Abstract
The purpose of this study is to quantify sex differences in 2-dimensional (2D) ankle stiffness during upright standing balance and investigate the mechanisms for the differences. A dual-axis robotic platform, capable of perturbing the ankle and measuring the corresponding ankle torques in both the sagittal and frontal planes, was used to reliably quantify the 2D ankle stiffness while healthy young human subjects perform a range of standing balance tasks, specifically, ankle muscle co-contraction tasks, weight-bearing tasks, and ankle torque generation tasks. In all task conditions and in both planes of ankle motion, ankle stiffness in males was consistently greater than that in females. Among all 26 experimental conditions, all but 2 conditions in the frontal plane showed statistically significant sex differences. Further investigation on the normalized ankle stiffness, scaled by weight times height, suggests that while sex differences in ankle stiffness in the sagittal plane could be explained by sex differences in anthropometric factors as well as neuromuscular factors, the differences in the frontal plane are mostly explained by anthropometric factors. This study also demonstrates that the sex differences in the sagittal plane were significantly higher as compared to those in the frontal plane. The results in this study will provide a foundation for not only characterizing sex differences in ankle stiffness during locomotion, but also investigating sex differences in lower body stability and risk of ankle injury.
Highlights
The human ankle is an essential joint which plays one of the most important roles in postural stability and locomotion (Winter, 1995)
Based on precise ankle torque and kinematic measurements, ankle stiffness was quantified with a high reliability in both the sagittal and frontal planes in all 40 subjects (Figure 2)
Previous studies have demonstrated that the incidence of musculoskeletal injuries at the ankle joint in females is significantly higher than in males participating in similar sports activities (Elias, 2001; Ristolainen et al, 2009)
Summary
The human ankle is an essential joint which plays one of the most important roles in postural stability and locomotion (Winter, 1995). It contributes to the movement and stabilization of the entire human body in both static and dynamic conditions (Robertson and Winter, 1980). The increased rate of musculoskeletal injury is largely associated with the greater range of motion (Beynnon et al, 2001), lower Young’s modulus (Kubo et al, 2003), and higher joint and ligamentous laxity (Wilkerson and Mason, 2000) in females. It is speculated that cyclic hormonal variations could cause a decrease in the strength of muscles and ligaments, and could increase in ligamentous
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