Ankle stiffness increases proportionally to weight borne on the ankle

Ankle sprains are one of the most common musculoskeletal injuries, leading to chronic ankle instability (CAI) in 30-40% of first-time sprains. Clinicians assess passive stability under non-weight-bearing conditions, but their relevance to weight-bearing conditions when ankle sprains occur is unclear. The objective of this study is to quantify how weight-bearing affects the stability of the ankle in the frontal plane. We assessed this by computing the stiffness of the ankle using small random angular displacements of the ankle in the frontal plane. We found that ankle stiffness increased with increasing weight borne on the ankle. At full body weight, the ankle was on average 3.7 times stiffer than during non-weight-bearing conditions. Thus, non-weight-bearing measures of ankle stability differ greatly from the ankle stability during the full body weight condition when sprains typically occur.