarsals and prevents excessive loading on any one part of the foot (Perry & Davids, 2010).

• Transverse Plane: The forefoot experiences abduction or internal rotation as it adjusts to the ground surface. This movement is an oppositional motion to the rear foot and contributes to the overall pronation mechanism.