A COMPARATIVE ANALYSIS OF SHOE SOLE HARDNESS AND ITS EFFECT ON KNEE JOINT LOADING DURING GAIT: A NARRATIVE REVIEW

Background: The way impact and propulsive forces are transferred through the lower limb can be altered by shoe sole hardness, also known as midsole mechanical compliance or stiffness. This could potentially change knee joint loading, which is a significant risk factor for discomfort and osteoarthritis (OA). Purpose: To compile data from earlier studies on the effects of sole hardness and associated sole designs on knee joint loads during running and walking. Methods: Peer-reviewed studies that report knee loading metrics, such as external knee adduction moment (KAM), knee flexion/extension moments, vertical ground-reaction force (vGRF), loading rates, knee stiffness, or patellofemoral surrogates, and examine footwear midsole hardness or functionally similar design features (e.g., rocker soles, variable-stiffness soles) are narratively reviewed. Results: Impact is modulated by midsole hardness: harsher soles tend to enhance impact peaks and loading rates, whereas softer soles tend to absorb impacts but can change joint strategy and occasionally increase knee moments, particularly while running. In some people with medial knee OA, functionally "softening" or dispersing load with rocker or laterally stiff/medial-compliant shoes might lower KAM, however reactions vary. Speed, strike pattern, task (walk vs. run), and participant characteristics all affect the effects. Although the effects vary by person and parameter, sole hardness has a significant impact on knee loads. Rocker or lateral stiffness designs help lower frontal- plane knee load (KAM) for people with knee OA while they walk. Softer, more cushioned soles for runners may lessen impact, but in some situations, they may also improve knee-dominant mechanics. It is advised to use task-specific testing and individualized selection.