Figure 2.

Alteration of Knee Flexion Angle at Heel Strike as a mechanical marker of OA risk. (a) Compared to young, healthy subjects, knee flexion during the heel strike phase of walking is increased in older subjects and in patients with knee OA.⁸ Knee flexion at heel strike is also increased compared to the contralateral side in patients after ACLR, and associated clinically with premature OA.⁹ Wearing high heels results in a similar increase in knee flexion at heel strike compared to the same person wearing flat shoes indicating complete reversibility in this group.¹⁰ The system metaphor (Fig. 1) helps to place the use and evaluation of a potential intervention in context. (b) In younger subjects the mechanical component (knee flexion) appears to be a promising target for intervention. (c) In older subjects there are biological changes that also need to be considered in assessing the effectiveness of potential interventions.