Ankle Muscle Stiffness in the Control of Balance During Quiet Standing

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Abstract
This research presents new data and reanalyzed information to refute the criticisms of our model of stiffness control during quiet standing. A re-review of their references to biomechanical research on muscle ankle stiffness confirmed muscle stiffness estimates of the ankle series elastic elements that agreed closely with our estimates. A new technique is presented that directly estimates the muscle stiffness from the ankle moment (N · m) and sway angle (deg). The linear regression of 10 subjects standing quietly for 10 s estimated the stiffness (N · m/deg) to be safely above the gravitational spring. The R 2 scores for this linear regression averaged 0.92, confirming how closely the model approached a perfect spring that would have an R 2 = 1. These results confirm our model of a simple muscle stiffness control and refutes the criticisms.