Restriction of Eversion During the Stance Phase


Role of Controlled Eversion in Efficient Gait

Controlled eversion of the subtalar joint during mid-stance provides several key biomechanical advantages for efficient human locomotion. This precisely regulated eversion facilitates appropriate medial rotation of the talus and the entire lower limb after heel contact, enhancing gait stability and efficiency.

The horizontal orientation of the subtalar joint’s articulating surfaces naturally promotes this biomechanical function. Without such a refined joint mechanism (controlled eversion), the plantar surface of the calcaneus, in conjunction with the medially rotating tibia, would behave like an unstable spinning top, causing an unsteady rotational movement on the ground.

To prevent this instability, systematic eccentric contractions of various supination muscles, including the posterior tibialis, regulate the speed of eversion and effectively limit excessive lowering of the medial longitudinal arch. Additionally, the finely controlled eversion of the subtalar joint optimizes midfoot flexibility, allowing the foot to adapt effectively to uneven terrain and slopes.

Clinical Implications of Abnormal Eversion

Extensive clinical reports highlight the influence of foot alignment on gait biomechanics. A particularly notable concern is excessive, improperly timed, or insufficiently regulated eversion at the subtalar joint during the stance phase. Various factors contribute to such abnormal movements, including:

  • Weakness of lower limb muscles
  • Dysfunction of the medial longitudinal arch’s support and control mechanisms
  • Structural abnormalities or atypical mobility of the tarsal bones

Regardless of the underlying cause, these cases commonly exhibit excessive calcaneal eversion after heel contact. Overpronation at the subtalar joint often serves as a compensatory mechanism for excessive or restricted movements in the frontal and transverse planes of the lower limb.

Interestingly, one of the most frequently observed structural deformities in individuals with excessive pronation is a relatively fixed rearfoot varus. Rearfoot varus refers to a condition where certain foot segments tilt inward toward the midline. As a compensatory mechanism, the subtalar joint undergoes excessive eversion during stance to ensure sufficient ground contact along the medial forefoot. This movement often exceeds normal ranges in both speed and magnitude. A similar compensatory pattern is observed in individuals with forefoot varus, though the precise causal relationship between these deformities and excessive rearfoot eversion remains unclear.

Kinematic Chain Effects and Knee Biomechanics

As previously discussed, excessive eversion of the rearfoot is closely linked to excessive transverse plane medial rotation of the talus and tibia during gait. These abnormal movements trigger a chain reaction of biomechanical compensations, altering the femorotibial joint’s contact area and increasing stress on the knee.

Excessive calcaneal eversion, in particular, intensifies valgus stress on the medial knee. Such abnormal biomechanics can contribute to patellofemoral pain syndrome (PFPS) and joint instability. Therefore, clinicians should carefully assess the subtalar joint's alignment during standing and gait when evaluating patients with knee pain or related functional impairments.

Complexity of Overpronation Mechanics

The underlying pathomechanics of excessive pronation are highly complex and multidimensional, with many aspects still not fully understood. These pathological phenomena extend beyond the intricate interactions of various foot joints, encompassing broader biomechanical relationships throughout the lower limb.

The origin of these pathological mechanisms may lie in proximal interactions between the hip and knee, or they may begin distally at the subtalar joint. Even if the issue is confined within the foot, the forefoot and rearfoot exhibit complex compensatory interactions for each other’s abnormal movements.

Furthermore, external factors such as footwear, orthotic devices, walking surfaces, and gait or running speed significantly influence foot and lower limb kinematics. Consequently, a comprehensive understanding of the entire lower limb’s biomechanical characteristics is essential for effectively treating patients with foot pain or alignment abnormalities.