Eversion at the Subtalar Joint

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Eversion Transition After Heel Contact

Immediately after the heel makes contact with the ground during walking (heel contact phase), the talocrural joint, initially in dorsiflexion, and the subtalar joint, which starts in slight inversion, rapidly transition into plantarflexion and eversion, respectively. Studies indicate an average eversion angle of approximately 2° relative to the neutral stance. However, other research conducted on asymptomatic individuals has reported ranges between 5° and 9°, highlighting inconsistencies in the literature.

These discrepancies arise due to variations in defining the neutral (0°) position of the subtalar joint, differences in sample sizes, and diverse measurement techniques. As a result, establishing a clear threshold for "abnormal" eversion during gait remains challenging.

Biomechanical Mechanisms of Eversion During Stance Phase

Eversion at the subtalar joint during the stance phase primarily occurs through two key biomechanical mechanisms:

  1. Ground Reaction Force Influence

    • Upon heel contact, an upward and slightly lateral ground reaction force acts on the central posterior aspect of the calcaneus, inducing a subtle lateral rotation.
    • Simultaneously, the impact forces at heel contact shift the talar head medially in the transverse plane and downward in the sagittal plane.
    • This relative movement between the talus and the calcaneus results in abduction and slight dorsiflexion at the subtalar joint, which aligns precisely with the formal definition of eversion.

  2. Lower Limb Rotation Contribution

    • During early stance, immediately after heel contact, the tibia and fibula, and to some extent the femur, rotate medially.
    • Due to the unique anatomical structure of the talocrural joint, this medial rotation of the lower leg further enhances eversion at the subtalar joint.
    • However, debate remains as to whether subtalar eversion passively follows tibial rotation or actively drives it. Both perspectives hold scientific merit and continue to be studied.

Magnitude and Speed of Eversion in Gait

The actual eversion at the subtalar joint during early to mid-stance is relatively small, averaging around 5°, and occurs rapidly over approximately 0.25 seconds at normal walking speeds. Despite its limited range and short duration, this movement has a profound impact on the kinematics of the proximal lower limb joints.

This biomechanical influence can be demonstrated by deliberately exaggerating and slowing down rearfoot eversion during the early loading phase of gait. When the lower leg is forcefully but slowly rotated medially while the foot is weight-bearing, eversion at the subtalar joint and the corresponding lowering of the medial longitudinal arch become clearly observable.

Effects on Lower Limb Kinetics

If the medial rotational force is strong enough, this kinetic sequence can lead to:

  • Medial rotation, slight flexion, and adduction at the hip joint.
  • Valgus stress at the knee joint, which places additional strain on the medial collateral ligament (MCL).

Although these mechanics are exaggerated in controlled demonstrations, they indicate how excessive or uncontrolled subtalar eversion can trigger compensatory movements in multiple joints throughout the lower limb kinetic chain.

Clinical Implications

In clinical practice, patients exhibiting excessive eversion in early stance often report medial knee pain. This is associated with increased valgus stress and potential overstretching of the MCL. However, whether excessive eversion directly causes MCL strain or vice versa remains unclear.

The kinematic relationship between excessive eversion and excessive medial tibial rotation is widely acknowledged, but the exact magnitude and temporal sequence of these movements have not been definitively established. Technological limitations make precise measurement of these interactions during dynamic gait challenging, and natural variations in movement patterns further complicate standardization.

A review of existing studies reveals methodological differences:

  • Some studies analyze the rotational movement of individual bones.
  • Others measure the relative rotation between adjacent bones.

Future Research and Clinical Relevance

Further research is needed to clarify the causal relationship between subtalar eversion and lower limb biomechanics. A precise understanding of these interactions is essential for developing targeted rehabilitation strategies, optimizing orthotic interventions, and effectively managing pain syndromes associated with excessive or unregulated eversion.